fs: Allow listmount() in foreign mount namespace

[linux.git] / fs / namespace.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 *  linux/fs/namespace.c
 *
 * (C) Copyright Al Viro 2000, 2001
 *
 * Based on code from fs/super.c, copyright Linus Torvalds and others.
 * Heavily rewritten.
 */

#include <linux/syscalls.h>
#include <linux/export.h>
#include <linux/capability.h>
#include <linux/mnt_namespace.h>
#include <linux/user_namespace.h>
#include <linux/namei.h>
#include <linux/security.h>
#include <linux/cred.h>
#include <linux/idr.h>
#include <linux/init.h>         /* init_rootfs */
#include <linux/fs_struct.h>    /* get_fs_root et.al. */
#include <linux/fsnotify.h>     /* fsnotify_vfsmount_delete */
#include <linux/file.h>
#include <linux/uaccess.h>
#include <linux/proc_ns.h>
#include <linux/magic.h>
#include <linux/memblock.h>
#include <linux/proc_fs.h>
#include <linux/task_work.h>
#include <linux/sched/task.h>
#include <uapi/linux/mount.h>
#include <linux/fs_context.h>
#include <linux/shmem_fs.h>
#include <linux/mnt_idmapping.h>
#include <linux/nospec.h>

#include "pnode.h"
#include "internal.h"

/* Maximum number of mounts in a mount namespace */
static unsigned int sysctl_mount_max __read_mostly = 100000;

static unsigned int m_hash_mask __ro_after_init;
static unsigned int m_hash_shift __ro_after_init;
static unsigned int mp_hash_mask __ro_after_init;
static unsigned int mp_hash_shift __ro_after_init;

static __initdata unsigned long mhash_entries;
static int __init set_mhash_entries(char *str)
{
        if (!str)
                return 0;
        mhash_entries = simple_strtoul(str, &str, 0);
        return 1;
}
__setup("mhash_entries=", set_mhash_entries);

static __initdata unsigned long mphash_entries;
static int __init set_mphash_entries(char *str)
{
        if (!str)
                return 0;
        mphash_entries = simple_strtoul(str, &str, 0);
        return 1;
}
__setup("mphash_entries=", set_mphash_entries);

static u64 event;
static DEFINE_IDA(mnt_id_ida);
static DEFINE_IDA(mnt_group_ida);

/* Don't allow confusion with old 32bit mount ID */
static atomic64_t mnt_id_ctr = ATOMIC64_INIT(1ULL << 32);

static struct hlist_head *mount_hashtable __ro_after_init;
static struct hlist_head *mountpoint_hashtable __ro_after_init;
static struct kmem_cache *mnt_cache __ro_after_init;
static DECLARE_RWSEM(namespace_sem);
static HLIST_HEAD(unmounted);   /* protected by namespace_sem */
static LIST_HEAD(ex_mountpoints); /* protected by namespace_sem */
static DEFINE_RWLOCK(mnt_ns_tree_lock);
static struct rb_root mnt_ns_tree = RB_ROOT; /* protected by mnt_ns_tree_lock */

struct mount_kattr {
        unsigned int attr_set;
        unsigned int attr_clr;
        unsigned int propagation;
        unsigned int lookup_flags;
        bool recurse;
        struct user_namespace *mnt_userns;
        struct mnt_idmap *mnt_idmap;
};

/* /sys/fs */
struct kobject *fs_kobj __ro_after_init;
EXPORT_SYMBOL_GPL(fs_kobj);

/*
 * vfsmount lock may be taken for read to prevent changes to the
 * vfsmount hash, ie. during mountpoint lookups or walking back
 * up the tree.
 *
 * It should be taken for write in all cases where the vfsmount
 * tree or hash is modified or when a vfsmount structure is modified.
 */
__cacheline_aligned_in_smp DEFINE_SEQLOCK(mount_lock);

static int mnt_ns_cmp(u64 seq, const struct mnt_namespace *ns)
{
        u64 seq_b = ns->seq;

        if (seq < seq_b)
                return -1;
        if (seq > seq_b)
                return 1;
        return 0;
}

static inline struct mnt_namespace *node_to_mnt_ns(const struct rb_node *node)
{
        if (!node)
                return NULL;
        return rb_entry(node, struct mnt_namespace, mnt_ns_tree_node);
}

static bool mnt_ns_less(struct rb_node *a, const struct rb_node *b)
{
        struct mnt_namespace *ns_a = node_to_mnt_ns(a);
        struct mnt_namespace *ns_b = node_to_mnt_ns(b);
        u64 seq_a = ns_a->seq;

        return mnt_ns_cmp(seq_a, ns_b) < 0;
}

static void mnt_ns_tree_add(struct mnt_namespace *ns)
{
        guard(write_lock)(&mnt_ns_tree_lock);
        rb_add(&ns->mnt_ns_tree_node, &mnt_ns_tree, mnt_ns_less);
}

static void mnt_ns_release(struct mnt_namespace *ns)
{
        lockdep_assert_not_held(&mnt_ns_tree_lock);

        /* keep alive for {list,stat}mount() */
        if (refcount_dec_and_test(&ns->passive)) {
                put_user_ns(ns->user_ns);
                kfree(ns);
        }
}
DEFINE_FREE(mnt_ns_release, struct mnt_namespace *, if (_T) mnt_ns_release(_T))

static void mnt_ns_tree_remove(struct mnt_namespace *ns)
{
        /* remove from global mount namespace list */
        if (!is_anon_ns(ns)) {
                guard(write_lock)(&mnt_ns_tree_lock);
                rb_erase(&ns->mnt_ns_tree_node, &mnt_ns_tree);
        }

        mnt_ns_release(ns);
}

/*
 * Returns the mount namespace which either has the specified id, or has the
 * next smallest id afer the specified one.
 */
static struct mnt_namespace *mnt_ns_find_id_at(u64 mnt_ns_id)
{
        struct rb_node *node = mnt_ns_tree.rb_node;
        struct mnt_namespace *ret = NULL;

        lockdep_assert_held(&mnt_ns_tree_lock);

        while (node) {
                struct mnt_namespace *n = node_to_mnt_ns(node);

                if (mnt_ns_id <= n->seq) {
                        ret = node_to_mnt_ns(node);
                        if (mnt_ns_id == n->seq)
                                break;
                        node = node->rb_left;
                } else {
                        node = node->rb_right;
                }
        }
        return ret;
}

/*
 * Lookup a mount namespace by id and take a passive reference count. Taking a
 * passive reference means the mount namespace can be emptied if e.g., the last
 * task holding an active reference exits. To access the mounts of the
 * namespace the @namespace_sem must first be acquired. If the namespace has
 * already shut down before acquiring @namespace_sem, {list,stat}mount() will
 * see that the mount rbtree of the namespace is empty.
 */
static struct mnt_namespace *lookup_mnt_ns(u64 mnt_ns_id)
{
       struct mnt_namespace *ns;

       guard(read_lock)(&mnt_ns_tree_lock);
       ns = mnt_ns_find_id_at(mnt_ns_id);
       if (!ns || ns->seq != mnt_ns_id)
               return NULL;

       refcount_inc(&ns->passive);
       return ns;
}

static inline void lock_mount_hash(void)
{
        write_seqlock(&mount_lock);
}

static inline void unlock_mount_hash(void)
{
        write_sequnlock(&mount_lock);
}

static inline struct hlist_head *m_hash(struct vfsmount *mnt, struct dentry *dentry)
{
        unsigned long tmp = ((unsigned long)mnt / L1_CACHE_BYTES);
        tmp += ((unsigned long)dentry / L1_CACHE_BYTES);
        tmp = tmp + (tmp >> m_hash_shift);
        return &mount_hashtable[tmp & m_hash_mask];
}

static inline struct hlist_head *mp_hash(struct dentry *dentry)
{
        unsigned long tmp = ((unsigned long)dentry / L1_CACHE_BYTES);
        tmp = tmp + (tmp >> mp_hash_shift);
        return &mountpoint_hashtable[tmp & mp_hash_mask];
}

static int mnt_alloc_id(struct mount *mnt)
{
        int res = ida_alloc(&mnt_id_ida, GFP_KERNEL);

        if (res < 0)
                return res;
        mnt->mnt_id = res;
        mnt->mnt_id_unique = atomic64_inc_return(&mnt_id_ctr);
        return 0;
}

static void mnt_free_id(struct mount *mnt)
{
        ida_free(&mnt_id_ida, mnt->mnt_id);
}

/*
 * Allocate a new peer group ID
 */
static int mnt_alloc_group_id(struct mount *mnt)
{
        int res = ida_alloc_min(&mnt_group_ida, 1, GFP_KERNEL);

        if (res < 0)
                return res;
        mnt->mnt_group_id = res;
        return 0;
}

/*
 * Release a peer group ID
 */
void mnt_release_group_id(struct mount *mnt)
{
        ida_free(&mnt_group_ida, mnt->mnt_group_id);
        mnt->mnt_group_id = 0;
}

/*
 * vfsmount lock must be held for read
 */
static inline void mnt_add_count(struct mount *mnt, int n)
{
#ifdef CONFIG_SMP
        this_cpu_add(mnt->mnt_pcp->mnt_count, n);
#else
        preempt_disable();
        mnt->mnt_count += n;
        preempt_enable();
#endif
}

/*
 * vfsmount lock must be held for write
 */
int mnt_get_count(struct mount *mnt)
{
#ifdef CONFIG_SMP
        int count = 0;
        int cpu;

        for_each_possible_cpu(cpu) {
                count += per_cpu_ptr(mnt->mnt_pcp, cpu)->mnt_count;
        }

        return count;
#else
        return mnt->mnt_count;
#endif
}

static struct mount *alloc_vfsmnt(const char *name)
{
        struct mount *mnt = kmem_cache_zalloc(mnt_cache, GFP_KERNEL);
        if (mnt) {
                int err;

                err = mnt_alloc_id(mnt);
                if (err)
                        goto out_free_cache;

                if (name) {
                        mnt->mnt_devname = kstrdup_const(name,
                                                         GFP_KERNEL_ACCOUNT);
                        if (!mnt->mnt_devname)
                                goto out_free_id;
                }

#ifdef CONFIG_SMP
                mnt->mnt_pcp = alloc_percpu(struct mnt_pcp);
                if (!mnt->mnt_pcp)
                        goto out_free_devname;

                this_cpu_add(mnt->mnt_pcp->mnt_count, 1);
#else
                mnt->mnt_count = 1;
                mnt->mnt_writers = 0;
#endif

                INIT_HLIST_NODE(&mnt->mnt_hash);
                INIT_LIST_HEAD(&mnt->mnt_child);
                INIT_LIST_HEAD(&mnt->mnt_mounts);
                INIT_LIST_HEAD(&mnt->mnt_list);
                INIT_LIST_HEAD(&mnt->mnt_expire);
                INIT_LIST_HEAD(&mnt->mnt_share);
                INIT_LIST_HEAD(&mnt->mnt_slave_list);
                INIT_LIST_HEAD(&mnt->mnt_slave);
                INIT_HLIST_NODE(&mnt->mnt_mp_list);
                INIT_LIST_HEAD(&mnt->mnt_umounting);
                INIT_HLIST_HEAD(&mnt->mnt_stuck_children);
                mnt->mnt.mnt_idmap = &nop_mnt_idmap;
        }
        return mnt;

#ifdef CONFIG_SMP
out_free_devname:
        kfree_const(mnt->mnt_devname);
#endif
out_free_id:
        mnt_free_id(mnt);
out_free_cache:
        kmem_cache_free(mnt_cache, mnt);
        return NULL;
}

/*
 * Most r/o checks on a fs are for operations that take
 * discrete amounts of time, like a write() or unlink().
 * We must keep track of when those operations start
 * (for permission checks) and when they end, so that
 * we can determine when writes are able to occur to
 * a filesystem.
 */
/*
 * __mnt_is_readonly: check whether a mount is read-only
 * @mnt: the mount to check for its write status
 *
 * This shouldn't be used directly ouside of the VFS.
 * It does not guarantee that the filesystem will stay
 * r/w, just that it is right *now*.  This can not and
 * should not be used in place of IS_RDONLY(inode).
 * mnt_want/drop_write() will _keep_ the filesystem
 * r/w.
 */
bool __mnt_is_readonly(struct vfsmount *mnt)
{
        return (mnt->mnt_flags & MNT_READONLY) || sb_rdonly(mnt->mnt_sb);
}
EXPORT_SYMBOL_GPL(__mnt_is_readonly);

static inline void mnt_inc_writers(struct mount *mnt)
{
#ifdef CONFIG_SMP
        this_cpu_inc(mnt->mnt_pcp->mnt_writers);
#else
        mnt->mnt_writers++;
#endif
}

static inline void mnt_dec_writers(struct mount *mnt)
{
#ifdef CONFIG_SMP
        this_cpu_dec(mnt->mnt_pcp->mnt_writers);
#else
        mnt->mnt_writers--;
#endif
}

static unsigned int mnt_get_writers(struct mount *mnt)
{
#ifdef CONFIG_SMP
        unsigned int count = 0;
        int cpu;

        for_each_possible_cpu(cpu) {
                count += per_cpu_ptr(mnt->mnt_pcp, cpu)->mnt_writers;
        }

        return count;
#else
        return mnt->mnt_writers;
#endif
}

static int mnt_is_readonly(struct vfsmount *mnt)
{
        if (READ_ONCE(mnt->mnt_sb->s_readonly_remount))
                return 1;
        /*
         * The barrier pairs with the barrier in sb_start_ro_state_change()
         * making sure if we don't see s_readonly_remount set yet, we also will
         * not see any superblock / mount flag changes done by remount.
         * It also pairs with the barrier in sb_end_ro_state_change()
         * assuring that if we see s_readonly_remount already cleared, we will
         * see the values of superblock / mount flags updated by remount.
         */
        smp_rmb();
        return __mnt_is_readonly(mnt);
}

/*
 * Most r/o & frozen checks on a fs are for operations that take discrete
 * amounts of time, like a write() or unlink().  We must keep track of when
 * those operations start (for permission checks) and when they end, so that we
 * can determine when writes are able to occur to a filesystem.
 */
/**
 * mnt_get_write_access - get write access to a mount without freeze protection
 * @m: the mount on which to take a write
 *
 * This tells the low-level filesystem that a write is about to be performed to
 * it, and makes sure that writes are allowed (mnt it read-write) before
 * returning success. This operation does not protect against filesystem being
 * frozen. When the write operation is finished, mnt_put_write_access() must be
 * called. This is effectively a refcount.
 */
int mnt_get_write_access(struct vfsmount *m)
{
        struct mount *mnt = real_mount(m);
        int ret = 0;

        preempt_disable();
        mnt_inc_writers(mnt);
        /*
         * The store to mnt_inc_writers must be visible before we pass
         * MNT_WRITE_HOLD loop below, so that the slowpath can see our
         * incremented count after it has set MNT_WRITE_HOLD.
         */
        smp_mb();
        might_lock(&mount_lock.lock);
        while (READ_ONCE(mnt->mnt.mnt_flags) & MNT_WRITE_HOLD) {
                if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
                        cpu_relax();
                } else {
                        /*
                         * This prevents priority inversion, if the task
                         * setting MNT_WRITE_HOLD got preempted on a remote
                         * CPU, and it prevents life lock if the task setting
                         * MNT_WRITE_HOLD has a lower priority and is bound to
                         * the same CPU as the task that is spinning here.
                         */
                        preempt_enable();
                        lock_mount_hash();
                        unlock_mount_hash();
                        preempt_disable();
                }
        }
        /*
         * The barrier pairs with the barrier sb_start_ro_state_change() making
         * sure that if we see MNT_WRITE_HOLD cleared, we will also see
         * s_readonly_remount set (or even SB_RDONLY / MNT_READONLY flags) in
         * mnt_is_readonly() and bail in case we are racing with remount
         * read-only.
         */
        smp_rmb();
        if (mnt_is_readonly(m)) {
                mnt_dec_writers(mnt);
                ret = -EROFS;
        }
        preempt_enable();

        return ret;
}
EXPORT_SYMBOL_GPL(mnt_get_write_access);

/**
 * mnt_want_write - get write access to a mount
 * @m: the mount on which to take a write
 *
 * This tells the low-level filesystem that a write is about to be performed to
 * it, and makes sure that writes are allowed (mount is read-write, filesystem
 * is not frozen) before returning success.  When the write operation is
 * finished, mnt_drop_write() must be called.  This is effectively a refcount.
 */
int mnt_want_write(struct vfsmount *m)
{
        int ret;

        sb_start_write(m->mnt_sb);
        ret = mnt_get_write_access(m);
        if (ret)
                sb_end_write(m->mnt_sb);
        return ret;
}
EXPORT_SYMBOL_GPL(mnt_want_write);

/**
 * mnt_get_write_access_file - get write access to a file's mount
 * @file: the file who's mount on which to take a write
 *
 * This is like mnt_get_write_access, but if @file is already open for write it
 * skips incrementing mnt_writers (since the open file already has a reference)
 * and instead only does the check for emergency r/o remounts.  This must be
 * paired with mnt_put_write_access_file.
 */
int mnt_get_write_access_file(struct file *file)
{
        if (file->f_mode & FMODE_WRITER) {
                /*
                 * Superblock may have become readonly while there are still
                 * writable fd's, e.g. due to a fs error with errors=remount-ro
                 */
                if (__mnt_is_readonly(file->f_path.mnt))
                        return -EROFS;
                return 0;
        }
        return mnt_get_write_access(file->f_path.mnt);
}

/**
 * mnt_want_write_file - get write access to a file's mount
 * @file: the file who's mount on which to take a write
 *
 * This is like mnt_want_write, but if the file is already open for writing it
 * skips incrementing mnt_writers (since the open file already has a reference)
 * and instead only does the freeze protection and the check for emergency r/o
 * remounts.  This must be paired with mnt_drop_write_file.
 */
int mnt_want_write_file(struct file *file)
{
        int ret;

        sb_start_write(file_inode(file)->i_sb);
        ret = mnt_get_write_access_file(file);
        if (ret)
                sb_end_write(file_inode(file)->i_sb);
        return ret;
}
EXPORT_SYMBOL_GPL(mnt_want_write_file);

/**
 * mnt_put_write_access - give up write access to a mount
 * @mnt: the mount on which to give up write access
 *
 * Tells the low-level filesystem that we are done
 * performing writes to it.  Must be matched with
 * mnt_get_write_access() call above.
 */
void mnt_put_write_access(struct vfsmount *mnt)
{
        preempt_disable();
        mnt_dec_writers(real_mount(mnt));
        preempt_enable();
}
EXPORT_SYMBOL_GPL(mnt_put_write_access);

/**
 * mnt_drop_write - give up write access to a mount
 * @mnt: the mount on which to give up write access
 *
 * Tells the low-level filesystem that we are done performing writes to it and
 * also allows filesystem to be frozen again.  Must be matched with
 * mnt_want_write() call above.
 */
void mnt_drop_write(struct vfsmount *mnt)
{
        mnt_put_write_access(mnt);
        sb_end_write(mnt->mnt_sb);
}
EXPORT_SYMBOL_GPL(mnt_drop_write);

void mnt_put_write_access_file(struct file *file)
{
        if (!(file->f_mode & FMODE_WRITER))
                mnt_put_write_access(file->f_path.mnt);
}

void mnt_drop_write_file(struct file *file)
{
        mnt_put_write_access_file(file);
        sb_end_write(file_inode(file)->i_sb);
}
EXPORT_SYMBOL(mnt_drop_write_file);

/**
 * mnt_hold_writers - prevent write access to the given mount
 * @mnt: mnt to prevent write access to
 *
 * Prevents write access to @mnt if there are no active writers for @mnt.
 * This function needs to be called and return successfully before changing
 * properties of @mnt that need to remain stable for callers with write access
 * to @mnt.
 *
 * After this functions has been called successfully callers must pair it with
 * a call to mnt_unhold_writers() in order to stop preventing write access to
 * @mnt.
 *
 * Context: This function expects lock_mount_hash() to be held serializing
 *          setting MNT_WRITE_HOLD.
 * Return: On success 0 is returned.
 *         On error, -EBUSY is returned.
 */
static inline int mnt_hold_writers(struct mount *mnt)
{
        mnt->mnt.mnt_flags |= MNT_WRITE_HOLD;
        /*
         * After storing MNT_WRITE_HOLD, we'll read the counters. This store
         * should be visible before we do.
         */
        smp_mb();

        /*
         * With writers on hold, if this value is zero, then there are
         * definitely no active writers (although held writers may subsequently
         * increment the count, they'll have to wait, and decrement it after
         * seeing MNT_READONLY).
         *
         * It is OK to have counter incremented on one CPU and decremented on
         * another: the sum will add up correctly. The danger would be when we
         * sum up each counter, if we read a counter before it is incremented,
         * but then read another CPU's count which it has been subsequently
         * decremented from -- we would see more decrements than we should.
         * MNT_WRITE_HOLD protects against this scenario, because
         * mnt_want_write first increments count, then smp_mb, then spins on
         * MNT_WRITE_HOLD, so it can't be decremented by another CPU while
         * we're counting up here.
         */
        if (mnt_get_writers(mnt) > 0)
                return -EBUSY;

        return 0;
}

/**
 * mnt_unhold_writers - stop preventing write access to the given mount
 * @mnt: mnt to stop preventing write access to
 *
 * Stop preventing write access to @mnt allowing callers to gain write access
 * to @mnt again.
 *
 * This function can only be called after a successful call to
 * mnt_hold_writers().
 *
 * Context: This function expects lock_mount_hash() to be held.
 */
static inline void mnt_unhold_writers(struct mount *mnt)
{
        /*
         * MNT_READONLY must become visible before ~MNT_WRITE_HOLD, so writers
         * that become unheld will see MNT_READONLY.
         */
        smp_wmb();
        mnt->mnt.mnt_flags &= ~MNT_WRITE_HOLD;
}

static int mnt_make_readonly(struct mount *mnt)
{
        int ret;

        ret = mnt_hold_writers(mnt);
        if (!ret)
                mnt->mnt.mnt_flags |= MNT_READONLY;
        mnt_unhold_writers(mnt);
        return ret;
}

int sb_prepare_remount_readonly(struct super_block *sb)
{
        struct mount *mnt;
        int err = 0;

        /* Racy optimization.  Recheck the counter under MNT_WRITE_HOLD */
        if (atomic_long_read(&sb->s_remove_count))
                return -EBUSY;

        lock_mount_hash();
        list_for_each_entry(mnt, &sb->s_mounts, mnt_instance) {
                if (!(mnt->mnt.mnt_flags & MNT_READONLY)) {
                        err = mnt_hold_writers(mnt);
                        if (err)
                                break;
                }
        }
        if (!err && atomic_long_read(&sb->s_remove_count))
                err = -EBUSY;

        if (!err)
                sb_start_ro_state_change(sb);
        list_for_each_entry(mnt, &sb->s_mounts, mnt_instance) {
                if (mnt->mnt.mnt_flags & MNT_WRITE_HOLD)
                        mnt->mnt.mnt_flags &= ~MNT_WRITE_HOLD;
        }
        unlock_mount_hash();

        return err;
}

static void free_vfsmnt(struct mount *mnt)
{
        mnt_idmap_put(mnt_idmap(&mnt->mnt));
        kfree_const(mnt->mnt_devname);
#ifdef CONFIG_SMP
        free_percpu(mnt->mnt_pcp);
#endif
        kmem_cache_free(mnt_cache, mnt);
}

static void delayed_free_vfsmnt(struct rcu_head *head)
{
        free_vfsmnt(container_of(head, struct mount, mnt_rcu));
}

/* call under rcu_read_lock */
int __legitimize_mnt(struct vfsmount *bastard, unsigned seq)
{
        struct mount *mnt;
        if (read_seqretry(&mount_lock, seq))
                return 1;
        if (bastard == NULL)
                return 0;
        mnt = real_mount(bastard);
        mnt_add_count(mnt, 1);
        smp_mb();                       // see mntput_no_expire()
        if (likely(!read_seqretry(&mount_lock, seq)))
                return 0;
        if (bastard->mnt_flags & MNT_SYNC_UMOUNT) {
                mnt_add_count(mnt, -1);
                return 1;
        }
        lock_mount_hash();
        if (unlikely(bastard->mnt_flags & MNT_DOOMED)) {
                mnt_add_count(mnt, -1);
                unlock_mount_hash();
                return 1;
        }
        unlock_mount_hash();
        /* caller will mntput() */
        return -1;
}

/* call under rcu_read_lock */
static bool legitimize_mnt(struct vfsmount *bastard, unsigned seq)
{
        int res = __legitimize_mnt(bastard, seq);
        if (likely(!res))
                return true;
        if (unlikely(res < 0)) {
                rcu_read_unlock();
                mntput(bastard);
                rcu_read_lock();
        }
        return false;
}

/**
 * __lookup_mnt - find first child mount
 * @mnt:        parent mount
 * @dentry:     mountpoint
 *
 * If @mnt has a child mount @c mounted @dentry find and return it.
 *
 * Note that the child mount @c need not be unique. There are cases
 * where shadow mounts are created. For example, during mount
 * propagation when a source mount @mnt whose root got overmounted by a
 * mount @o after path lookup but before @namespace_sem could be
 * acquired gets copied and propagated. So @mnt gets copied including
 * @o. When @mnt is propagated to a destination mount @d that already
 * has another mount @n mounted at the same mountpoint then the source
 * mount @mnt will be tucked beneath @n, i.e., @n will be mounted on
 * @mnt and @mnt mounted on @d. Now both @n and @o are mounted at @mnt
 * on @dentry.
 *
 * Return: The first child of @mnt mounted @dentry or NULL.
 */
struct mount *__lookup_mnt(struct vfsmount *mnt, struct dentry *dentry)
{
        struct hlist_head *head = m_hash(mnt, dentry);
        struct mount *p;

        hlist_for_each_entry_rcu(p, head, mnt_hash)
                if (&p->mnt_parent->mnt == mnt && p->mnt_mountpoint == dentry)
                        return p;
        return NULL;
}

/*
 * lookup_mnt - Return the first child mount mounted at path
 *
 * "First" means first mounted chronologically.  If you create the
 * following mounts:
 *
 * mount /dev/sda1 /mnt
 * mount /dev/sda2 /mnt
 * mount /dev/sda3 /mnt
 *
 * Then lookup_mnt() on the base /mnt dentry in the root mount will
 * return successively the root dentry and vfsmount of /dev/sda1, then
 * /dev/sda2, then /dev/sda3, then NULL.
 *
 * lookup_mnt takes a reference to the found vfsmount.
 */
struct vfsmount *lookup_mnt(const struct path *path)
{
        struct mount *child_mnt;
        struct vfsmount *m;
        unsigned seq;

        rcu_read_lock();
        do {
                seq = read_seqbegin(&mount_lock);
                child_mnt = __lookup_mnt(path->mnt, path->dentry);
                m = child_mnt ? &child_mnt->mnt : NULL;
        } while (!legitimize_mnt(m, seq));
        rcu_read_unlock();
        return m;
}

/*
 * __is_local_mountpoint - Test to see if dentry is a mountpoint in the
 *                         current mount namespace.
 *
 * The common case is dentries are not mountpoints at all and that
 * test is handled inline.  For the slow case when we are actually
 * dealing with a mountpoint of some kind, walk through all of the
 * mounts in the current mount namespace and test to see if the dentry
 * is a mountpoint.
 *
 * The mount_hashtable is not usable in the context because we
 * need to identify all mounts that may be in the current mount
 * namespace not just a mount that happens to have some specified
 * parent mount.
 */
bool __is_local_mountpoint(struct dentry *dentry)
{
        struct mnt_namespace *ns = current->nsproxy->mnt_ns;
        struct mount *mnt, *n;
        bool is_covered = false;

        down_read(&namespace_sem);
        rbtree_postorder_for_each_entry_safe(mnt, n, &ns->mounts, mnt_node) {
                is_covered = (mnt->mnt_mountpoint == dentry);
                if (is_covered)
                        break;
        }
        up_read(&namespace_sem);

        return is_covered;
}

static struct mountpoint *lookup_mountpoint(struct dentry *dentry)
{
        struct hlist_head *chain = mp_hash(dentry);
        struct mountpoint *mp;

        hlist_for_each_entry(mp, chain, m_hash) {
                if (mp->m_dentry == dentry) {
                        mp->m_count++;
                        return mp;
                }
        }
        return NULL;
}

static struct mountpoint *get_mountpoint(struct dentry *dentry)
{
        struct mountpoint *mp, *new = NULL;
        int ret;

        if (d_mountpoint(dentry)) {
                /* might be worth a WARN_ON() */
                if (d_unlinked(dentry))
                        return ERR_PTR(-ENOENT);
mountpoint:
                read_seqlock_excl(&mount_lock);
                mp = lookup_mountpoint(dentry);
                read_sequnlock_excl(&mount_lock);
                if (mp)
                        goto done;
        }

        if (!new)
                new = kmalloc(sizeof(struct mountpoint), GFP_KERNEL);
        if (!new)
                return ERR_PTR(-ENOMEM);


        /* Exactly one processes may set d_mounted */
        ret = d_set_mounted(dentry);

        /* Someone else set d_mounted? */
        if (ret == -EBUSY)
                goto mountpoint;

        /* The dentry is not available as a mountpoint? */
        mp = ERR_PTR(ret);
        if (ret)
                goto done;

        /* Add the new mountpoint to the hash table */
        read_seqlock_excl(&mount_lock);
        new->m_dentry = dget(dentry);
        new->m_count = 1;
        hlist_add_head(&new->m_hash, mp_hash(dentry));
        INIT_HLIST_HEAD(&new->m_list);
        read_sequnlock_excl(&mount_lock);

        mp = new;
        new = NULL;
done:
        kfree(new);
        return mp;
}

/*
 * vfsmount lock must be held.  Additionally, the caller is responsible
 * for serializing calls for given disposal list.
 */
static void __put_mountpoint(struct mountpoint *mp, struct list_head *list)
{
        if (!--mp->m_count) {
                struct dentry *dentry = mp->m_dentry;
                BUG_ON(!hlist_empty(&mp->m_list));
                spin_lock(&dentry->d_lock);
                dentry->d_flags &= ~DCACHE_MOUNTED;
                spin_unlock(&dentry->d_lock);
                dput_to_list(dentry, list);
                hlist_del(&mp->m_hash);
                kfree(mp);
        }
}

/* called with namespace_lock and vfsmount lock */
static void put_mountpoint(struct mountpoint *mp)
{
        __put_mountpoint(mp, &ex_mountpoints);
}

static inline int check_mnt(struct mount *mnt)
{
        return mnt->mnt_ns == current->nsproxy->mnt_ns;
}

/*
 * vfsmount lock must be held for write
 */
static void touch_mnt_namespace(struct mnt_namespace *ns)
{
        if (ns) {
                ns->event = ++event;
                wake_up_interruptible(&ns->poll);
        }
}

/*
 * vfsmount lock must be held for write
 */
static void __touch_mnt_namespace(struct mnt_namespace *ns)
{
        if (ns && ns->event != event) {
                ns->event = event;
                wake_up_interruptible(&ns->poll);
        }
}

/*
 * vfsmount lock must be held for write
 */
static struct mountpoint *unhash_mnt(struct mount *mnt)
{
        struct mountpoint *mp;
        mnt->mnt_parent = mnt;
        mnt->mnt_mountpoint = mnt->mnt.mnt_root;
        list_del_init(&mnt->mnt_child);
        hlist_del_init_rcu(&mnt->mnt_hash);
        hlist_del_init(&mnt->mnt_mp_list);
        mp = mnt->mnt_mp;
        mnt->mnt_mp = NULL;
        return mp;
}

/*
 * vfsmount lock must be held for write
 */
static void umount_mnt(struct mount *mnt)
{
        put_mountpoint(unhash_mnt(mnt));
}

/*
 * vfsmount lock must be held for write
 */
void mnt_set_mountpoint(struct mount *mnt,
                        struct mountpoint *mp,
                        struct mount *child_mnt)
{
        mp->m_count++;
        mnt_add_count(mnt, 1);  /* essentially, that's mntget */
        child_mnt->mnt_mountpoint = mp->m_dentry;
        child_mnt->mnt_parent = mnt;
        child_mnt->mnt_mp = mp;
        hlist_add_head(&child_mnt->mnt_mp_list, &mp->m_list);
}

/**
 * mnt_set_mountpoint_beneath - mount a mount beneath another one
 *
 * @new_parent: the source mount
 * @top_mnt:    the mount beneath which @new_parent is mounted
 * @new_mp:     the new mountpoint of @top_mnt on @new_parent
 *
 * Remove @top_mnt from its current mountpoint @top_mnt->mnt_mp and
 * parent @top_mnt->mnt_parent and mount it on top of @new_parent at
 * @new_mp. And mount @new_parent on the old parent and old
 * mountpoint of @top_mnt.
 *
 * Context: This function expects namespace_lock() and lock_mount_hash()
 *          to have been acquired in that order.
 */
static void mnt_set_mountpoint_beneath(struct mount *new_parent,
                                       struct mount *top_mnt,
                                       struct mountpoint *new_mp)
{
        struct mount *old_top_parent = top_mnt->mnt_parent;
        struct mountpoint *old_top_mp = top_mnt->mnt_mp;

        mnt_set_mountpoint(old_top_parent, old_top_mp, new_parent);
        mnt_change_mountpoint(new_parent, new_mp, top_mnt);
}


static void __attach_mnt(struct mount *mnt, struct mount *parent)
{
        hlist_add_head_rcu(&mnt->mnt_hash,
                           m_hash(&parent->mnt, mnt->mnt_mountpoint));
        list_add_tail(&mnt->mnt_child, &parent->mnt_mounts);
}

/**
 * attach_mnt - mount a mount, attach to @mount_hashtable and parent's
 *              list of child mounts
 * @parent:  the parent
 * @mnt:     the new mount
 * @mp:      the new mountpoint
 * @beneath: whether to mount @mnt beneath or on top of @parent
 *
 * If @beneath is false, mount @mnt at @mp on @parent. Then attach @mnt
 * to @parent's child mount list and to @mount_hashtable.
 *
 * If @beneath is true, remove @mnt from its current parent and
 * mountpoint and mount it on @mp on @parent, and mount @parent on the
 * old parent and old mountpoint of @mnt. Finally, attach @parent to
 * @mnt_hashtable and @parent->mnt_parent->mnt_mounts.
 *
 * Note, when __attach_mnt() is called @mnt->mnt_parent already points
 * to the correct parent.
 *
 * Context: This function expects namespace_lock() and lock_mount_hash()
 *          to have been acquired in that order.
 */
static void attach_mnt(struct mount *mnt, struct mount *parent,
                       struct mountpoint *mp, bool beneath)
{
        if (beneath)
                mnt_set_mountpoint_beneath(mnt, parent, mp);
        else
                mnt_set_mountpoint(parent, mp, mnt);
        /*
         * Note, @mnt->mnt_parent has to be used. If @mnt was mounted
         * beneath @parent then @mnt will need to be attached to
         * @parent's old parent, not @parent. IOW, @mnt->mnt_parent
         * isn't the same mount as @parent.
         */
        __attach_mnt(mnt, mnt->mnt_parent);
}

void mnt_change_mountpoint(struct mount *parent, struct mountpoint *mp, struct mount *mnt)
{
        struct mountpoint *old_mp = mnt->mnt_mp;
        struct mount *old_parent = mnt->mnt_parent;

        list_del_init(&mnt->mnt_child);
        hlist_del_init(&mnt->mnt_mp_list);
        hlist_del_init_rcu(&mnt->mnt_hash);

        attach_mnt(mnt, parent, mp, false);

        put_mountpoint(old_mp);
        mnt_add_count(old_parent, -1);
}

static inline struct mount *node_to_mount(struct rb_node *node)
{
        return node ? rb_entry(node, struct mount, mnt_node) : NULL;
}

static void mnt_add_to_ns(struct mnt_namespace *ns, struct mount *mnt)
{
        struct rb_node **link = &ns->mounts.rb_node;
        struct rb_node *parent = NULL;

        WARN_ON(mnt->mnt.mnt_flags & MNT_ONRB);
        mnt->mnt_ns = ns;
        while (*link) {
                parent = *link;
                if (mnt->mnt_id_unique < node_to_mount(parent)->mnt_id_unique)
                        link = &parent->rb_left;
                else
                        link = &parent->rb_right;
        }
        rb_link_node(&mnt->mnt_node, parent, link);
        rb_insert_color(&mnt->mnt_node, &ns->mounts);
        mnt->mnt.mnt_flags |= MNT_ONRB;
}

/*
 * vfsmount lock must be held for write
 */
static void commit_tree(struct mount *mnt)
{
        struct mount *parent = mnt->mnt_parent;
        struct mount *m;
        LIST_HEAD(head);
        struct mnt_namespace *n = parent->mnt_ns;

        BUG_ON(parent == mnt);

        list_add_tail(&head, &mnt->mnt_list);
        while (!list_empty(&head)) {
                m = list_first_entry(&head, typeof(*m), mnt_list);
                list_del(&m->mnt_list);

                mnt_add_to_ns(n, m);
        }
        n->nr_mounts += n->pending_mounts;
        n->pending_mounts = 0;

        __attach_mnt(mnt, parent);
        touch_mnt_namespace(n);
}

static struct mount *next_mnt(struct mount *p, struct mount *root)
{
        struct list_head *next = p->mnt_mounts.next;
        if (next == &p->mnt_mounts) {
                while (1) {
                        if (p == root)
                                return NULL;
                        next = p->mnt_child.next;
                        if (next != &p->mnt_parent->mnt_mounts)
                                break;
                        p = p->mnt_parent;
                }
        }
        return list_entry(next, struct mount, mnt_child);
}

static struct mount *skip_mnt_tree(struct mount *p)
{
        struct list_head *prev = p->mnt_mounts.prev;
        while (prev != &p->mnt_mounts) {
                p = list_entry(prev, struct mount, mnt_child);
                prev = p->mnt_mounts.prev;
        }
        return p;
}

/**
 * vfs_create_mount - Create a mount for a configured superblock
 * @fc: The configuration context with the superblock attached
 *
 * Create a mount to an already configured superblock.  If necessary, the
 * caller should invoke vfs_get_tree() before calling this.
 *
 * Note that this does not attach the mount to anything.
 */
struct vfsmount *vfs_create_mount(struct fs_context *fc)
{
        struct mount *mnt;

        if (!fc->root)
                return ERR_PTR(-EINVAL);

        mnt = alloc_vfsmnt(fc->source ?: "none");
        if (!mnt)
                return ERR_PTR(-ENOMEM);

        if (fc->sb_flags & SB_KERNMOUNT)
                mnt->mnt.mnt_flags = MNT_INTERNAL;

        atomic_inc(&fc->root->d_sb->s_active);
        mnt->mnt.mnt_sb         = fc->root->d_sb;
        mnt->mnt.mnt_root       = dget(fc->root);
        mnt->mnt_mountpoint     = mnt->mnt.mnt_root;
        mnt->mnt_parent         = mnt;

        lock_mount_hash();
        list_add_tail(&mnt->mnt_instance, &mnt->mnt.mnt_sb->s_mounts);
        unlock_mount_hash();
        return &mnt->mnt;
}
EXPORT_SYMBOL(vfs_create_mount);

struct vfsmount *fc_mount(struct fs_context *fc)
{
        int err = vfs_get_tree(fc);
        if (!err) {
                up_write(&fc->root->d_sb->s_umount);
                return vfs_create_mount(fc);
        }
        return ERR_PTR(err);
}
EXPORT_SYMBOL(fc_mount);

struct vfsmount *vfs_kern_mount(struct file_system_type *type,
                                int flags, const char *name,
                                void *data)
{
        struct fs_context *fc;
        struct vfsmount *mnt;
        int ret = 0;

        if (!type)
                return ERR_PTR(-EINVAL);

        fc = fs_context_for_mount(type, flags);
        if (IS_ERR(fc))
                return ERR_CAST(fc);

        if (name)
                ret = vfs_parse_fs_string(fc, "source",
                                          name, strlen(name));
        if (!ret)
                ret = parse_monolithic_mount_data(fc, data);
        if (!ret)
                mnt = fc_mount(fc);
        else
                mnt = ERR_PTR(ret);

        put_fs_context(fc);
        return mnt;
}
EXPORT_SYMBOL_GPL(vfs_kern_mount);

struct vfsmount *
vfs_submount(const struct dentry *mountpoint, struct file_system_type *type,
             const char *name, void *data)
{
        /* Until it is worked out how to pass the user namespace
         * through from the parent mount to the submount don't support
         * unprivileged mounts with submounts.
         */
        if (mountpoint->d_sb->s_user_ns != &init_user_ns)
                return ERR_PTR(-EPERM);

        return vfs_kern_mount(type, SB_SUBMOUNT, name, data);
}
EXPORT_SYMBOL_GPL(vfs_submount);

static struct mount *clone_mnt(struct mount *old, struct dentry *root,
                                        int flag)
{
        struct super_block *sb = old->mnt.mnt_sb;
        struct mount *mnt;
        int err;

        mnt = alloc_vfsmnt(old->mnt_devname);
        if (!mnt)
                return ERR_PTR(-ENOMEM);

        if (flag & (CL_SLAVE | CL_PRIVATE | CL_SHARED_TO_SLAVE))
                mnt->mnt_group_id = 0; /* not a peer of original */
        else
                mnt->mnt_group_id = old->mnt_group_id;

        if ((flag & CL_MAKE_SHARED) && !mnt->mnt_group_id) {
                err = mnt_alloc_group_id(mnt);
                if (err)
                        goto out_free;
        }

        mnt->mnt.mnt_flags = old->mnt.mnt_flags;
        mnt->mnt.mnt_flags &= ~(MNT_WRITE_HOLD|MNT_MARKED|MNT_INTERNAL|MNT_ONRB);

        atomic_inc(&sb->s_active);
        mnt->mnt.mnt_idmap = mnt_idmap_get(mnt_idmap(&old->mnt));

        mnt->mnt.mnt_sb = sb;
        mnt->mnt.mnt_root = dget(root);
        mnt->mnt_mountpoint = mnt->mnt.mnt_root;
        mnt->mnt_parent = mnt;
        lock_mount_hash();
        list_add_tail(&mnt->mnt_instance, &sb->s_mounts);
        unlock_mount_hash();

        if ((flag & CL_SLAVE) ||
            ((flag & CL_SHARED_TO_SLAVE) && IS_MNT_SHARED(old))) {
                list_add(&mnt->mnt_slave, &old->mnt_slave_list);
                mnt->mnt_master = old;
                CLEAR_MNT_SHARED(mnt);
        } else if (!(flag & CL_PRIVATE)) {
                if ((flag & CL_MAKE_SHARED) || IS_MNT_SHARED(old))
                        list_add(&mnt->mnt_share, &old->mnt_share);
                if (IS_MNT_SLAVE(old))
                        list_add(&mnt->mnt_slave, &old->mnt_slave);
                mnt->mnt_master = old->mnt_master;
        } else {
                CLEAR_MNT_SHARED(mnt);
        }
        if (flag & CL_MAKE_SHARED)
                set_mnt_shared(mnt);

        /* stick the duplicate mount on the same expiry list
         * as the original if that was on one */
        if (flag & CL_EXPIRE) {
                if (!list_empty(&old->mnt_expire))
                        list_add(&mnt->mnt_expire, &old->mnt_expire);
        }

        return mnt;

 out_free:
        mnt_free_id(mnt);
        free_vfsmnt(mnt);
        return ERR_PTR(err);
}

static void cleanup_mnt(struct mount *mnt)
{
        struct hlist_node *p;
        struct mount *m;
        /*
         * The warning here probably indicates that somebody messed
         * up a mnt_want/drop_write() pair.  If this happens, the
         * filesystem was probably unable to make r/w->r/o transitions.
         * The locking used to deal with mnt_count decrement provides barriers,
         * so mnt_get_writers() below is safe.
         */
        WARN_ON(mnt_get_writers(mnt));
        if (unlikely(mnt->mnt_pins.first))
                mnt_pin_kill(mnt);
        hlist_for_each_entry_safe(m, p, &mnt->mnt_stuck_children, mnt_umount) {
                hlist_del(&m->mnt_umount);
                mntput(&m->mnt);
        }
        fsnotify_vfsmount_delete(&mnt->mnt);
        dput(mnt->mnt.mnt_root);
        deactivate_super(mnt->mnt.mnt_sb);
        mnt_free_id(mnt);
        call_rcu(&mnt->mnt_rcu, delayed_free_vfsmnt);
}

static void __cleanup_mnt(struct rcu_head *head)
{
        cleanup_mnt(container_of(head, struct mount, mnt_rcu));
}

static LLIST_HEAD(delayed_mntput_list);
static void delayed_mntput(struct work_struct *unused)
{
        struct llist_node *node = llist_del_all(&delayed_mntput_list);
        struct mount *m, *t;

        llist_for_each_entry_safe(m, t, node, mnt_llist)
                cleanup_mnt(m);
}
static DECLARE_DELAYED_WORK(delayed_mntput_work, delayed_mntput);

static void mntput_no_expire(struct mount *mnt)
{
        LIST_HEAD(list);
        int count;

        rcu_read_lock();
        if (likely(READ_ONCE(mnt->mnt_ns))) {
                /*
                 * Since we don't do lock_mount_hash() here,
                 * ->mnt_ns can change under us.  However, if it's
                 * non-NULL, then there's a reference that won't
                 * be dropped until after an RCU delay done after
                 * turning ->mnt_ns NULL.  So if we observe it
                 * non-NULL under rcu_read_lock(), the reference
                 * we are dropping is not the final one.
                 */
                mnt_add_count(mnt, -1);
                rcu_read_unlock();
                return;
        }
        lock_mount_hash();
        /*
         * make sure that if __legitimize_mnt() has not seen us grab
         * mount_lock, we'll see their refcount increment here.
         */
        smp_mb();
        mnt_add_count(mnt, -1);
        count = mnt_get_count(mnt);
        if (count != 0) {
                WARN_ON(count < 0);
                rcu_read_unlock();
                unlock_mount_hash();
                return;
        }
        if (unlikely(mnt->mnt.mnt_flags & MNT_DOOMED)) {
                rcu_read_unlock();
                unlock_mount_hash();
                return;
        }
        mnt->mnt.mnt_flags |= MNT_DOOMED;
        rcu_read_unlock();

        list_del(&mnt->mnt_instance);

        if (unlikely(!list_empty(&mnt->mnt_mounts))) {
                struct mount *p, *tmp;
                list_for_each_entry_safe(p, tmp, &mnt->mnt_mounts,  mnt_child) {
                        __put_mountpoint(unhash_mnt(p), &list);
                        hlist_add_head(&p->mnt_umount, &mnt->mnt_stuck_children);
                }
        }
        unlock_mount_hash();
        shrink_dentry_list(&list);

        if (likely(!(mnt->mnt.mnt_flags & MNT_INTERNAL))) {
                struct task_struct *task = current;
                if (likely(!(task->flags & PF_KTHREAD))) {
                        init_task_work(&mnt->mnt_rcu, __cleanup_mnt);
                        if (!task_work_add(task, &mnt->mnt_rcu, TWA_RESUME))
                                return;
                }
                if (llist_add(&mnt->mnt_llist, &delayed_mntput_list))
                        schedule_delayed_work(&delayed_mntput_work, 1);
                return;
        }
        cleanup_mnt(mnt);
}

void mntput(struct vfsmount *mnt)
{
        if (mnt) {
                struct mount *m = real_mount(mnt);
                /* avoid cacheline pingpong */
                if (unlikely(m->mnt_expiry_mark))
                        WRITE_ONCE(m->mnt_expiry_mark, 0);
                mntput_no_expire(m);
        }
}
EXPORT_SYMBOL(mntput);

struct vfsmount *mntget(struct vfsmount *mnt)
{
        if (mnt)
                mnt_add_count(real_mount(mnt), 1);
        return mnt;
}
EXPORT_SYMBOL(mntget);

/*
 * Make a mount point inaccessible to new lookups.
 * Because there may still be current users, the caller MUST WAIT
 * for an RCU grace period before destroying the mount point.
 */
void mnt_make_shortterm(struct vfsmount *mnt)
{
        if (mnt)
                real_mount(mnt)->mnt_ns = NULL;
}

/**
 * path_is_mountpoint() - Check if path is a mount in the current namespace.
 * @path: path to check
 *
 *  d_mountpoint() can only be used reliably to establish if a dentry is
 *  not mounted in any namespace and that common case is handled inline.
 *  d_mountpoint() isn't aware of the possibility there may be multiple
 *  mounts using a given dentry in a different namespace. This function
 *  checks if the passed in path is a mountpoint rather than the dentry
 *  alone.
 */
bool path_is_mountpoint(const struct path *path)
{
        unsigned seq;
        bool res;

        if (!d_mountpoint(path->dentry))
                return false;

        rcu_read_lock();
        do {
                seq = read_seqbegin(&mount_lock);
                res = __path_is_mountpoint(path);
        } while (read_seqretry(&mount_lock, seq));
        rcu_read_unlock();

        return res;
}
EXPORT_SYMBOL(path_is_mountpoint);

struct vfsmount *mnt_clone_internal(const struct path *path)
{
        struct mount *p;
        p = clone_mnt(real_mount(path->mnt), path->dentry, CL_PRIVATE);
        if (IS_ERR(p))
                return ERR_CAST(p);
        p->mnt.mnt_flags |= MNT_INTERNAL;
        return &p->mnt;
}

/*
 * Returns the mount which either has the specified mnt_id, or has the next
 * smallest id afer the specified one.
 */
static struct mount *mnt_find_id_at(struct mnt_namespace *ns, u64 mnt_id)
{
        struct rb_node *node = ns->mounts.rb_node;
        struct mount *ret = NULL;

        while (node) {
                struct mount *m = node_to_mount(node);

                if (mnt_id <= m->mnt_id_unique) {
                        ret = node_to_mount(node);
                        if (mnt_id == m->mnt_id_unique)
                                break;
                        node = node->rb_left;
                } else {
                        node = node->rb_right;
                }
        }
        return ret;
}

/*
 * Returns the mount which either has the specified mnt_id, or has the next
 * greater id before the specified one.
 */
static struct mount *mnt_find_id_at_reverse(struct mnt_namespace *ns, u64 mnt_id)
{
        struct rb_node *node = ns->mounts.rb_node;
        struct mount *ret = NULL;

        while (node) {
                struct mount *m = node_to_mount(node);

                if (mnt_id >= m->mnt_id_unique) {
                        ret = node_to_mount(node);
                        if (mnt_id == m->mnt_id_unique)
                                break;
                        node = node->rb_right;
                } else {
                        node = node->rb_left;
                }
        }
        return ret;
}

#ifdef CONFIG_PROC_FS

/* iterator; we want it to have access to namespace_sem, thus here... */
static void *m_start(struct seq_file *m, loff_t *pos)
{
        struct proc_mounts *p = m->private;

        down_read(&namespace_sem);

        return mnt_find_id_at(p->ns, *pos);
}

static void *m_next(struct seq_file *m, void *v, loff_t *pos)
{
        struct mount *next = NULL, *mnt = v;
        struct rb_node *node = rb_next(&mnt->mnt_node);

        ++*pos;
        if (node) {
                next = node_to_mount(node);
                *pos = next->mnt_id_unique;
        }
        return next;
}

static void m_stop(struct seq_file *m, void *v)
{
        up_read(&namespace_sem);
}

static int m_show(struct seq_file *m, void *v)
{
        struct proc_mounts *p = m->private;
        struct mount *r = v;
        return p->show(m, &r->mnt);
}

const struct seq_operations mounts_op = {
        .start  = m_start,
        .next   = m_next,
        .stop   = m_stop,
        .show   = m_show,
};

#endif  /* CONFIG_PROC_FS */

/**
 * may_umount_tree - check if a mount tree is busy
 * @m: root of mount tree
 *
 * This is called to check if a tree of mounts has any
 * open files, pwds, chroots or sub mounts that are
 * busy.
 */
int may_umount_tree(struct vfsmount *m)
{
        struct mount *mnt = real_mount(m);
        int actual_refs = 0;
        int minimum_refs = 0;
        struct mount *p;
        BUG_ON(!m);

        /* write lock needed for mnt_get_count */
        lock_mount_hash();
        for (p = mnt; p; p = next_mnt(p, mnt)) {
                actual_refs += mnt_get_count(p);
                minimum_refs += 2;
        }
        unlock_mount_hash();

        if (actual_refs > minimum_refs)
                return 0;

        return 1;
}

EXPORT_SYMBOL(may_umount_tree);

/**
 * may_umount - check if a mount point is busy
 * @mnt: root of mount
 *
 * This is called to check if a mount point has any
 * open files, pwds, chroots or sub mounts. If the
 * mount has sub mounts this will return busy
 * regardless of whether the sub mounts are busy.
 *
 * Doesn't take quota and stuff into account. IOW, in some cases it will
 * give false negatives. The main reason why it's here is that we need
 * a non-destructive way to look for easily umountable filesystems.
 */
int may_umount(struct vfsmount *mnt)
{
        int ret = 1;
        down_read(&namespace_sem);
        lock_mount_hash();
        if (propagate_mount_busy(real_mount(mnt), 2))
                ret = 0;
        unlock_mount_hash();
        up_read(&namespace_sem);
        return ret;
}

EXPORT_SYMBOL(may_umount);

static void namespace_unlock(void)
{
        struct hlist_head head;
        struct hlist_node *p;
        struct mount *m;
        LIST_HEAD(list);

        hlist_move_list(&unmounted, &head);
        list_splice_init(&ex_mountpoints, &list);

        up_write(&namespace_sem);

        shrink_dentry_list(&list);

        if (likely(hlist_empty(&head)))
                return;

        synchronize_rcu_expedited();

        hlist_for_each_entry_safe(m, p, &head, mnt_umount) {
                hlist_del(&m->mnt_umount);
                mntput(&m->mnt);
        }
}

static inline void namespace_lock(void)
{
        down_write(&namespace_sem);
}

enum umount_tree_flags {
        UMOUNT_SYNC = 1,
        UMOUNT_PROPAGATE = 2,
        UMOUNT_CONNECTED = 4,
};

static bool disconnect_mount(struct mount *mnt, enum umount_tree_flags how)
{
        /* Leaving mounts connected is only valid for lazy umounts */
        if (how & UMOUNT_SYNC)
                return true;

        /* A mount without a parent has nothing to be connected to */
        if (!mnt_has_parent(mnt))
                return true;

        /* Because the reference counting rules change when mounts are
         * unmounted and connected, umounted mounts may not be
         * connected to mounted mounts.
         */
        if (!(mnt->mnt_parent->mnt.mnt_flags & MNT_UMOUNT))
                return true;

        /* Has it been requested that the mount remain connected? */
        if (how & UMOUNT_CONNECTED)
                return false;

        /* Is the mount locked such that it needs to remain connected? */
        if (IS_MNT_LOCKED(mnt))
                return false;

        /* By default disconnect the mount */
        return true;
}

/*
 * mount_lock must be held
 * namespace_sem must be held for write
 */
static void umount_tree(struct mount *mnt, enum umount_tree_flags how)
{
        LIST_HEAD(tmp_list);
        struct mount *p;

        if (how & UMOUNT_PROPAGATE)
                propagate_mount_unlock(mnt);

        /* Gather the mounts to umount */
        for (p = mnt; p; p = next_mnt(p, mnt)) {
                p->mnt.mnt_flags |= MNT_UMOUNT;
                if (p->mnt.mnt_flags & MNT_ONRB)
                        move_from_ns(p, &tmp_list);
                else
                        list_move(&p->mnt_list, &tmp_list);
        }

        /* Hide the mounts from mnt_mounts */
        list_for_each_entry(p, &tmp_list, mnt_list) {
                list_del_init(&p->mnt_child);
        }

        /* Add propogated mounts to the tmp_list */
        if (how & UMOUNT_PROPAGATE)
                propagate_umount(&tmp_list);

        while (!list_empty(&tmp_list)) {
                struct mnt_namespace *ns;
                bool disconnect;
                p = list_first_entry(&tmp_list, struct mount, mnt_list);
                list_del_init(&p->mnt_expire);
                list_del_init(&p->mnt_list);
                ns = p->mnt_ns;
                if (ns) {
                        ns->nr_mounts--;
                        __touch_mnt_namespace(ns);
                }
                p->mnt_ns = NULL;
                if (how & UMOUNT_SYNC)
                        p->mnt.mnt_flags |= MNT_SYNC_UMOUNT;

                disconnect = disconnect_mount(p, how);
                if (mnt_has_parent(p)) {
                        mnt_add_count(p->mnt_parent, -1);
                        if (!disconnect) {
                                /* Don't forget about p */
                                list_add_tail(&p->mnt_child, &p->mnt_parent->mnt_mounts);
                        } else {
                                umount_mnt(p);
                        }
                }
                change_mnt_propagation(p, MS_PRIVATE);
                if (disconnect)
                        hlist_add_head(&p->mnt_umount, &unmounted);
        }
}

static void shrink_submounts(struct mount *mnt);

static int do_umount_root(struct super_block *sb)
{
        int ret = 0;

        down_write(&sb->s_umount);
        if (!sb_rdonly(sb)) {
                struct fs_context *fc;

                fc = fs_context_for_reconfigure(sb->s_root, SB_RDONLY,
                                                SB_RDONLY);
                if (IS_ERR(fc)) {
                        ret = PTR_ERR(fc);
                } else {
                        ret = parse_monolithic_mount_data(fc, NULL);
                        if (!ret)
                                ret = reconfigure_super(fc);
                        put_fs_context(fc);
                }
        }
        up_write(&sb->s_umount);
        return ret;
}

static int do_umount(struct mount *mnt, int flags)
{
        struct super_block *sb = mnt->mnt.mnt_sb;
        int retval;

        retval = security_sb_umount(&mnt->mnt, flags);
        if (retval)
                return retval;

        /*
         * Allow userspace to request a mountpoint be expired rather than
         * unmounting unconditionally. Unmount only happens if:
         *  (1) the mark is already set (the mark is cleared by mntput())
         *  (2) the usage count == 1 [parent vfsmount] + 1 [sys_umount]
         */
        if (flags & MNT_EXPIRE) {
                if (&mnt->mnt == current->fs->root.mnt ||
                    flags & (MNT_FORCE | MNT_DETACH))
                        return -EINVAL;

                /*
                 * probably don't strictly need the lock here if we examined
                 * all race cases, but it's a slowpath.
                 */
                lock_mount_hash();
                if (mnt_get_count(mnt) != 2) {
                        unlock_mount_hash();
                        return -EBUSY;
                }
                unlock_mount_hash();

                if (!xchg(&mnt->mnt_expiry_mark, 1))
                        return -EAGAIN;
        }

        /*
         * If we may have to abort operations to get out of this
         * mount, and they will themselves hold resources we must
         * allow the fs to do things. In the Unix tradition of
         * 'Gee thats tricky lets do it in userspace' the umount_begin
         * might fail to complete on the first run through as other tasks
         * must return, and the like. Thats for the mount program to worry
         * about for the moment.
         */

        if (flags & MNT_FORCE && sb->s_op->umount_begin) {
                sb->s_op->umount_begin(sb);
        }

        /*
         * No sense to grab the lock for this test, but test itself looks
         * somewhat bogus. Suggestions for better replacement?
         * Ho-hum... In principle, we might treat that as umount + switch
         * to rootfs. GC would eventually take care of the old vfsmount.
         * Actually it makes sense, especially if rootfs would contain a
         * /reboot - static binary that would close all descriptors and
         * call reboot(9). Then init(8) could umount root and exec /reboot.
         */
        if (&mnt->mnt == current->fs->root.mnt && !(flags & MNT_DETACH)) {
                /*
                 * Special case for "unmounting" root ...
                 * we just try to remount it readonly.
                 */
                if (!ns_capable(sb->s_user_ns, CAP_SYS_ADMIN))
                        return -EPERM;
                return do_umount_root(sb);
        }

        namespace_lock();
        lock_mount_hash();

        /* Recheck MNT_LOCKED with the locks held */
        retval = -EINVAL;
        if (mnt->mnt.mnt_flags & MNT_LOCKED)
                goto out;

        event++;
        if (flags & MNT_DETACH) {
                if (mnt->mnt.mnt_flags & MNT_ONRB ||
                    !list_empty(&mnt->mnt_list))
                        umount_tree(mnt, UMOUNT_PROPAGATE);
                retval = 0;
        } else {
                shrink_submounts(mnt);
                retval = -EBUSY;
                if (!propagate_mount_busy(mnt, 2)) {
                        if (mnt->mnt.mnt_flags & MNT_ONRB ||
                            !list_empty(&mnt->mnt_list))
                                umount_tree(mnt, UMOUNT_PROPAGATE|UMOUNT_SYNC);
                        retval = 0;
                }
        }
out:
        unlock_mount_hash();
        namespace_unlock();
        return retval;
}

/*
 * __detach_mounts - lazily unmount all mounts on the specified dentry
 *
 * During unlink, rmdir, and d_drop it is possible to loose the path
 * to an existing mountpoint, and wind up leaking the mount.
 * detach_mounts allows lazily unmounting those mounts instead of
 * leaking them.
 *
 * The caller may hold dentry->d_inode->i_mutex.
 */
void __detach_mounts(struct dentry *dentry)
{
        struct mountpoint *mp;
        struct mount *mnt;

        namespace_lock();
        lock_mount_hash();
        mp = lookup_mountpoint(dentry);
        if (!mp)
                goto out_unlock;

        event++;
        while (!hlist_empty(&mp->m_list)) {
                mnt = hlist_entry(mp->m_list.first, struct mount, mnt_mp_list);
                if (mnt->mnt.mnt_flags & MNT_UMOUNT) {
                        umount_mnt(mnt);
                        hlist_add_head(&mnt->mnt_umount, &unmounted);
                }
                else umount_tree(mnt, UMOUNT_CONNECTED);
        }
        put_mountpoint(mp);
out_unlock:
        unlock_mount_hash();
        namespace_unlock();
}

/*
 * Is the caller allowed to modify his namespace?
 */
bool may_mount(void)
{
        return ns_capable(current->nsproxy->mnt_ns->user_ns, CAP_SYS_ADMIN);
}

/**
 * path_mounted - check whether path is mounted
 * @path: path to check
 *
 * Determine whether @path refers to the root of a mount.
 *
 * Return: true if @path is the root of a mount, false if not.
 */
static inline bool path_mounted(const struct path *path)
{
        return path->mnt->mnt_root == path->dentry;
}

static void warn_mandlock(void)
{
        pr_warn_once("=======================================================\n"
                     "WARNING: The mand mount option has been deprecated and\n"
                     "         and is ignored by this kernel. Remove the mand\n"
                     "         option from the mount to silence this warning.\n"
                     "=======================================================\n");
}

static int can_umount(const struct path *path, int flags)
{
        struct mount *mnt = real_mount(path->mnt);

        if (!may_mount())
                return -EPERM;
        if (!path_mounted(path))
                return -EINVAL;
        if (!check_mnt(mnt))
                return -EINVAL;
        if (mnt->mnt.mnt_flags & MNT_LOCKED) /* Check optimistically */
                return -EINVAL;
        if (flags & MNT_FORCE && !capable(CAP_SYS_ADMIN))
                return -EPERM;
        return 0;
}

// caller is responsible for flags being sane
int path_umount(struct path *path, int flags)
{
        struct mount *mnt = real_mount(path->mnt);
        int ret;

        ret = can_umount(path, flags);
        if (!ret)
                ret = do_umount(mnt, flags);

        /* we mustn't call path_put() as that would clear mnt_expiry_mark */
        dput(path->dentry);
        mntput_no_expire(mnt);
        return ret;
}

static int ksys_umount(char __user *name, int flags)
{
        int lookup_flags = LOOKUP_MOUNTPOINT;
        struct path path;
        int ret;

        // basic validity checks done first
        if (flags & ~(MNT_FORCE | MNT_DETACH | MNT_EXPIRE | UMOUNT_NOFOLLOW))
                return -EINVAL;

        if (!(flags & UMOUNT_NOFOLLOW))
                lookup_flags |= LOOKUP_FOLLOW;
        ret = user_path_at(AT_FDCWD, name, lookup_flags, &path);
        if (ret)
                return ret;
        return path_umount(&path, flags);
}

SYSCALL_DEFINE2(umount, char __user *, name, int, flags)
{
        return ksys_umount(name, flags);
}

#ifdef __ARCH_WANT_SYS_OLDUMOUNT

/*
 *      The 2.0 compatible umount. No flags.
 */
SYSCALL_DEFINE1(oldumount, char __user *, name)
{
        return ksys_umount(name, 0);
}

#endif

static bool is_mnt_ns_file(struct dentry *dentry)
{
        /* Is this a proxy for a mount namespace? */
        return dentry->d_op == &ns_dentry_operations &&
               dentry->d_fsdata == &mntns_operations;
}

static struct mnt_namespace *to_mnt_ns(struct ns_common *ns)
{
        return container_of(ns, struct mnt_namespace, ns);
}

struct ns_common *from_mnt_ns(struct mnt_namespace *mnt)
{
        return &mnt->ns;
}

static bool mnt_ns_loop(struct dentry *dentry)
{
        /* Could bind mounting the mount namespace inode cause a
         * mount namespace loop?
         */
        struct mnt_namespace *mnt_ns;
        if (!is_mnt_ns_file(dentry))
                return false;

        mnt_ns = to_mnt_ns(get_proc_ns(dentry->d_inode));
        return current->nsproxy->mnt_ns->seq >= mnt_ns->seq;
}

struct mount *copy_tree(struct mount *mnt, struct dentry *dentry,
                                        int flag)
{
        struct mount *res, *p, *q, *r, *parent;

        if (!(flag & CL_COPY_UNBINDABLE) && IS_MNT_UNBINDABLE(mnt))
                return ERR_PTR(-EINVAL);

        if (!(flag & CL_COPY_MNT_NS_FILE) && is_mnt_ns_file(dentry))
                return ERR_PTR(-EINVAL);

        res = q = clone_mnt(mnt, dentry, flag);
        if (IS_ERR(q))
                return q;

        q->mnt_mountpoint = mnt->mnt_mountpoint;

        p = mnt;
        list_for_each_entry(r, &mnt->mnt_mounts, mnt_child) {
                struct mount *s;
                if (!is_subdir(r->mnt_mountpoint, dentry))
                        continue;

                for (s = r; s; s = next_mnt(s, r)) {
                        if (!(flag & CL_COPY_UNBINDABLE) &&
                            IS_MNT_UNBINDABLE(s)) {
                                if (s->mnt.mnt_flags & MNT_LOCKED) {
                                        /* Both unbindable and locked. */
                                        q = ERR_PTR(-EPERM);
                                        goto out;
                                } else {
                                        s = skip_mnt_tree(s);
                                        continue;
                                }
                        }
                        if (!(flag & CL_COPY_MNT_NS_FILE) &&
                            is_mnt_ns_file(s->mnt.mnt_root)) {
                                s = skip_mnt_tree(s);
                                continue;
                        }
                        while (p != s->mnt_parent) {
                                p = p->mnt_parent;
                                q = q->mnt_parent;
                        }
                        p = s;
                        parent = q;
                        q = clone_mnt(p, p->mnt.mnt_root, flag);
                        if (IS_ERR(q))
                                goto out;
                        lock_mount_hash();
                        list_add_tail(&q->mnt_list, &res->mnt_list);
                        attach_mnt(q, parent, p->mnt_mp, false);
                        unlock_mount_hash();
                }
        }
        return res;
out:
        if (res) {
                lock_mount_hash();
                umount_tree(res, UMOUNT_SYNC);
                unlock_mount_hash();
        }
        return q;
}

/* Caller should check returned pointer for errors */

struct vfsmount *collect_mounts(const struct path *path)
{
        struct mount *tree;
        namespace_lock();
        if (!check_mnt(real_mount(path->mnt)))
                tree = ERR_PTR(-EINVAL);
        else
                tree = copy_tree(real_mount(path->mnt), path->dentry,
                                 CL_COPY_ALL | CL_PRIVATE);
        namespace_unlock();
        if (IS_ERR(tree))
                return ERR_CAST(tree);
        return &tree->mnt;
}

static void free_mnt_ns(struct mnt_namespace *);
static struct mnt_namespace *alloc_mnt_ns(struct user_namespace *, bool);

void dissolve_on_fput(struct vfsmount *mnt)
{
        struct mnt_namespace *ns;
        namespace_lock();
        lock_mount_hash();
        ns = real_mount(mnt)->mnt_ns;
        if (ns) {
                if (is_anon_ns(ns))
                        umount_tree(real_mount(mnt), UMOUNT_CONNECTED);
                else
                        ns = NULL;
        }
        unlock_mount_hash();
        namespace_unlock();
        if (ns)
                free_mnt_ns(ns);
}

void drop_collected_mounts(struct vfsmount *mnt)
{
        namespace_lock();
        lock_mount_hash();
        umount_tree(real_mount(mnt), 0);
        unlock_mount_hash();
        namespace_unlock();
}

static bool has_locked_children(struct mount *mnt, struct dentry *dentry)
{
        struct mount *child;

        list_for_each_entry(child, &mnt->mnt_mounts, mnt_child) {
                if (!is_subdir(child->mnt_mountpoint, dentry))
                        continue;

                if (child->mnt.mnt_flags & MNT_LOCKED)
                        return true;
        }
        return false;
}

/**
 * clone_private_mount - create a private clone of a path
 * @path: path to clone
 *
 * This creates a new vfsmount, which will be the clone of @path.  The new mount
 * will not be attached anywhere in the namespace and will be private (i.e.
 * changes to the originating mount won't be propagated into this).
 *
 * Release with mntput().
 */
struct vfsmount *clone_private_mount(const struct path *path)
{
        struct mount *old_mnt = real_mount(path->mnt);
        struct mount *new_mnt;

        down_read(&namespace_sem);
        if (IS_MNT_UNBINDABLE(old_mnt))
                goto invalid;

        if (!check_mnt(old_mnt))
                goto invalid;

        if (has_locked_children(old_mnt, path->dentry))
                goto invalid;

        new_mnt = clone_mnt(old_mnt, path->dentry, CL_PRIVATE);
        up_read(&namespace_sem);

        if (IS_ERR(new_mnt))
                return ERR_CAST(new_mnt);

        /* Longterm mount to be removed by kern_unmount*() */
        new_mnt->mnt_ns = MNT_NS_INTERNAL;

        return &new_mnt->mnt;

invalid:
        up_read(&namespace_sem);
        return ERR_PTR(-EINVAL);
}
EXPORT_SYMBOL_GPL(clone_private_mount);

int iterate_mounts(int (*f)(struct vfsmount *, void *), void *arg,
                   struct vfsmount *root)
{
        struct mount *mnt;
        int res = f(root, arg);
        if (res)
                return res;
        list_for_each_entry(mnt, &real_mount(root)->mnt_list, mnt_list) {
                res = f(&mnt->mnt, arg);
                if (res)
                        return res;
        }
        return 0;
}

static void lock_mnt_tree(struct mount *mnt)
{
        struct mount *p;

        for (p = mnt; p; p = next_mnt(p, mnt)) {
                int flags = p->mnt.mnt_flags;
                /* Don't allow unprivileged users to change mount flags */
                flags |= MNT_LOCK_ATIME;

                if (flags & MNT_READONLY)
                        flags |= MNT_LOCK_READONLY;

                if (flags & MNT_NODEV)
                        flags |= MNT_LOCK_NODEV;

                if (flags & MNT_NOSUID)
                        flags |= MNT_LOCK_NOSUID;

                if (flags & MNT_NOEXEC)
                        flags |= MNT_LOCK_NOEXEC;
                /* Don't allow unprivileged users to reveal what is under a mount */
                if (list_empty(&p->mnt_expire))
                        flags |= MNT_LOCKED;
                p->mnt.mnt_flags = flags;
        }
}

static void cleanup_group_ids(struct mount *mnt, struct mount *end)
{
        struct mount *p;

        for (p = mnt; p != end; p = next_mnt(p, mnt)) {
                if (p->mnt_group_id && !IS_MNT_SHARED(p))
                        mnt_release_group_id(p);
        }
}

static int invent_group_ids(struct mount *mnt, bool recurse)
{
        struct mount *p;

        for (p = mnt; p; p = recurse ? next_mnt(p, mnt) : NULL) {
                if (!p->mnt_group_id && !IS_MNT_SHARED(p)) {
                        int err = mnt_alloc_group_id(p);
                        if (err) {
                                cleanup_group_ids(mnt, p);
                                return err;
                        }
                }
        }

        return 0;
}

int count_mounts(struct mnt_namespace *ns, struct mount *mnt)
{
        unsigned int max = READ_ONCE(sysctl_mount_max);
        unsigned int mounts = 0;
        struct mount *p;

        if (ns->nr_mounts >= max)
                return -ENOSPC;
        max -= ns->nr_mounts;
        if (ns->pending_mounts >= max)
                return -ENOSPC;
        max -= ns->pending_mounts;

        for (p = mnt; p; p = next_mnt(p, mnt))
                mounts++;

        if (mounts > max)
                return -ENOSPC;

        ns->pending_mounts += mounts;
        return 0;
}

enum mnt_tree_flags_t {
        MNT_TREE_MOVE = BIT(0),
        MNT_TREE_BENEATH = BIT(1),
};

/**
 * attach_recursive_mnt - attach a source mount tree
 * @source_mnt: mount tree to be attached
 * @top_mnt:    mount that @source_mnt will be mounted on or mounted beneath
 * @dest_mp:    the mountpoint @source_mnt will be mounted at
 * @flags:      modify how @source_mnt is supposed to be attached
 *
 *  NOTE: in the table below explains the semantics when a source mount
 *  of a given type is attached to a destination mount of a given type.
 * ---------------------------------------------------------------------------
 * |         BIND MOUNT OPERATION                                            |
 * |**************************************************************************
 * | source-->| shared        |       private  |       slave    | unbindable |
 * | dest     |               |                |                |            |
 * |   |      |               |                |                |            |
 * |   v      |               |                |                |            |
 * |**************************************************************************
 * |  shared  | shared (++)   |     shared (+) |     shared(+++)|  invalid   |
 * |          |               |                |                |            |
 * |non-shared| shared (+)    |      private   |      slave (*) |  invalid   |
 * ***************************************************************************
 * A bind operation clones the source mount and mounts the clone on the
 * destination mount.
 *
 * (++)  the cloned mount is propagated to all the mounts in the propagation
 *       tree of the destination mount and the cloned mount is added to
 *       the peer group of the source mount.
 * (+)   the cloned mount is created under the destination mount and is marked
 *       as shared. The cloned mount is added to the peer group of the source
 *       mount.
 * (+++) the mount is propagated to all the mounts in the propagation tree
 *       of the destination mount and the cloned mount is made slave
 *       of the same master as that of the source mount. The cloned mount
 *       is marked as 'shared and slave'.
 * (*)   the cloned mount is made a slave of the same master as that of the
 *       source mount.
 *
 * ---------------------------------------------------------------------------
 * |                    MOVE MOUNT OPERATION                                 |
 * |**************************************************************************
 * | source-->| shared        |       private  |       slave    | unbindable |
 * | dest     |               |                |                |            |
 * |   |      |               |                |                |            |
 * |   v      |               |                |                |            |
 * |**************************************************************************
 * |  shared  | shared (+)    |     shared (+) |    shared(+++) |  invalid   |
 * |          |               |                |                |            |
 * |non-shared| shared (+*)   |      private   |    slave (*)   | unbindable |
 * ***************************************************************************
 *
 * (+)  the mount is moved to the destination. And is then propagated to
 *      all the mounts in the propagation tree of the destination mount.
 * (+*)  the mount is moved to the destination.
 * (+++)  the mount is moved to the destination and is then propagated to
 *      all the mounts belonging to the destination mount's propagation tree.
 *      the mount is marked as 'shared and slave'.
 * (*)  the mount continues to be a slave at the new location.
 *
 * if the source mount is a tree, the operations explained above is
 * applied to each mount in the tree.
 * Must be called without spinlocks held, since this function can sleep
 * in allocations.
 *
 * Context: The function expects namespace_lock() to be held.
 * Return: If @source_mnt was successfully attached 0 is returned.
 *         Otherwise a negative error code is returned.
 */
static int attach_recursive_mnt(struct mount *source_mnt,
                                struct mount *top_mnt,
                                struct mountpoint *dest_mp,
                                enum mnt_tree_flags_t flags)
{
        struct user_namespace *user_ns = current->nsproxy->mnt_ns->user_ns;
        HLIST_HEAD(tree_list);
        struct mnt_namespace *ns = top_mnt->mnt_ns;
        struct mountpoint *smp;
        struct mount *child, *dest_mnt, *p;
        struct hlist_node *n;
        int err = 0;
        bool moving = flags & MNT_TREE_MOVE, beneath = flags & MNT_TREE_BENEATH;

        /*
         * Preallocate a mountpoint in case the new mounts need to be
         * mounted beneath mounts on the same mountpoint.
         */
        smp = get_mountpoint(source_mnt->mnt.mnt_root);
        if (IS_ERR(smp))
                return PTR_ERR(smp);

        /* Is there space to add these mounts to the mount namespace? */
        if (!moving) {
                err = count_mounts(ns, source_mnt);
                if (err)
                        goto out;
        }

        if (beneath)
                dest_mnt = top_mnt->mnt_parent;
        else
                dest_mnt = top_mnt;

        if (IS_MNT_SHARED(dest_mnt)) {
                err = invent_group_ids(source_mnt, true);
                if (err)
                        goto out;
                err = propagate_mnt(dest_mnt, dest_mp, source_mnt, &tree_list);
        }
        lock_mount_hash();
        if (err)
                goto out_cleanup_ids;

        if (IS_MNT_SHARED(dest_mnt)) {
                for (p = source_mnt; p; p = next_mnt(p, source_mnt))
                        set_mnt_shared(p);
        }

        if (moving) {
                if (beneath)
                        dest_mp = smp;
                unhash_mnt(source_mnt);
                attach_mnt(source_mnt, top_mnt, dest_mp, beneath);
                touch_mnt_namespace(source_mnt->mnt_ns);
        } else {
                if (source_mnt->mnt_ns) {
                        LIST_HEAD(head);

                        /* move from anon - the caller will destroy */
                        for (p = source_mnt; p; p = next_mnt(p, source_mnt))
                                move_from_ns(p, &head);
                        list_del_init(&head);
                }
                if (beneath)
                        mnt_set_mountpoint_beneath(source_mnt, top_mnt, smp);
                else
                        mnt_set_mountpoint(dest_mnt, dest_mp, source_mnt);
                commit_tree(source_mnt);
        }

        hlist_for_each_entry_safe(child, n, &tree_list, mnt_hash) {
                struct mount *q;
                hlist_del_init(&child->mnt_hash);
                q = __lookup_mnt(&child->mnt_parent->mnt,
                                 child->mnt_mountpoint);
                if (q)
                        mnt_change_mountpoint(child, smp, q);
                /* Notice when we are propagating across user namespaces */
                if (child->mnt_parent->mnt_ns->user_ns != user_ns)
                        lock_mnt_tree(child);
                child->mnt.mnt_flags &= ~MNT_LOCKED;
                commit_tree(child);
        }
        put_mountpoint(smp);
        unlock_mount_hash();

        return 0;

 out_cleanup_ids:
        while (!hlist_empty(&tree_list)) {
                child = hlist_entry(tree_list.first, struct mount, mnt_hash);
                child->mnt_parent->mnt_ns->pending_mounts = 0;
                umount_tree(child, UMOUNT_SYNC);
        }
        unlock_mount_hash();
        cleanup_group_ids(source_mnt, NULL);
 out:
        ns->pending_mounts = 0;

        read_seqlock_excl(&mount_lock);
        put_mountpoint(smp);
        read_sequnlock_excl(&mount_lock);

        return err;
}

/**
 * do_lock_mount - lock mount and mountpoint
 * @path:    target path
 * @beneath: whether the intention is to mount beneath @path
 *
 * Follow the mount stack on @path until the top mount @mnt is found. If
 * the initial @path->{mnt,dentry} is a mountpoint lookup the first
 * mount stacked on top of it. Then simply follow @{mnt,mnt->mnt_root}
 * until nothing is stacked on top of it anymore.
 *
 * Acquire the inode_lock() on the top mount's ->mnt_root to protect
 * against concurrent removal of the new mountpoint from another mount
 * namespace.
 *
 * If @beneath is requested, acquire inode_lock() on @mnt's mountpoint
 * @mp on @mnt->mnt_parent must be acquired. This protects against a
 * concurrent unlink of @mp->mnt_dentry from another mount namespace
 * where @mnt doesn't have a child mount mounted @mp. A concurrent
 * removal of @mnt->mnt_root doesn't matter as nothing will be mounted
 * on top of it for @beneath.
 *
 * In addition, @beneath needs to make sure that @mnt hasn't been
 * unmounted or moved from its current mountpoint in between dropping
 * @mount_lock and acquiring @namespace_sem. For the !@beneath case @mnt
 * being unmounted would be detected later by e.g., calling
 * check_mnt(mnt) in the function it's called from. For the @beneath
 * case however, it's useful to detect it directly in do_lock_mount().
 * If @mnt hasn't been unmounted then @mnt->mnt_mountpoint still points
 * to @mnt->mnt_mp->m_dentry. But if @mnt has been unmounted it will
 * point to @mnt->mnt_root and @mnt->mnt_mp will be NULL.
 *
 * Return: Either the target mountpoint on the top mount or the top
 *         mount's mountpoint.
 */
static struct mountpoint *do_lock_mount(struct path *path, bool beneath)
{
        struct vfsmount *mnt = path->mnt;
        struct dentry *dentry;
        struct mountpoint *mp = ERR_PTR(-ENOENT);

        for (;;) {
                struct mount *m;

                if (beneath) {
                        m = real_mount(mnt);
                        read_seqlock_excl(&mount_lock);
                        dentry = dget(m->mnt_mountpoint);
                        read_sequnlock_excl(&mount_lock);
                } else {
                        dentry = path->dentry;
                }

                inode_lock(dentry->d_inode);
                if (unlikely(cant_mount(dentry))) {
                        inode_unlock(dentry->d_inode);
                        goto out;
                }

                namespace_lock();

                if (beneath && (!is_mounted(mnt) || m->mnt_mountpoint != dentry)) {
                        namespace_unlock();
                        inode_unlock(dentry->d_inode);
                        goto out;
                }

                mnt = lookup_mnt(path);
                if (likely(!mnt))
                        break;

                namespace_unlock();
                inode_unlock(dentry->d_inode);
                if (beneath)
                        dput(dentry);
                path_put(path);
                path->mnt = mnt;
                path->dentry = dget(mnt->mnt_root);
        }

        mp = get_mountpoint(dentry);
        if (IS_ERR(mp)) {
                namespace_unlock();
                inode_unlock(dentry->d_inode);
        }

out:
        if (beneath)
                dput(dentry);

        return mp;
}

static inline struct mountpoint *lock_mount(struct path *path)
{
        return do_lock_mount(path, false);
}

static void unlock_mount(struct mountpoint *where)
{
        struct dentry *dentry = where->m_dentry;

        read_seqlock_excl(&mount_lock);
        put_mountpoint(where);
        read_sequnlock_excl(&mount_lock);

        namespace_unlock();
        inode_unlock(dentry->d_inode);
}

static int graft_tree(struct mount *mnt, struct mount *p, struct mountpoint *mp)
{
        if (mnt->mnt.mnt_sb->s_flags & SB_NOUSER)
                return -EINVAL;

        if (d_is_dir(mp->m_dentry) !=
              d_is_dir(mnt->mnt.mnt_root))
                return -ENOTDIR;

        return attach_recursive_mnt(mnt, p, mp, 0);
}

/*
 * Sanity check the flags to change_mnt_propagation.
 */

static int flags_to_propagation_type(int ms_flags)
{
        int type = ms_flags & ~(MS_REC | MS_SILENT);

        /* Fail if any non-propagation flags are set */
        if (type & ~(MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
                return 0;
        /* Only one propagation flag should be set */
        if (!is_power_of_2(type))
                return 0;
        return type;
}

/*
 * recursively change the type of the mountpoint.
 */
static int do_change_type(struct path *path, int ms_flags)
{
        struct mount *m;
        struct mount *mnt = real_mount(path->mnt);
        int recurse = ms_flags & MS_REC;
        int type;
        int err = 0;

        if (!path_mounted(path))
                return -EINVAL;

        type = flags_to_propagation_type(ms_flags);
        if (!type)
                return -EINVAL;

        namespace_lock();
        if (type == MS_SHARED) {
                err = invent_group_ids(mnt, recurse);
                if (err)
                        goto out_unlock;
        }

        lock_mount_hash();
        for (m = mnt; m; m = (recurse ? next_mnt(m, mnt) : NULL))
                change_mnt_propagation(m, type);
        unlock_mount_hash();

 out_unlock:
        namespace_unlock();
        return err;
}

static struct mount *__do_loopback(struct path *old_path, int recurse)
{
        struct mount *mnt = ERR_PTR(-EINVAL), *old = real_mount(old_path->mnt);

        if (IS_MNT_UNBINDABLE(old))
                return mnt;

        if (!check_mnt(old) && old_path->dentry->d_op != &ns_dentry_operations)
                return mnt;

        if (!recurse && has_locked_children(old, old_path->dentry))
                return mnt;

        if (recurse)
                mnt = copy_tree(old, old_path->dentry, CL_COPY_MNT_NS_FILE);
        else
                mnt = clone_mnt(old, old_path->dentry, 0);

        if (!IS_ERR(mnt))
                mnt->mnt.mnt_flags &= ~MNT_LOCKED;

        return mnt;
}

/*
 * do loopback mount.
 */
static int do_loopback(struct path *path, const char *old_name,
                                int recurse)
{
        struct path old_path;
        struct mount *mnt = NULL, *parent;
        struct mountpoint *mp;
        int err;
        if (!old_name || !*old_name)
                return -EINVAL;
        err = kern_path(old_name, LOOKUP_FOLLOW|LOOKUP_AUTOMOUNT, &old_path);
        if (err)
                return err;

        err = -EINVAL;
        if (mnt_ns_loop(old_path.dentry))
                goto out;

        mp = lock_mount(path);
        if (IS_ERR(mp)) {
                err = PTR_ERR(mp);
                goto out;
        }

        parent = real_mount(path->mnt);
        if (!check_mnt(parent))
                goto out2;

        mnt = __do_loopback(&old_path, recurse);
        if (IS_ERR(mnt)) {
                err = PTR_ERR(mnt);
                goto out2;
        }

        err = graft_tree(mnt, parent, mp);
        if (err) {
                lock_mount_hash();
                umount_tree(mnt, UMOUNT_SYNC);
                unlock_mount_hash();
        }
out2:
        unlock_mount(mp);
out:
        path_put(&old_path);
        return err;
}

static struct file *open_detached_copy(struct path *path, bool recursive)
{
        struct user_namespace *user_ns = current->nsproxy->mnt_ns->user_ns;
        struct mnt_namespace *ns = alloc_mnt_ns(user_ns, true);
        struct mount *mnt, *p;
        struct file *file;

        if (IS_ERR(ns))
                return ERR_CAST(ns);

        namespace_lock();
        mnt = __do_loopback(path, recursive);
        if (IS_ERR(mnt)) {
                namespace_unlock();
                free_mnt_ns(ns);
                return ERR_CAST(mnt);
        }

        lock_mount_hash();
        for (p = mnt; p; p = next_mnt(p, mnt)) {
                mnt_add_to_ns(ns, p);
                ns->nr_mounts++;
        }
        ns->root = mnt;
        mntget(&mnt->mnt);
        unlock_mount_hash();
        namespace_unlock();

        mntput(path->mnt);
        path->mnt = &mnt->mnt;
        file = dentry_open(path, O_PATH, current_cred());
        if (IS_ERR(file))
                dissolve_on_fput(path->mnt);
        else
                file->f_mode |= FMODE_NEED_UNMOUNT;
        return file;
}

SYSCALL_DEFINE3(open_tree, int, dfd, const char __user *, filename, unsigned, flags)
{
        struct file *file;
        struct path path;
        int lookup_flags = LOOKUP_AUTOMOUNT | LOOKUP_FOLLOW;
        bool detached = flags & OPEN_TREE_CLONE;
        int error;
        int fd;

        BUILD_BUG_ON(OPEN_TREE_CLOEXEC != O_CLOEXEC);

        if (flags & ~(AT_EMPTY_PATH | AT_NO_AUTOMOUNT | AT_RECURSIVE |
                      AT_SYMLINK_NOFOLLOW | OPEN_TREE_CLONE |
                      OPEN_TREE_CLOEXEC))
                return -EINVAL;

        if ((flags & (AT_RECURSIVE | OPEN_TREE_CLONE)) == AT_RECURSIVE)
                return -EINVAL;

        if (flags & AT_NO_AUTOMOUNT)
                lookup_flags &= ~LOOKUP_AUTOMOUNT;
        if (flags & AT_SYMLINK_NOFOLLOW)
                lookup_flags &= ~LOOKUP_FOLLOW;
        if (flags & AT_EMPTY_PATH)
                lookup_flags |= LOOKUP_EMPTY;

        if (detached && !may_mount())
                return -EPERM;

        fd = get_unused_fd_flags(flags & O_CLOEXEC);
        if (fd < 0)
                return fd;

        error = user_path_at(dfd, filename, lookup_flags, &path);
        if (unlikely(error)) {
                file = ERR_PTR(error);
        } else {
                if (detached)
                        file = open_detached_copy(&path, flags & AT_RECURSIVE);
                else
                        file = dentry_open(&path, O_PATH, current_cred());
                path_put(&path);
        }
        if (IS_ERR(file)) {
                put_unused_fd(fd);
                return PTR_ERR(file);
        }
        fd_install(fd, file);
        return fd;
}

/*
 * Don't allow locked mount flags to be cleared.
 *
 * No locks need to be held here while testing the various MNT_LOCK
 * flags because those flags can never be cleared once they are set.
 */
static bool can_change_locked_flags(struct mount *mnt, unsigned int mnt_flags)
{
        unsigned int fl = mnt->mnt.mnt_flags;

        if ((fl & MNT_LOCK_READONLY) &&
            !(mnt_flags & MNT_READONLY))
                return false;

        if ((fl & MNT_LOCK_NODEV) &&
            !(mnt_flags & MNT_NODEV))
                return false;

        if ((fl & MNT_LOCK_NOSUID) &&
            !(mnt_flags & MNT_NOSUID))
                return false;

        if ((fl & MNT_LOCK_NOEXEC) &&
            !(mnt_flags & MNT_NOEXEC))
                return false;

        if ((fl & MNT_LOCK_ATIME) &&
            ((fl & MNT_ATIME_MASK) != (mnt_flags & MNT_ATIME_MASK)))
                return false;

        return true;
}

static int change_mount_ro_state(struct mount *mnt, unsigned int mnt_flags)
{
        bool readonly_request = (mnt_flags & MNT_READONLY);

        if (readonly_request == __mnt_is_readonly(&mnt->mnt))
                return 0;

        if (readonly_request)
                return mnt_make_readonly(mnt);

        mnt->mnt.mnt_flags &= ~MNT_READONLY;
        return 0;
}

static void set_mount_attributes(struct mount *mnt, unsigned int mnt_flags)
{
        mnt_flags |= mnt->mnt.mnt_flags & ~MNT_USER_SETTABLE_MASK;
        mnt->mnt.mnt_flags = mnt_flags;
        touch_mnt_namespace(mnt->mnt_ns);
}

static void mnt_warn_timestamp_expiry(struct path *mountpoint, struct vfsmount *mnt)
{
        struct super_block *sb = mnt->mnt_sb;

        if (!__mnt_is_readonly(mnt) &&
           (!(sb->s_iflags & SB_I_TS_EXPIRY_WARNED)) &&
           (ktime_get_real_seconds() + TIME_UPTIME_SEC_MAX > sb->s_time_max)) {
                char *buf = (char *)__get_free_page(GFP_KERNEL);
                char *mntpath = buf ? d_path(mountpoint, buf, PAGE_SIZE) : ERR_PTR(-ENOMEM);

                pr_warn("%s filesystem being %s at %s supports timestamps until %ptTd (0x%llx)\n",
                        sb->s_type->name,
                        is_mounted(mnt) ? "remounted" : "mounted",
                        mntpath, &sb->s_time_max,
                        (unsigned long long)sb->s_time_max);

                free_page((unsigned long)buf);
                sb->s_iflags |= SB_I_TS_EXPIRY_WARNED;
        }
}

/*
 * Handle reconfiguration of the mountpoint only without alteration of the
 * superblock it refers to.  This is triggered by specifying MS_REMOUNT|MS_BIND
 * to mount(2).
 */
static int do_reconfigure_mnt(struct path *path, unsigned int mnt_flags)
{
        struct super_block *sb = path->mnt->mnt_sb;
        struct mount *mnt = real_mount(path->mnt);
        int ret;

        if (!check_mnt(mnt))
                return -EINVAL;

        if (!path_mounted(path))
                return -EINVAL;

        if (!can_change_locked_flags(mnt, mnt_flags))
                return -EPERM;

        /*
         * We're only checking whether the superblock is read-only not
         * changing it, so only take down_read(&sb->s_umount).
         */
        down_read(&sb->s_umount);
        lock_mount_hash();
        ret = change_mount_ro_state(mnt, mnt_flags);
        if (ret == 0)
                set_mount_attributes(mnt, mnt_flags);
        unlock_mount_hash();
        up_read(&sb->s_umount);

        mnt_warn_timestamp_expiry(path, &mnt->mnt);

        return ret;
}

/*
 * change filesystem flags. dir should be a physical root of filesystem.
 * If you've mounted a non-root directory somewhere and want to do remount
 * on it - tough luck.
 */
static int do_remount(struct path *path, int ms_flags, int sb_flags,
                      int mnt_flags, void *data)
{
        int err;
        struct super_block *sb = path->mnt->mnt_sb;
        struct mount *mnt = real_mount(path->mnt);
        struct fs_context *fc;

        if (!check_mnt(mnt))
                return -EINVAL;

        if (!path_mounted(path))
                return -EINVAL;

        if (!can_change_locked_flags(mnt, mnt_flags))
                return -EPERM;

        fc = fs_context_for_reconfigure(path->dentry, sb_flags, MS_RMT_MASK);
        if (IS_ERR(fc))
                return PTR_ERR(fc);

        /*
         * Indicate to the filesystem that the remount request is coming
         * from the legacy mount system call.
         */
        fc->oldapi = true;

        err = parse_monolithic_mount_data(fc, data);
        if (!err) {
                down_write(&sb->s_umount);
                err = -EPERM;
                if (ns_capable(sb->s_user_ns, CAP_SYS_ADMIN)) {
                        err = reconfigure_super(fc);
                        if (!err) {
                                lock_mount_hash();
                                set_mount_attributes(mnt, mnt_flags);
                                unlock_mount_hash();
                        }
                }
                up_write(&sb->s_umount);
        }

        mnt_warn_timestamp_expiry(path, &mnt->mnt);

        put_fs_context(fc);
        return err;
}

static inline int tree_contains_unbindable(struct mount *mnt)
{
        struct mount *p;
        for (p = mnt; p; p = next_mnt(p, mnt)) {
                if (IS_MNT_UNBINDABLE(p))
                        return 1;
        }
        return 0;
}

/*
 * Check that there aren't references to earlier/same mount namespaces in the
 * specified subtree.  Such references can act as pins for mount namespaces
 * that aren't checked by the mount-cycle checking code, thereby allowing
 * cycles to be made.
 */
static bool check_for_nsfs_mounts(struct mount *subtree)
{
        struct mount *p;
        bool ret = false;

        lock_mount_hash();
        for (p = subtree; p; p = next_mnt(p, subtree))
                if (mnt_ns_loop(p->mnt.mnt_root))
                        goto out;

        ret = true;
out:
        unlock_mount_hash();
        return ret;
}

static int do_set_group(struct path *from_path, struct path *to_path)
{
        struct mount *from, *to;
        int err;

        from = real_mount(from_path->mnt);
        to = real_mount(to_path->mnt);

        namespace_lock();

        err = -EINVAL;
        /* To and From must be mounted */
        if (!is_mounted(&from->mnt))
                goto out;
        if (!is_mounted(&to->mnt))
                goto out;

        err = -EPERM;
        /* We should be allowed to modify mount namespaces of both mounts */
        if (!ns_capable(from->mnt_ns->user_ns, CAP_SYS_ADMIN))
                goto out;
        if (!ns_capable(to->mnt_ns->user_ns, CAP_SYS_ADMIN))
                goto out;

        err = -EINVAL;
        /* To and From paths should be mount roots */
        if (!path_mounted(from_path))
                goto out;
        if (!path_mounted(to_path))
                goto out;

        /* Setting sharing groups is only allowed across same superblock */
        if (from->mnt.mnt_sb != to->mnt.mnt_sb)
                goto out;

        /* From mount root should be wider than To mount root */
        if (!is_subdir(to->mnt.mnt_root, from->mnt.mnt_root))
                goto out;

        /* From mount should not have locked children in place of To's root */
        if (has_locked_children(from, to->mnt.mnt_root))
                goto out;

        /* Setting sharing groups is only allowed on private mounts */
        if (IS_MNT_SHARED(to) || IS_MNT_SLAVE(to))
                goto out;

        /* From should not be private */
        if (!IS_MNT_SHARED(from) && !IS_MNT_SLAVE(from))
                goto out;

        if (IS_MNT_SLAVE(from)) {
                struct mount *m = from->mnt_master;

                list_add(&to->mnt_slave, &m->mnt_slave_list);
                to->mnt_master = m;
        }

        if (IS_MNT_SHARED(from)) {
                to->mnt_group_id = from->mnt_group_id;
                list_add(&to->mnt_share, &from->mnt_share);
                lock_mount_hash();
                set_mnt_shared(to);
                unlock_mount_hash();
        }

        err = 0;
out:
        namespace_unlock();
        return err;
}

/**
 * path_overmounted - check if path is overmounted
 * @path: path to check
 *
 * Check if path is overmounted, i.e., if there's a mount on top of
 * @path->mnt with @path->dentry as mountpoint.
 *
 * Context: This function expects namespace_lock() to be held.
 * Return: If path is overmounted true is returned, false if not.
 */
static inline bool path_overmounted(const struct path *path)
{
        rcu_read_lock();
        if (unlikely(__lookup_mnt(path->mnt, path->dentry))) {
                rcu_read_unlock();
                return true;
        }
        rcu_read_unlock();
        return false;
}

/**
 * can_move_mount_beneath - check that we can mount beneath the top mount
 * @from: mount to mount beneath
 * @to:   mount under which to mount
 * @mp:   mountpoint of @to
 *
 * - Make sure that @to->dentry is actually the root of a mount under
 *   which we can mount another mount.
 * - Make sure that nothing can be mounted beneath the caller's current
 *   root or the rootfs of the namespace.
 * - Make sure that the caller can unmount the topmost mount ensuring
 *   that the caller could reveal the underlying mountpoint.
 * - Ensure that nothing has been mounted on top of @from before we
 *   grabbed @namespace_sem to avoid creating pointless shadow mounts.
 * - Prevent mounting beneath a mount if the propagation relationship
 *   between the source mount, parent mount, and top mount would lead to
 *   nonsensical mount trees.
 *
 * Context: This function expects namespace_lock() to be held.
 * Return: On success 0, and on error a negative error code is returned.
 */
static int can_move_mount_beneath(const struct path *from,
                                  const struct path *to,
                                  const struct mountpoint *mp)
{
        struct mount *mnt_from = real_mount(from->mnt),
                     *mnt_to = real_mount(to->mnt),
                     *parent_mnt_to = mnt_to->mnt_parent;

        if (!mnt_has_parent(mnt_to))
                return -EINVAL;

        if (!path_mounted(to))
                return -EINVAL;

        if (IS_MNT_LOCKED(mnt_to))
                return -EINVAL;

        /* Avoid creating shadow mounts during mount propagation. */
        if (path_overmounted(from))
                return -EINVAL;

        /*
         * Mounting beneath the rootfs only makes sense when the
         * semantics of pivot_root(".", ".") are used.
         */
        if (&mnt_to->mnt == current->fs->root.mnt)
                return -EINVAL;
        if (parent_mnt_to == current->nsproxy->mnt_ns->root)
                return -EINVAL;

        for (struct mount *p = mnt_from; mnt_has_parent(p); p = p->mnt_parent)
                if (p == mnt_to)
                        return -EINVAL;

        /*
         * If the parent mount propagates to the child mount this would
         * mean mounting @mnt_from on @mnt_to->mnt_parent and then
         * propagating a copy @c of @mnt_from on top of @mnt_to. This
         * defeats the whole purpose of mounting beneath another mount.
         */
        if (propagation_would_overmount(parent_mnt_to, mnt_to, mp))
                return -EINVAL;

        /*
         * If @mnt_to->mnt_parent propagates to @mnt_from this would
         * mean propagating a copy @c of @mnt_from on top of @mnt_from.
         * Afterwards @mnt_from would be mounted on top of
         * @mnt_to->mnt_parent and @mnt_to would be unmounted from
         * @mnt->mnt_parent and remounted on @mnt_from. But since @c is
         * already mounted on @mnt_from, @mnt_to would ultimately be
         * remounted on top of @c. Afterwards, @mnt_from would be
         * covered by a copy @c of @mnt_from and @c would be covered by
         * @mnt_from itself. This defeats the whole purpose of mounting
         * @mnt_from beneath @mnt_to.
         */
        if (propagation_would_overmount(parent_mnt_to, mnt_from, mp))
                return -EINVAL;

        return 0;
}

static int do_move_mount(struct path *old_path, struct path *new_path,
                         bool beneath)
{
        struct mnt_namespace *ns;
        struct mount *p;
        struct mount *old;
        struct mount *parent;
        struct mountpoint *mp, *old_mp;
        int err;
        bool attached;
        enum mnt_tree_flags_t flags = 0;

        mp = do_lock_mount(new_path, beneath);
        if (IS_ERR(mp))
                return PTR_ERR(mp);

        old = real_mount(old_path->mnt);
        p = real_mount(new_path->mnt);
        parent = old->mnt_parent;
        attached = mnt_has_parent(old);
        if (attached)
                flags |= MNT_TREE_MOVE;
        old_mp = old->mnt_mp;
        ns = old->mnt_ns;

        err = -EINVAL;
        /* The mountpoint must be in our namespace. */
        if (!check_mnt(p))
                goto out;

        /* The thing moved must be mounted... */
        if (!is_mounted(&old->mnt))
                goto out;

        /* ... and either ours or the root of anon namespace */
        if (!(attached ? check_mnt(old) : is_anon_ns(ns)))
                goto out;

        if (old->mnt.mnt_flags & MNT_LOCKED)
                goto out;

        if (!path_mounted(old_path))
                goto out;

        if (d_is_dir(new_path->dentry) !=
            d_is_dir(old_path->dentry))
                goto out;
        /*
         * Don't move a mount residing in a shared parent.
         */
        if (attached && IS_MNT_SHARED(parent))
                goto out;

        if (beneath) {
                err = can_move_mount_beneath(old_path, new_path, mp);
                if (err)
                        goto out;

                err = -EINVAL;
                p = p->mnt_parent;
                flags |= MNT_TREE_BENEATH;
        }

        /*
         * Don't move a mount tree containing unbindable mounts to a destination
         * mount which is shared.
         */
        if (IS_MNT_SHARED(p) && tree_contains_unbindable(old))
                goto out;
        err = -ELOOP;
        if (!check_for_nsfs_mounts(old))
                goto out;
        for (; mnt_has_parent(p); p = p->mnt_parent)
                if (p == old)
                        goto out;

        err = attach_recursive_mnt(old, real_mount(new_path->mnt), mp, flags);
        if (err)
                goto out;

        /* if the mount is moved, it should no longer be expire
         * automatically */
        list_del_init(&old->mnt_expire);
        if (attached)
                put_mountpoint(old_mp);
out:
        unlock_mount(mp);
        if (!err) {
                if (attached)
                        mntput_no_expire(parent);
                else
                        free_mnt_ns(ns);
        }
        return err;
}

static int do_move_mount_old(struct path *path, const char *old_name)
{
        struct path old_path;
        int err;

        if (!old_name || !*old_name)
                return -EINVAL;

        err = kern_path(old_name, LOOKUP_FOLLOW, &old_path);
        if (err)
                return err;

        err = do_move_mount(&old_path, path, false);
        path_put(&old_path);
        return err;
}

/*
 * add a mount into a namespace's mount tree
 */
static int do_add_mount(struct mount *newmnt, struct mountpoint *mp,
                        const struct path *path, int mnt_flags)
{
        struct mount *parent = real_mount(path->mnt);

        mnt_flags &= ~MNT_INTERNAL_FLAGS;

        if (unlikely(!check_mnt(parent))) {
                /* that's acceptable only for automounts done in private ns */
                if (!(mnt_flags & MNT_SHRINKABLE))
                        return -EINVAL;
                /* ... and for those we'd better have mountpoint still alive */
                if (!parent->mnt_ns)
                        return -EINVAL;
        }

        /* Refuse the same filesystem on the same mount point */
        if (path->mnt->mnt_sb == newmnt->mnt.mnt_sb && path_mounted(path))
                return -EBUSY;

        if (d_is_symlink(newmnt->mnt.mnt_root))
                return -EINVAL;

        newmnt->mnt.mnt_flags = mnt_flags;
        return graft_tree(newmnt, parent, mp);
}

static bool mount_too_revealing(const struct super_block *sb, int *new_mnt_flags);

/*
 * Create a new mount using a superblock configuration and request it
 * be added to the namespace tree.
 */
static int do_new_mount_fc(struct fs_context *fc, struct path *mountpoint,
                           unsigned int mnt_flags)
{
        struct vfsmount *mnt;
        struct mountpoint *mp;
        struct super_block *sb = fc->root->d_sb;
        int error;

        error = security_sb_kern_mount(sb);
        if (!error && mount_too_revealing(sb, &mnt_flags))
                error = -EPERM;

        if (unlikely(error)) {
                fc_drop_locked(fc);
                return error;
        }

        up_write(&sb->s_umount);

        mnt = vfs_create_mount(fc);
        if (IS_ERR(mnt))
                return PTR_ERR(mnt);

        mnt_warn_timestamp_expiry(mountpoint, mnt);

        mp = lock_mount(mountpoint);
        if (IS_ERR(mp)) {
                mntput(mnt);
                return PTR_ERR(mp);
        }
        error = do_add_mount(real_mount(mnt), mp, mountpoint, mnt_flags);
        unlock_mount(mp);
        if (error < 0)
                mntput(mnt);
        return error;
}

/*
 * create a new mount for userspace and request it to be added into the
 * namespace's tree
 */
static int do_new_mount(struct path *path, const char *fstype, int sb_flags,
                        int mnt_flags, const char *name, void *data)
{
        struct file_system_type *type;
        struct fs_context *fc;
        const char *subtype = NULL;
        int err = 0;

        if (!fstype)
                return -EINVAL;

        type = get_fs_type(fstype);
        if (!type)
                return -ENODEV;

        if (type->fs_flags & FS_HAS_SUBTYPE) {
                subtype = strchr(fstype, '.');
                if (subtype) {
                        subtype++;
                        if (!*subtype) {
                                put_filesystem(type);
                                return -EINVAL;
                        }
                }
        }

        fc = fs_context_for_mount(type, sb_flags);
        put_filesystem(type);
        if (IS_ERR(fc))
                return PTR_ERR(fc);

        /*
         * Indicate to the filesystem that the mount request is coming
         * from the legacy mount system call.
         */
        fc->oldapi = true;

        if (subtype)
                err = vfs_parse_fs_string(fc, "subtype",
                                          subtype, strlen(subtype));
        if (!err && name)
                err = vfs_parse_fs_string(fc, "source", name, strlen(name));
        if (!err)
                err = parse_monolithic_mount_data(fc, data);
        if (!err && !mount_capable(fc))
                err = -EPERM;
        if (!err)
                err = vfs_get_tree(fc);
        if (!err)
                err = do_new_mount_fc(fc, path, mnt_flags);

        put_fs_context(fc);
        return err;
}

int finish_automount(struct vfsmount *m, const struct path *path)
{
        struct dentry *dentry = path->dentry;
        struct mountpoint *mp;
        struct mount *mnt;
        int err;

        if (!m)
                return 0;
        if (IS_ERR(m))
                return PTR_ERR(m);

        mnt = real_mount(m);
        /* The new mount record should have at least 2 refs to prevent it being
         * expired before we get a chance to add it
         */
        BUG_ON(mnt_get_count(mnt) < 2);

        if (m->mnt_sb == path->mnt->mnt_sb &&
            m->mnt_root == dentry) {
                err = -ELOOP;
                goto discard;
        }

        /*
         * we don't want to use lock_mount() - in this case finding something
         * that overmounts our mountpoint to be means "quitely drop what we've
         * got", not "try to mount it on top".
         */
        inode_lock(dentry->d_inode);
        namespace_lock();
        if (unlikely(cant_mount(dentry))) {
                err = -ENOENT;
                goto discard_locked;
        }
        if (path_overmounted(path)) {
                err = 0;
                goto discard_locked;
        }
        mp = get_mountpoint(dentry);
        if (IS_ERR(mp)) {
                err = PTR_ERR(mp);
                goto discard_locked;
        }

        err = do_add_mount(mnt, mp, path, path->mnt->mnt_flags | MNT_SHRINKABLE);
        unlock_mount(mp);
        if (unlikely(err))
                goto discard;
        mntput(m);
        return 0;

discard_locked:
        namespace_unlock();
        inode_unlock(dentry->d_inode);
discard:
        /* remove m from any expiration list it may be on */
        if (!list_empty(&mnt->mnt_expire)) {
                namespace_lock();
                list_del_init(&mnt->mnt_expire);
                namespace_unlock();
        }
        mntput(m);
        mntput(m);
        return err;
}

/**
 * mnt_set_expiry - Put a mount on an expiration list
 * @mnt: The mount to list.
 * @expiry_list: The list to add the mount to.
 */
void mnt_set_expiry(struct vfsmount *mnt, struct list_head *expiry_list)
{
        namespace_lock();

        list_add_tail(&real_mount(mnt)->mnt_expire, expiry_list);

        namespace_unlock();
}
EXPORT_SYMBOL(mnt_set_expiry);

/*
 * process a list of expirable mountpoints with the intent of discarding any
 * mountpoints that aren't in use and haven't been touched since last we came
 * here
 */
void mark_mounts_for_expiry(struct list_head *mounts)
{
        struct mount *mnt, *next;
        LIST_HEAD(graveyard);

        if (list_empty(mounts))
                return;

        namespace_lock();
        lock_mount_hash();

        /* extract from the expiration list every vfsmount that matches the
         * following criteria:
         * - only referenced by its parent vfsmount
         * - still marked for expiry (marked on the last call here; marks are
         *   cleared by mntput())
         */
        list_for_each_entry_safe(mnt, next, mounts, mnt_expire) {
                if (!xchg(&mnt->mnt_expiry_mark, 1) ||
                        propagate_mount_busy(mnt, 1))
                        continue;
                list_move(&mnt->mnt_expire, &graveyard);
        }
        while (!list_empty(&graveyard)) {
                mnt = list_first_entry(&graveyard, struct mount, mnt_expire);
                touch_mnt_namespace(mnt->mnt_ns);
                umount_tree(mnt, UMOUNT_PROPAGATE|UMOUNT_SYNC);
        }
        unlock_mount_hash();
        namespace_unlock();
}

EXPORT_SYMBOL_GPL(mark_mounts_for_expiry);

/*
 * Ripoff of 'select_parent()'
 *
 * search the list of submounts for a given mountpoint, and move any
 * shrinkable submounts to the 'graveyard' list.
 */
static int select_submounts(struct mount *parent, struct list_head *graveyard)
{
        struct mount *this_parent = parent;
        struct list_head *next;
        int found = 0;

repeat:
        next = this_parent->mnt_mounts.next;
resume:
        while (next != &this_parent->mnt_mounts) {
                struct list_head *tmp = next;
                struct mount *mnt = list_entry(tmp, struct mount, mnt_child);

                next = tmp->next;
                if (!(mnt->mnt.mnt_flags & MNT_SHRINKABLE))
                        continue;
                /*
                 * Descend a level if the d_mounts list is non-empty.
                 */
                if (!list_empty(&mnt->mnt_mounts)) {
                        this_parent = mnt;
                        goto repeat;
                }

                if (!propagate_mount_busy(mnt, 1)) {
                        list_move_tail(&mnt->mnt_expire, graveyard);
                        found++;
                }
        }
        /*
         * All done at this level ... ascend and resume the search
         */
        if (this_parent != parent) {
                next = this_parent->mnt_child.next;
                this_parent = this_parent->mnt_parent;
                goto resume;
        }
        return found;
}

/*
 * process a list of expirable mountpoints with the intent of discarding any
 * submounts of a specific parent mountpoint
 *
 * mount_lock must be held for write
 */
static void shrink_submounts(struct mount *mnt)
{
        LIST_HEAD(graveyard);
        struct mount *m;

        /* extract submounts of 'mountpoint' from the expiration list */
        while (select_submounts(mnt, &graveyard)) {
                while (!list_empty(&graveyard)) {
                        m = list_first_entry(&graveyard, struct mount,
                                                mnt_expire);
                        touch_mnt_namespace(m->mnt_ns);
                        umount_tree(m, UMOUNT_PROPAGATE|UMOUNT_SYNC);
                }
        }
}

static void *copy_mount_options(const void __user * data)
{
        char *copy;
        unsigned left, offset;

        if (!data)
                return NULL;

        copy = kmalloc(PAGE_SIZE, GFP_KERNEL);
        if (!copy)
                return ERR_PTR(-ENOMEM);

        left = copy_from_user(copy, data, PAGE_SIZE);

        /*
         * Not all architectures have an exact copy_from_user(). Resort to
         * byte at a time.
         */
        offset = PAGE_SIZE - left;
        while (left) {
                char c;
                if (get_user(c, (const char __user *)data + offset))
                        break;
                copy[offset] = c;
                left--;
                offset++;
        }

        if (left == PAGE_SIZE) {
                kfree(copy);
                return ERR_PTR(-EFAULT);
        }

        return copy;
}

static char *copy_mount_string(const void __user *data)
{
        return data ? strndup_user(data, PATH_MAX) : NULL;
}

/*
 * Flags is a 32-bit value that allows up to 31 non-fs dependent flags to
 * be given to the mount() call (ie: read-only, no-dev, no-suid etc).
 *
 * data is a (void *) that can point to any structure up to
 * PAGE_SIZE-1 bytes, which can contain arbitrary fs-dependent
 * information (or be NULL).
 *
 * Pre-0.97 versions of mount() didn't have a flags word.
 * When the flags word was introduced its top half was required
 * to have the magic value 0xC0ED, and this remained so until 2.4.0-test9.
 * Therefore, if this magic number is present, it carries no information
 * and must be discarded.
 */
int path_mount(const char *dev_name, struct path *path,
                const char *type_page, unsigned long flags, void *data_page)
{
        unsigned int mnt_flags = 0, sb_flags;
        int ret;

        /* Discard magic */
        if ((flags & MS_MGC_MSK) == MS_MGC_VAL)
                flags &= ~MS_MGC_MSK;

        /* Basic sanity checks */
        if (data_page)
                ((char *)data_page)[PAGE_SIZE - 1] = 0;

        if (flags & MS_NOUSER)
                return -EINVAL;

        ret = security_sb_mount(dev_name, path, type_page, flags, data_page);
        if (ret)
                return ret;
        if (!may_mount())
                return -EPERM;
        if (flags & SB_MANDLOCK)
                warn_mandlock();

        /* Default to relatime unless overriden */
        if (!(flags & MS_NOATIME))
                mnt_flags |= MNT_RELATIME;

        /* Separate the per-mountpoint flags */
        if (flags & MS_NOSUID)
                mnt_flags |= MNT_NOSUID;
        if (flags & MS_NODEV)
                mnt_flags |= MNT_NODEV;
        if (flags & MS_NOEXEC)
                mnt_flags |= MNT_NOEXEC;
        if (flags & MS_NOATIME)
                mnt_flags |= MNT_NOATIME;
        if (flags & MS_NODIRATIME)
                mnt_flags |= MNT_NODIRATIME;
        if (flags & MS_STRICTATIME)
                mnt_flags &= ~(MNT_RELATIME | MNT_NOATIME);
        if (flags & MS_RDONLY)
                mnt_flags |= MNT_READONLY;
        if (flags & MS_NOSYMFOLLOW)
                mnt_flags |= MNT_NOSYMFOLLOW;

        /* The default atime for remount is preservation */
        if ((flags & MS_REMOUNT) &&
            ((flags & (MS_NOATIME | MS_NODIRATIME | MS_RELATIME |
                       MS_STRICTATIME)) == 0)) {
                mnt_flags &= ~MNT_ATIME_MASK;
                mnt_flags |= path->mnt->mnt_flags & MNT_ATIME_MASK;
        }

        sb_flags = flags & (SB_RDONLY |
                            SB_SYNCHRONOUS |
                            SB_MANDLOCK |
                            SB_DIRSYNC |
                            SB_SILENT |
                            SB_POSIXACL |
                            SB_LAZYTIME |
                            SB_I_VERSION);

        if ((flags & (MS_REMOUNT | MS_BIND)) == (MS_REMOUNT | MS_BIND))
                return do_reconfigure_mnt(path, mnt_flags);
        if (flags & MS_REMOUNT)
                return do_remount(path, flags, sb_flags, mnt_flags, data_page);
        if (flags & MS_BIND)
                return do_loopback(path, dev_name, flags & MS_REC);
        if (flags & (MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
                return do_change_type(path, flags);
        if (flags & MS_MOVE)
                return do_move_mount_old(path, dev_name);

        return do_new_mount(path, type_page, sb_flags, mnt_flags, dev_name,
                            data_page);
}

long do_mount(const char *dev_name, const char __user *dir_name,
                const char *type_page, unsigned long flags, void *data_page)
{
        struct path path;
        int ret;

        ret = user_path_at(AT_FDCWD, dir_name, LOOKUP_FOLLOW, &path);
        if (ret)
                return ret;
        ret = path_mount(dev_name, &path, type_page, flags, data_page);
        path_put(&path);
        return ret;
}

static struct ucounts *inc_mnt_namespaces(struct user_namespace *ns)
{
        return inc_ucount(ns, current_euid(), UCOUNT_MNT_NAMESPACES);
}

static void dec_mnt_namespaces(struct ucounts *ucounts)
{
        dec_ucount(ucounts, UCOUNT_MNT_NAMESPACES);
}

static void free_mnt_ns(struct mnt_namespace *ns)
{
        if (!is_anon_ns(ns))
                ns_free_inum(&ns->ns);
        dec_mnt_namespaces(ns->ucounts);
        mnt_ns_tree_remove(ns);
}

/*
 * Assign a sequence number so we can detect when we attempt to bind
 * mount a reference to an older mount namespace into the current
 * mount namespace, preventing reference counting loops.  A 64bit
 * number incrementing at 10Ghz will take 12,427 years to wrap which
 * is effectively never, so we can ignore the possibility.
 */
static atomic64_t mnt_ns_seq = ATOMIC64_INIT(1);

static struct mnt_namespace *alloc_mnt_ns(struct user_namespace *user_ns, bool anon)
{
        struct mnt_namespace *new_ns;
        struct ucounts *ucounts;
        int ret;

        ucounts = inc_mnt_namespaces(user_ns);
        if (!ucounts)
                return ERR_PTR(-ENOSPC);

        new_ns = kzalloc(sizeof(struct mnt_namespace), GFP_KERNEL_ACCOUNT);
        if (!new_ns) {
                dec_mnt_namespaces(ucounts);
                return ERR_PTR(-ENOMEM);
        }
        if (!anon) {
                ret = ns_alloc_inum(&new_ns->ns);
                if (ret) {
                        kfree(new_ns);
                        dec_mnt_namespaces(ucounts);
                        return ERR_PTR(ret);
                }
        }
        new_ns->ns.ops = &mntns_operations;
        if (!anon)
                new_ns->seq = atomic64_add_return(1, &mnt_ns_seq);
        refcount_set(&new_ns->ns.count, 1);
        refcount_set(&new_ns->passive, 1);
        new_ns->mounts = RB_ROOT;
        RB_CLEAR_NODE(&new_ns->mnt_ns_tree_node);
        init_waitqueue_head(&new_ns->poll);
        new_ns->user_ns = get_user_ns(user_ns);
        new_ns->ucounts = ucounts;
        return new_ns;
}

__latent_entropy
struct mnt_namespace *copy_mnt_ns(unsigned long flags, struct mnt_namespace *ns,
                struct user_namespace *user_ns, struct fs_struct *new_fs)
{
        struct mnt_namespace *new_ns;
        struct vfsmount *rootmnt = NULL, *pwdmnt = NULL;
        struct mount *p, *q;
        struct mount *old;
        struct mount *new;
        int copy_flags;

        BUG_ON(!ns);

        if (likely(!(flags & CLONE_NEWNS))) {
                get_mnt_ns(ns);
                return ns;
        }

        old = ns->root;

        new_ns = alloc_mnt_ns(user_ns, false);
        if (IS_ERR(new_ns))
                return new_ns;

        namespace_lock();
        /* First pass: copy the tree topology */
        copy_flags = CL_COPY_UNBINDABLE | CL_EXPIRE;
        if (user_ns != ns->user_ns)
                copy_flags |= CL_SHARED_TO_SLAVE;
        new = copy_tree(old, old->mnt.mnt_root, copy_flags);
        if (IS_ERR(new)) {
                namespace_unlock();
                free_mnt_ns(new_ns);
                return ERR_CAST(new);
        }
        if (user_ns != ns->user_ns) {
                lock_mount_hash();
                lock_mnt_tree(new);
                unlock_mount_hash();
        }
        new_ns->root = new;

        /*
         * Second pass: switch the tsk->fs->* elements and mark new vfsmounts
         * as belonging to new namespace.  We have already acquired a private
         * fs_struct, so tsk->fs->lock is not needed.
         */
        p = old;
        q = new;
        while (p) {
                mnt_add_to_ns(new_ns, q);
                new_ns->nr_mounts++;
                if (new_fs) {
                        if (&p->mnt == new_fs->root.mnt) {
                                new_fs->root.mnt = mntget(&q->mnt);
                                rootmnt = &p->mnt;
                        }
                        if (&p->mnt == new_fs->pwd.mnt) {
                                new_fs->pwd.mnt = mntget(&q->mnt);
                                pwdmnt = &p->mnt;
                        }
                }
                p = next_mnt(p, old);
                q = next_mnt(q, new);
                if (!q)
                        break;
                // an mntns binding we'd skipped?
                while (p->mnt.mnt_root != q->mnt.mnt_root)
                        p = next_mnt(skip_mnt_tree(p), old);
        }
        mnt_ns_tree_add(new_ns);
        namespace_unlock();

        if (rootmnt)
                mntput(rootmnt);
        if (pwdmnt)
                mntput(pwdmnt);

        return new_ns;
}

struct dentry *mount_subtree(struct vfsmount *m, const char *name)
{
        struct mount *mnt = real_mount(m);
        struct mnt_namespace *ns;
        struct super_block *s;
        struct path path;
        int err;

        ns = alloc_mnt_ns(&init_user_ns, true);
        if (IS_ERR(ns)) {
                mntput(m);
                return ERR_CAST(ns);
        }
        ns->root = mnt;
        ns->nr_mounts++;
        mnt_add_to_ns(ns, mnt);

        err = vfs_path_lookup(m->mnt_root, m,
                        name, LOOKUP_FOLLOW|LOOKUP_AUTOMOUNT, &path);

        put_mnt_ns(ns);

        if (err)
                return ERR_PTR(err);

        /* trade a vfsmount reference for active sb one */
        s = path.mnt->mnt_sb;
        atomic_inc(&s->s_active);
        mntput(path.mnt);
        /* lock the sucker */
        down_write(&s->s_umount);
        /* ... and return the root of (sub)tree on it */
        return path.dentry;
}
EXPORT_SYMBOL(mount_subtree);

SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,
                char __user *, type, unsigned long, flags, void __user *, data)
{
        int ret;
        char *kernel_type;
        char *kernel_dev;
        void *options;

        kernel_type = copy_mount_string(type);
        ret = PTR_ERR(kernel_type);
        if (IS_ERR(kernel_type))
                goto out_type;

        kernel_dev = copy_mount_string(dev_name);
        ret = PTR_ERR(kernel_dev);
        if (IS_ERR(kernel_dev))
                goto out_dev;

        options = copy_mount_options(data);
        ret = PTR_ERR(options);
        if (IS_ERR(options))
                goto out_data;

        ret = do_mount(kernel_dev, dir_name, kernel_type, flags, options);

        kfree(options);
out_data:
        kfree(kernel_dev);
out_dev:
        kfree(kernel_type);
out_type:
        return ret;
}

#define FSMOUNT_VALID_FLAGS                                                    \
        (MOUNT_ATTR_RDONLY | MOUNT_ATTR_NOSUID | MOUNT_ATTR_NODEV |            \
         MOUNT_ATTR_NOEXEC | MOUNT_ATTR__ATIME | MOUNT_ATTR_NODIRATIME |       \
         MOUNT_ATTR_NOSYMFOLLOW)

#define MOUNT_SETATTR_VALID_FLAGS (FSMOUNT_VALID_FLAGS | MOUNT_ATTR_IDMAP)

#define MOUNT_SETATTR_PROPAGATION_FLAGS \
        (MS_UNBINDABLE | MS_PRIVATE | MS_SLAVE | MS_SHARED)

static unsigned int attr_flags_to_mnt_flags(u64 attr_flags)
{
        unsigned int mnt_flags = 0;

        if (attr_flags & MOUNT_ATTR_RDONLY)
                mnt_flags |= MNT_READONLY;
        if (attr_flags & MOUNT_ATTR_NOSUID)
                mnt_flags |= MNT_NOSUID;
        if (attr_flags & MOUNT_ATTR_NODEV)
                mnt_flags |= MNT_NODEV;
        if (attr_flags & MOUNT_ATTR_NOEXEC)
                mnt_flags |= MNT_NOEXEC;
        if (attr_flags & MOUNT_ATTR_NODIRATIME)
                mnt_flags |= MNT_NODIRATIME;
        if (attr_flags & MOUNT_ATTR_NOSYMFOLLOW)
                mnt_flags |= MNT_NOSYMFOLLOW;

        return mnt_flags;
}

/*
 * Create a kernel mount representation for a new, prepared superblock
 * (specified by fs_fd) and attach to an open_tree-like file descriptor.
 */
SYSCALL_DEFINE3(fsmount, int, fs_fd, unsigned int, flags,
                unsigned int, attr_flags)
{
        struct mnt_namespace *ns;
        struct fs_context *fc;
        struct file *file;
        struct path newmount;
        struct mount *mnt;
        struct fd f;
        unsigned int mnt_flags = 0;
        long ret;

        if (!may_mount())
                return -EPERM;

        if ((flags & ~(FSMOUNT_CLOEXEC)) != 0)
                return -EINVAL;

        if (attr_flags & ~FSMOUNT_VALID_FLAGS)
                return -EINVAL;

        mnt_flags = attr_flags_to_mnt_flags(attr_flags);

        switch (attr_flags & MOUNT_ATTR__ATIME) {
        case MOUNT_ATTR_STRICTATIME:
                break;
        case MOUNT_ATTR_NOATIME:
                mnt_flags |= MNT_NOATIME;
                break;
        case MOUNT_ATTR_RELATIME:
                mnt_flags |= MNT_RELATIME;
                break;
        default:
                return -EINVAL;
        }

        f = fdget(fs_fd);
        if (!f.file)
                return -EBADF;

        ret = -EINVAL;
        if (f.file->f_op != &fscontext_fops)
                goto err_fsfd;

        fc = f.file->private_data;

        ret = mutex_lock_interruptible(&fc->uapi_mutex);
        if (ret < 0)
                goto err_fsfd;

        /* There must be a valid superblock or we can't mount it */
        ret = -EINVAL;
        if (!fc->root)
                goto err_unlock;

        ret = -EPERM;
        if (mount_too_revealing(fc->root->d_sb, &mnt_flags)) {
                pr_warn("VFS: Mount too revealing\n");
                goto err_unlock;
        }

        ret = -EBUSY;
        if (fc->phase != FS_CONTEXT_AWAITING_MOUNT)
                goto err_unlock;

        if (fc->sb_flags & SB_MANDLOCK)
                warn_mandlock();

        newmount.mnt = vfs_create_mount(fc);
        if (IS_ERR(newmount.mnt)) {
                ret = PTR_ERR(newmount.mnt);
                goto err_unlock;
        }
        newmount.dentry = dget(fc->root);
        newmount.mnt->mnt_flags = mnt_flags;

        /* We've done the mount bit - now move the file context into more or
         * less the same state as if we'd done an fspick().  We don't want to
         * do any memory allocation or anything like that at this point as we
         * don't want to have to handle any errors incurred.
         */
        vfs_clean_context(fc);

        ns = alloc_mnt_ns(current->nsproxy->mnt_ns->user_ns, true);
        if (IS_ERR(ns)) {
                ret = PTR_ERR(ns);
                goto err_path;
        }
        mnt = real_mount(newmount.mnt);
        ns->root = mnt;
        ns->nr_mounts = 1;
        mnt_add_to_ns(ns, mnt);
        mntget(newmount.mnt);

        /* Attach to an apparent O_PATH fd with a note that we need to unmount
         * it, not just simply put it.
         */
        file = dentry_open(&newmount, O_PATH, fc->cred);
        if (IS_ERR(file)) {
                dissolve_on_fput(newmount.mnt);
                ret = PTR_ERR(file);
                goto err_path;
        }
        file->f_mode |= FMODE_NEED_UNMOUNT;

        ret = get_unused_fd_flags((flags & FSMOUNT_CLOEXEC) ? O_CLOEXEC : 0);
        if (ret >= 0)
                fd_install(ret, file);
        else
                fput(file);

err_path:
        path_put(&newmount);
err_unlock:
        mutex_unlock(&fc->uapi_mutex);
err_fsfd:
        fdput(f);
        return ret;
}

/*
 * Move a mount from one place to another.  In combination with
 * fsopen()/fsmount() this is used to install a new mount and in combination
 * with open_tree(OPEN_TREE_CLONE [| AT_RECURSIVE]) it can be used to copy
 * a mount subtree.
 *
 * Note the flags value is a combination of MOVE_MOUNT_* flags.
 */
SYSCALL_DEFINE5(move_mount,
                int, from_dfd, const char __user *, from_pathname,
                int, to_dfd, const char __user *, to_pathname,
                unsigned int, flags)
{
        struct path from_path, to_path;
        unsigned int lflags;
        int ret = 0;

        if (!may_mount())
                return -EPERM;

        if (flags & ~MOVE_MOUNT__MASK)
                return -EINVAL;

        if ((flags & (MOVE_MOUNT_BENEATH | MOVE_MOUNT_SET_GROUP)) ==
            (MOVE_MOUNT_BENEATH | MOVE_MOUNT_SET_GROUP))
                return -EINVAL;

        /* If someone gives a pathname, they aren't permitted to move
         * from an fd that requires unmount as we can't get at the flag
         * to clear it afterwards.
         */
        lflags = 0;
        if (flags & MOVE_MOUNT_F_SYMLINKS)      lflags |= LOOKUP_FOLLOW;
        if (flags & MOVE_MOUNT_F_AUTOMOUNTS)    lflags |= LOOKUP_AUTOMOUNT;
        if (flags & MOVE_MOUNT_F_EMPTY_PATH)    lflags |= LOOKUP_EMPTY;

        ret = user_path_at(from_dfd, from_pathname, lflags, &from_path);
        if (ret < 0)
                return ret;

        lflags = 0;
        if (flags & MOVE_MOUNT_T_SYMLINKS)      lflags |= LOOKUP_FOLLOW;
        if (flags & MOVE_MOUNT_T_AUTOMOUNTS)    lflags |= LOOKUP_AUTOMOUNT;
        if (flags & MOVE_MOUNT_T_EMPTY_PATH)    lflags |= LOOKUP_EMPTY;

        ret = user_path_at(to_dfd, to_pathname, lflags, &to_path);
        if (ret < 0)
                goto out_from;

        ret = security_move_mount(&from_path, &to_path);
        if (ret < 0)
                goto out_to;

        if (flags & MOVE_MOUNT_SET_GROUP)
                ret = do_set_group(&from_path, &to_path);
        else
                ret = do_move_mount(&from_path, &to_path,
                                    (flags & MOVE_MOUNT_BENEATH));

out_to:
        path_put(&to_path);
out_from:
        path_put(&from_path);
        return ret;
}

/*
 * Return true if path is reachable from root
 *
 * namespace_sem or mount_lock is held
 */
bool is_path_reachable(struct mount *mnt, struct dentry *dentry,
                         const struct path *root)
{
        while (&mnt->mnt != root->mnt && mnt_has_parent(mnt)) {
                dentry = mnt->mnt_mountpoint;
                mnt = mnt->mnt_parent;
        }
        return &mnt->mnt == root->mnt && is_subdir(dentry, root->dentry);
}

bool path_is_under(const struct path *path1, const struct path *path2)
{
        bool res;
        read_seqlock_excl(&mount_lock);
        res = is_path_reachable(real_mount(path1->mnt), path1->dentry, path2);
        read_sequnlock_excl(&mount_lock);
        return res;
}
EXPORT_SYMBOL(path_is_under);

/*
 * pivot_root Semantics:
 * Moves the root file system of the current process to the directory put_old,
 * makes new_root as the new root file system of the current process, and sets
 * root/cwd of all processes which had them on the current root to new_root.
 *
 * Restrictions:
 * The new_root and put_old must be directories, and  must not be on the
 * same file  system as the current process root. The put_old  must  be
 * underneath new_root,  i.e. adding a non-zero number of /.. to the string
 * pointed to by put_old must yield the same directory as new_root. No other
 * file system may be mounted on put_old. After all, new_root is a mountpoint.
 *
 * Also, the current root cannot be on the 'rootfs' (initial ramfs) filesystem.
 * See Documentation/filesystems/ramfs-rootfs-initramfs.rst for alternatives
 * in this situation.
 *
 * Notes:
 *  - we don't move root/cwd if they are not at the root (reason: if something
 *    cared enough to change them, it's probably wrong to force them elsewhere)
 *  - it's okay to pick a root that isn't the root of a file system, e.g.
 *    /nfs/my_root where /nfs is the mount point. It must be a mountpoint,
 *    though, so you may need to say mount --bind /nfs/my_root /nfs/my_root
 *    first.
 */
SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
                const char __user *, put_old)
{
        struct path new, old, root;
        struct mount *new_mnt, *root_mnt, *old_mnt, *root_parent, *ex_parent;
        struct mountpoint *old_mp, *root_mp;
        int error;

        if (!may_mount())
                return -EPERM;

        error = user_path_at(AT_FDCWD, new_root,
                             LOOKUP_FOLLOW | LOOKUP_DIRECTORY, &new);
        if (error)
                goto out0;

        error = user_path_at(AT_FDCWD, put_old,
                             LOOKUP_FOLLOW | LOOKUP_DIRECTORY, &old);
        if (error)
                goto out1;

        error = security_sb_pivotroot(&old, &new);
        if (error)
                goto out2;

        get_fs_root(current->fs, &root);
        old_mp = lock_mount(&old);
        error = PTR_ERR(old_mp);
        if (IS_ERR(old_mp))
                goto out3;

        error = -EINVAL;
        new_mnt = real_mount(new.mnt);
        root_mnt = real_mount(root.mnt);
        old_mnt = real_mount(old.mnt);
        ex_parent = new_mnt->mnt_parent;
        root_parent = root_mnt->mnt_parent;
        if (IS_MNT_SHARED(old_mnt) ||
                IS_MNT_SHARED(ex_parent) ||
                IS_MNT_SHARED(root_parent))
                goto out4;
        if (!check_mnt(root_mnt) || !check_mnt(new_mnt))
                goto out4;
        if (new_mnt->mnt.mnt_flags & MNT_LOCKED)
                goto out4;
        error = -ENOENT;
        if (d_unlinked(new.dentry))
                goto out4;
        error = -EBUSY;
        if (new_mnt == root_mnt || old_mnt == root_mnt)
                goto out4; /* loop, on the same file system  */
        error = -EINVAL;
        if (!path_mounted(&root))
                goto out4; /* not a mountpoint */
        if (!mnt_has_parent(root_mnt))
                goto out4; /* not attached */
        if (!path_mounted(&new))
                goto out4; /* not a mountpoint */
        if (!mnt_has_parent(new_mnt))
                goto out4; /* not attached */
        /* make sure we can reach put_old from new_root */
        if (!is_path_reachable(old_mnt, old.dentry, &new))
                goto out4;
        /* make certain new is below the root */
        if (!is_path_reachable(new_mnt, new.dentry, &root))
                goto out4;
        lock_mount_hash();
        umount_mnt(new_mnt);
        root_mp = unhash_mnt(root_mnt);  /* we'll need its mountpoint */
        if (root_mnt->mnt.mnt_flags & MNT_LOCKED) {
                new_mnt->mnt.mnt_flags |= MNT_LOCKED;
                root_mnt->mnt.mnt_flags &= ~MNT_LOCKED;
        }
        /* mount old root on put_old */
        attach_mnt(root_mnt, old_mnt, old_mp, false);
        /* mount new_root on / */
        attach_mnt(new_mnt, root_parent, root_mp, false);
        mnt_add_count(root_parent, -1);
        touch_mnt_namespace(current->nsproxy->mnt_ns);
        /* A moved mount should not expire automatically */
        list_del_init(&new_mnt->mnt_expire);
        put_mountpoint(root_mp);
        unlock_mount_hash();
        chroot_fs_refs(&root, &new);
        error = 0;
out4:
        unlock_mount(old_mp);
        if (!error)
                mntput_no_expire(ex_parent);
out3:
        path_put(&root);
out2:
        path_put(&old);
out1:
        path_put(&new);
out0:
        return error;
}

static unsigned int recalc_flags(struct mount_kattr *kattr, struct mount *mnt)
{
        unsigned int flags = mnt->mnt.mnt_flags;

        /*  flags to clear */
        flags &= ~kattr->attr_clr;
        /* flags to raise */
        flags |= kattr->attr_set;

        return flags;
}

static int can_idmap_mount(const struct mount_kattr *kattr, struct mount *mnt)
{
        struct vfsmount *m = &mnt->mnt;
        struct user_namespace *fs_userns = m->mnt_sb->s_user_ns;

        if (!kattr->mnt_idmap)
                return 0;

        /*
         * Creating an idmapped mount with the filesystem wide idmapping
         * doesn't make sense so block that. We don't allow mushy semantics.
         */
        if (kattr->mnt_userns == m->mnt_sb->s_user_ns)
                return -EINVAL;

        /*
         * Once a mount has been idmapped we don't allow it to change its
         * mapping. It makes things simpler and callers can just create
         * another bind-mount they can idmap if they want to.
         */
        if (is_idmapped_mnt(m))
                return -EPERM;

        /* The underlying filesystem doesn't support idmapped mounts yet. */
        if (!(m->mnt_sb->s_type->fs_flags & FS_ALLOW_IDMAP))
                return -EINVAL;

        /* We're not controlling the superblock. */
        if (!ns_capable(fs_userns, CAP_SYS_ADMIN))
                return -EPERM;

        /* Mount has already been visible in the filesystem hierarchy. */
        if (!is_anon_ns(mnt->mnt_ns))
                return -EINVAL;

        return 0;
}

/**
 * mnt_allow_writers() - check whether the attribute change allows writers
 * @kattr: the new mount attributes
 * @mnt: the mount to which @kattr will be applied
 *
 * Check whether thew new mount attributes in @kattr allow concurrent writers.
 *
 * Return: true if writers need to be held, false if not
 */
static inline bool mnt_allow_writers(const struct mount_kattr *kattr,
                                     const struct mount *mnt)
{
        return (!(kattr->attr_set & MNT_READONLY) ||
                (mnt->mnt.mnt_flags & MNT_READONLY)) &&
               !kattr->mnt_idmap;
}

static int mount_setattr_prepare(struct mount_kattr *kattr, struct mount *mnt)
{
        struct mount *m;
        int err;

        for (m = mnt; m; m = next_mnt(m, mnt)) {
                if (!can_change_locked_flags(m, recalc_flags(kattr, m))) {
                        err = -EPERM;
                        break;
                }

                err = can_idmap_mount(kattr, m);
                if (err)
                        break;

                if (!mnt_allow_writers(kattr, m)) {
                        err = mnt_hold_writers(m);
                        if (err)
                                break;
                }

                if (!kattr->recurse)
                        return 0;
        }

        if (err) {
                struct mount *p;

                /*
                 * If we had to call mnt_hold_writers() MNT_WRITE_HOLD will
                 * be set in @mnt_flags. The loop unsets MNT_WRITE_HOLD for all
                 * mounts and needs to take care to include the first mount.
                 */
                for (p = mnt; p; p = next_mnt(p, mnt)) {
                        /* If we had to hold writers unblock them. */
                        if (p->mnt.mnt_flags & MNT_WRITE_HOLD)
                                mnt_unhold_writers(p);

                        /*
                         * We're done once the first mount we changed got
                         * MNT_WRITE_HOLD unset.
                         */
                        if (p == m)
                                break;
                }
        }
        return err;
}

static void do_idmap_mount(const struct mount_kattr *kattr, struct mount *mnt)
{
        if (!kattr->mnt_idmap)
                return;

        /*
         * Pairs with smp_load_acquire() in mnt_idmap().
         *
         * Since we only allow a mount to change the idmapping once and
         * verified this in can_idmap_mount() we know that the mount has
         * @nop_mnt_idmap attached to it. So there's no need to drop any
         * references.
         */
        smp_store_release(&mnt->mnt.mnt_idmap, mnt_idmap_get(kattr->mnt_idmap));
}

static void mount_setattr_commit(struct mount_kattr *kattr, struct mount *mnt)
{
        struct mount *m;

        for (m = mnt; m; m = next_mnt(m, mnt)) {
                unsigned int flags;

                do_idmap_mount(kattr, m);
                flags = recalc_flags(kattr, m);
                WRITE_ONCE(m->mnt.mnt_flags, flags);

                /* If we had to hold writers unblock them. */
                if (m->mnt.mnt_flags & MNT_WRITE_HOLD)
                        mnt_unhold_writers(m);

                if (kattr->propagation)
                        change_mnt_propagation(m, kattr->propagation);
                if (!kattr->recurse)
                        break;
        }
        touch_mnt_namespace(mnt->mnt_ns);
}

static int do_mount_setattr(struct path *path, struct mount_kattr *kattr)
{
        struct mount *mnt = real_mount(path->mnt);
        int err = 0;

        if (!path_mounted(path))
                return -EINVAL;

        if (kattr->mnt_userns) {
                struct mnt_idmap *mnt_idmap;

                mnt_idmap = alloc_mnt_idmap(kattr->mnt_userns);
                if (IS_ERR(mnt_idmap))
                        return PTR_ERR(mnt_idmap);
                kattr->mnt_idmap = mnt_idmap;
        }

        if (kattr->propagation) {
                /*
                 * Only take namespace_lock() if we're actually changing
                 * propagation.
                 */
                namespace_lock();
                if (kattr->propagation == MS_SHARED) {
                        err = invent_group_ids(mnt, kattr->recurse);
                        if (err) {
                                namespace_unlock();
                                return err;
                        }
                }
        }

        err = -EINVAL;
        lock_mount_hash();

        /* Ensure that this isn't anything purely vfs internal. */
        if (!is_mounted(&mnt->mnt))
                goto out;

        /*
         * If this is an attached mount make sure it's located in the callers
         * mount namespace. If it's not don't let the caller interact with it.
         *
         * If this mount doesn't have a parent it's most often simply a
         * detached mount with an anonymous mount namespace. IOW, something
         * that's simply not attached yet. But there are apparently also users
         * that do change mount properties on the rootfs itself. That obviously
         * neither has a parent nor is it a detached mount so we cannot
         * unconditionally check for detached mounts.
         */
        if ((mnt_has_parent(mnt) || !is_anon_ns(mnt->mnt_ns)) && !check_mnt(mnt))
                goto out;

        /*
         * First, we get the mount tree in a shape where we can change mount
         * properties without failure. If we succeeded to do so we commit all
         * changes and if we failed we clean up.
         */
        err = mount_setattr_prepare(kattr, mnt);
        if (!err)
                mount_setattr_commit(kattr, mnt);

out:
        unlock_mount_hash();

        if (kattr->propagation) {
                if (err)
                        cleanup_group_ids(mnt, NULL);
                namespace_unlock();
        }

        return err;
}

static int build_mount_idmapped(const struct mount_attr *attr, size_t usize,
                                struct mount_kattr *kattr, unsigned int flags)
{
        int err = 0;
        struct ns_common *ns;
        struct user_namespace *mnt_userns;
        struct fd f;

        if (!((attr->attr_set | attr->attr_clr) & MOUNT_ATTR_IDMAP))
                return 0;

        /*
         * We currently do not support clearing an idmapped mount. If this ever
         * is a use-case we can revisit this but for now let's keep it simple
         * and not allow it.
         */
        if (attr->attr_clr & MOUNT_ATTR_IDMAP)
                return -EINVAL;

        if (attr->userns_fd > INT_MAX)
                return -EINVAL;

        f = fdget(attr->userns_fd);
        if (!f.file)
                return -EBADF;

        if (!proc_ns_file(f.file)) {
                err = -EINVAL;
                goto out_fput;
        }

        ns = get_proc_ns(file_inode(f.file));
        if (ns->ops->type != CLONE_NEWUSER) {
                err = -EINVAL;
                goto out_fput;
        }

        /*
         * The initial idmapping cannot be used to create an idmapped
         * mount. We use the initial idmapping as an indicator of a mount
         * that is not idmapped. It can simply be passed into helpers that
         * are aware of idmapped mounts as a convenient shortcut. A user
         * can just create a dedicated identity mapping to achieve the same
         * result.
         */
        mnt_userns = container_of(ns, struct user_namespace, ns);
        if (mnt_userns == &init_user_ns) {
                err = -EPERM;
                goto out_fput;
        }

        /* We're not controlling the target namespace. */
        if (!ns_capable(mnt_userns, CAP_SYS_ADMIN)) {
                err = -EPERM;
                goto out_fput;
        }

        kattr->mnt_userns = get_user_ns(mnt_userns);

out_fput:
        fdput(f);
        return err;
}

static int build_mount_kattr(const struct mount_attr *attr, size_t usize,
                             struct mount_kattr *kattr, unsigned int flags)
{
        unsigned int lookup_flags = LOOKUP_AUTOMOUNT | LOOKUP_FOLLOW;

        if (flags & AT_NO_AUTOMOUNT)
                lookup_flags &= ~LOOKUP_AUTOMOUNT;
        if (flags & AT_SYMLINK_NOFOLLOW)
                lookup_flags &= ~LOOKUP_FOLLOW;
        if (flags & AT_EMPTY_PATH)
                lookup_flags |= LOOKUP_EMPTY;

        *kattr = (struct mount_kattr) {
                .lookup_flags   = lookup_flags,
                .recurse        = !!(flags & AT_RECURSIVE),
        };

        if (attr->propagation & ~MOUNT_SETATTR_PROPAGATION_FLAGS)
                return -EINVAL;
        if (hweight32(attr->propagation & MOUNT_SETATTR_PROPAGATION_FLAGS) > 1)
                return -EINVAL;
        kattr->propagation = attr->propagation;

        if ((attr->attr_set | attr->attr_clr) & ~MOUNT_SETATTR_VALID_FLAGS)
                return -EINVAL;

        kattr->attr_set = attr_flags_to_mnt_flags(attr->attr_set);
        kattr->attr_clr = attr_flags_to_mnt_flags(attr->attr_clr);

        /*
         * Since the MOUNT_ATTR_<atime> values are an enum, not a bitmap,
         * users wanting to transition to a different atime setting cannot
         * simply specify the atime setting in @attr_set, but must also
         * specify MOUNT_ATTR__ATIME in the @attr_clr field.
         * So ensure that MOUNT_ATTR__ATIME can't be partially set in
         * @attr_clr and that @attr_set can't have any atime bits set if
         * MOUNT_ATTR__ATIME isn't set in @attr_clr.
         */
        if (attr->attr_clr & MOUNT_ATTR__ATIME) {
                if ((attr->attr_clr & MOUNT_ATTR__ATIME) != MOUNT_ATTR__ATIME)
                        return -EINVAL;

                /*
                 * Clear all previous time settings as they are mutually
                 * exclusive.
                 */
                kattr->attr_clr |= MNT_RELATIME | MNT_NOATIME;
                switch (attr->attr_set & MOUNT_ATTR__ATIME) {
                case MOUNT_ATTR_RELATIME:
                        kattr->attr_set |= MNT_RELATIME;
                        break;
                case MOUNT_ATTR_NOATIME:
                        kattr->attr_set |= MNT_NOATIME;
                        break;
                case MOUNT_ATTR_STRICTATIME:
                        break;
                default:
                        return -EINVAL;
                }
        } else {
                if (attr->attr_set & MOUNT_ATTR__ATIME)
                        return -EINVAL;
        }

        return build_mount_idmapped(attr, usize, kattr, flags);
}

static void finish_mount_kattr(struct mount_kattr *kattr)
{
        put_user_ns(kattr->mnt_userns);
        kattr->mnt_userns = NULL;

        if (kattr->mnt_idmap)
                mnt_idmap_put(kattr->mnt_idmap);
}

SYSCALL_DEFINE5(mount_setattr, int, dfd, const char __user *, path,
                unsigned int, flags, struct mount_attr __user *, uattr,
                size_t, usize)
{
        int err;
        struct path target;
        struct mount_attr attr;
        struct mount_kattr kattr;

        BUILD_BUG_ON(sizeof(struct mount_attr) != MOUNT_ATTR_SIZE_VER0);

        if (flags & ~(AT_EMPTY_PATH |
                      AT_RECURSIVE |
                      AT_SYMLINK_NOFOLLOW |
                      AT_NO_AUTOMOUNT))
                return -EINVAL;

        if (unlikely(usize > PAGE_SIZE))
                return -E2BIG;
        if (unlikely(usize < MOUNT_ATTR_SIZE_VER0))
                return -EINVAL;

        if (!may_mount())
                return -EPERM;

        err = copy_struct_from_user(&attr, sizeof(attr), uattr, usize);
        if (err)
                return err;

        /* Don't bother walking through the mounts if this is a nop. */
        if (attr.attr_set == 0 &&
            attr.attr_clr == 0 &&
            attr.propagation == 0)
                return 0;

        err = build_mount_kattr(&attr, usize, &kattr, flags);
        if (err)
                return err;

        err = user_path_at(dfd, path, kattr.lookup_flags, &target);
        if (!err) {
                err = do_mount_setattr(&target, &kattr);
                path_put(&target);
        }
        finish_mount_kattr(&kattr);
        return err;
}

int show_path(struct seq_file *m, struct dentry *root)
{
        if (root->d_sb->s_op->show_path)
                return root->d_sb->s_op->show_path(m, root);

        seq_dentry(m, root, " \t\n\\");
        return 0;
}

static struct vfsmount *lookup_mnt_in_ns(u64 id, struct mnt_namespace *ns)
{
        struct mount *mnt = mnt_find_id_at(ns, id);

        if (!mnt || mnt->mnt_id_unique != id)
                return NULL;

        return &mnt->mnt;
}

struct kstatmount {
        struct statmount __user *buf;
        size_t bufsize;
        struct vfsmount *mnt;
        u64 mask;
        struct path root;
        struct statmount sm;
        struct seq_file seq;
};

static u64 mnt_to_attr_flags(struct vfsmount *mnt)
{
        unsigned int mnt_flags = READ_ONCE(mnt->mnt_flags);
        u64 attr_flags = 0;

        if (mnt_flags & MNT_READONLY)
                attr_flags |= MOUNT_ATTR_RDONLY;
        if (mnt_flags & MNT_NOSUID)
                attr_flags |= MOUNT_ATTR_NOSUID;
        if (mnt_flags & MNT_NODEV)
                attr_flags |= MOUNT_ATTR_NODEV;
        if (mnt_flags & MNT_NOEXEC)
                attr_flags |= MOUNT_ATTR_NOEXEC;
        if (mnt_flags & MNT_NODIRATIME)
                attr_flags |= MOUNT_ATTR_NODIRATIME;
        if (mnt_flags & MNT_NOSYMFOLLOW)
                attr_flags |= MOUNT_ATTR_NOSYMFOLLOW;

        if (mnt_flags & MNT_NOATIME)
                attr_flags |= MOUNT_ATTR_NOATIME;
        else if (mnt_flags & MNT_RELATIME)
                attr_flags |= MOUNT_ATTR_RELATIME;
        else
                attr_flags |= MOUNT_ATTR_STRICTATIME;

        if (is_idmapped_mnt(mnt))
                attr_flags |= MOUNT_ATTR_IDMAP;

        return attr_flags;
}

static u64 mnt_to_propagation_flags(struct mount *m)
{
        u64 propagation = 0;

        if (IS_MNT_SHARED(m))
                propagation |= MS_SHARED;
        if (IS_MNT_SLAVE(m))
                propagation |= MS_SLAVE;
        if (IS_MNT_UNBINDABLE(m))
                propagation |= MS_UNBINDABLE;
        if (!propagation)
                propagation |= MS_PRIVATE;

        return propagation;
}

static void statmount_sb_basic(struct kstatmount *s)
{
        struct super_block *sb = s->mnt->mnt_sb;

        s->sm.mask |= STATMOUNT_SB_BASIC;
        s->sm.sb_dev_major = MAJOR(sb->s_dev);
        s->sm.sb_dev_minor = MINOR(sb->s_dev);
        s->sm.sb_magic = sb->s_magic;
        s->sm.sb_flags = sb->s_flags & (SB_RDONLY|SB_SYNCHRONOUS|SB_DIRSYNC|SB_LAZYTIME);
}

static void statmount_mnt_basic(struct kstatmount *s)
{
        struct mount *m = real_mount(s->mnt);

        s->sm.mask |= STATMOUNT_MNT_BASIC;
        s->sm.mnt_id = m->mnt_id_unique;
        s->sm.mnt_parent_id = m->mnt_parent->mnt_id_unique;
        s->sm.mnt_id_old = m->mnt_id;
        s->sm.mnt_parent_id_old = m->mnt_parent->mnt_id;
        s->sm.mnt_attr = mnt_to_attr_flags(&m->mnt);
        s->sm.mnt_propagation = mnt_to_propagation_flags(m);
        s->sm.mnt_peer_group = IS_MNT_SHARED(m) ? m->mnt_group_id : 0;
        s->sm.mnt_master = IS_MNT_SLAVE(m) ? m->mnt_master->mnt_group_id : 0;
}

static void statmount_propagate_from(struct kstatmount *s)
{
        struct mount *m = real_mount(s->mnt);

        s->sm.mask |= STATMOUNT_PROPAGATE_FROM;
        if (IS_MNT_SLAVE(m))
                s->sm.propagate_from = get_dominating_id(m, &current->fs->root);
}

static int statmount_mnt_root(struct kstatmount *s, struct seq_file *seq)
{
        int ret;
        size_t start = seq->count;

        ret = show_path(seq, s->mnt->mnt_root);
        if (ret)
                return ret;

        if (unlikely(seq_has_overflowed(seq)))
                return -EAGAIN;

        /*
         * Unescape the result. It would be better if supplied string was not
         * escaped in the first place, but that's a pretty invasive change.
         */
        seq->buf[seq->count] = '\0';
        seq->count = start;
        seq_commit(seq, string_unescape_inplace(seq->buf + start, UNESCAPE_OCTAL));
        return 0;
}

static int statmount_mnt_point(struct kstatmount *s, struct seq_file *seq)
{
        struct vfsmount *mnt = s->mnt;
        struct path mnt_path = { .dentry = mnt->mnt_root, .mnt = mnt };
        int err;

        err = seq_path_root(seq, &mnt_path, &s->root, "");
        return err == SEQ_SKIP ? 0 : err;
}

static int statmount_fs_type(struct kstatmount *s, struct seq_file *seq)
{
        struct super_block *sb = s->mnt->mnt_sb;

        seq_puts(seq, sb->s_type->name);
        return 0;
}

static void statmount_mnt_ns_id(struct kstatmount *s)
{
        struct mnt_namespace *ns = current->nsproxy->mnt_ns;

        s->sm.mask |= STATMOUNT_MNT_NS_ID;
        s->sm.mnt_ns_id = ns->seq;
}

static int statmount_string(struct kstatmount *s, u64 flag)
{
        int ret;
        size_t kbufsize;
        struct seq_file *seq = &s->seq;
        struct statmount *sm = &s->sm;

        switch (flag) {
        case STATMOUNT_FS_TYPE:
                sm->fs_type = seq->count;
                ret = statmount_fs_type(s, seq);
                break;
        case STATMOUNT_MNT_ROOT:
                sm->mnt_root = seq->count;
                ret = statmount_mnt_root(s, seq);
                break;
        case STATMOUNT_MNT_POINT:
                sm->mnt_point = seq->count;
                ret = statmount_mnt_point(s, seq);
                break;
        default:
                WARN_ON_ONCE(true);
                return -EINVAL;
        }

        if (unlikely(check_add_overflow(sizeof(*sm), seq->count, &kbufsize)))
                return -EOVERFLOW;
        if (kbufsize >= s->bufsize)
                return -EOVERFLOW;

        /* signal a retry */
        if (unlikely(seq_has_overflowed(seq)))
                return -EAGAIN;

        if (ret)
                return ret;

        seq->buf[seq->count++] = '\0';
        sm->mask |= flag;
        return 0;
}

static int copy_statmount_to_user(struct kstatmount *s)
{
        struct statmount *sm = &s->sm;
        struct seq_file *seq = &s->seq;
        char __user *str = ((char __user *)s->buf) + sizeof(*sm);
        size_t copysize = min_t(size_t, s->bufsize, sizeof(*sm));

        if (seq->count && copy_to_user(str, seq->buf, seq->count))
                return -EFAULT;

        /* Return the number of bytes copied to the buffer */
        sm->size = copysize + seq->count;
        if (copy_to_user(s->buf, sm, copysize))
                return -EFAULT;

        return 0;
}

static int do_statmount(struct kstatmount *s)
{
        struct mount *m = real_mount(s->mnt);
        struct mnt_namespace *ns = m->mnt_ns;
        int err;

        /*
         * Don't trigger audit denials. We just want to determine what
         * mounts to show users.
         */
        if (!is_path_reachable(m, m->mnt.mnt_root, &s->root) &&
            !ns_capable_noaudit(ns->user_ns, CAP_SYS_ADMIN))
                return -EPERM;

        err = security_sb_statfs(s->mnt->mnt_root);
        if (err)
                return err;

        if (s->mask & STATMOUNT_SB_BASIC)
                statmount_sb_basic(s);

        if (s->mask & STATMOUNT_MNT_BASIC)
                statmount_mnt_basic(s);

        if (s->mask & STATMOUNT_PROPAGATE_FROM)
                statmount_propagate_from(s);

        if (s->mask & STATMOUNT_FS_TYPE)
                err = statmount_string(s, STATMOUNT_FS_TYPE);

        if (!err && s->mask & STATMOUNT_MNT_ROOT)
                err = statmount_string(s, STATMOUNT_MNT_ROOT);

        if (!err && s->mask & STATMOUNT_MNT_POINT)
                err = statmount_string(s, STATMOUNT_MNT_POINT);

        if (!err && s->mask & STATMOUNT_MNT_NS_ID)
                statmount_mnt_ns_id(s);

        if (err)
                return err;

        return 0;
}

static inline bool retry_statmount(const long ret, size_t *seq_size)
{
        if (likely(ret != -EAGAIN))
                return false;
        if (unlikely(check_mul_overflow(*seq_size, 2, seq_size)))
                return false;
        if (unlikely(*seq_size > MAX_RW_COUNT))
                return false;
        return true;
}

static int prepare_kstatmount(struct kstatmount *ks, struct mnt_id_req *kreq,
                              struct statmount __user *buf, size_t bufsize,
                              size_t seq_size)
{
        if (!access_ok(buf, bufsize))
                return -EFAULT;

        memset(ks, 0, sizeof(*ks));
        ks->mask = kreq->param;
        ks->buf = buf;
        ks->bufsize = bufsize;
        ks->seq.size = seq_size;
        ks->seq.buf = kvmalloc(seq_size, GFP_KERNEL_ACCOUNT);
        if (!ks->seq.buf)
                return -ENOMEM;
        return 0;
}

static int copy_mnt_id_req(const struct mnt_id_req __user *req,
                           struct mnt_id_req *kreq)
{
        int ret;
        size_t usize;

        BUILD_BUG_ON(sizeof(struct mnt_id_req) != MNT_ID_REQ_SIZE_VER1);

        ret = get_user(usize, &req->size);
        if (ret)
                return -EFAULT;
        if (unlikely(usize > PAGE_SIZE))
                return -E2BIG;
        if (unlikely(usize < MNT_ID_REQ_SIZE_VER0))
                return -EINVAL;
        memset(kreq, 0, sizeof(*kreq));
        ret = copy_struct_from_user(kreq, sizeof(*kreq), req, usize);
        if (ret)
                return ret;
        if (kreq->spare != 0)
                return -EINVAL;
        return 0;
}

static struct mount *listmnt_next(struct mount *curr, bool reverse)
{
        struct rb_node *node;

        if (reverse)
                node = rb_prev(&curr->mnt_node);
        else
                node = rb_next(&curr->mnt_node);

        return node_to_mount(node);
}

static int grab_requested_root(struct mnt_namespace *ns, struct path *root)
{
        struct mount *first;

        rwsem_assert_held(&namespace_sem);

        /* We're looking at our own ns, just use get_fs_root. */
        if (ns == current->nsproxy->mnt_ns) {
                get_fs_root(current->fs, root);
                return 0;
        }

        /*
         * We have to find the first mount in our ns and use that, however it
         * may not exist, so handle that properly.
         */
        if (RB_EMPTY_ROOT(&ns->mounts))
                return -ENOENT;

        first = listmnt_next(ns->root, false);
        if (!first)
                return -ENOENT;
        root->mnt = mntget(&first->mnt);
        root->dentry = dget(root->mnt->mnt_root);
        return 0;
}

/*
 * If the user requested a specific mount namespace id, look that up and return
 * that, or if not simply grab a passive reference on our mount namespace and
 * return that.
 */
static struct mnt_namespace *grab_requested_mnt_ns(u64 mnt_ns_id)
{
        if (mnt_ns_id)
                return lookup_mnt_ns(mnt_ns_id);
        refcount_inc(&current->nsproxy->mnt_ns->passive);
        return current->nsproxy->mnt_ns;
}

SYSCALL_DEFINE4(statmount, const struct mnt_id_req __user *, req,
                struct statmount __user *, buf, size_t, bufsize,
                unsigned int, flags)
{
        struct vfsmount *mnt;
        struct mnt_id_req kreq;
        struct kstatmount ks;
        /* We currently support retrieval of 3 strings. */
        size_t seq_size = 3 * PATH_MAX;
        int ret;

        if (flags)
                return -EINVAL;

        ret = copy_mnt_id_req(req, &kreq);
        if (ret)
                return ret;

retry:
        ret = prepare_kstatmount(&ks, &kreq, buf, bufsize, seq_size);
        if (ret)
                return ret;

        down_read(&namespace_sem);
        mnt = lookup_mnt_in_ns(kreq.mnt_id, current->nsproxy->mnt_ns);
        if (!mnt) {
                up_read(&namespace_sem);
                kvfree(ks.seq.buf);
                return -ENOENT;
        }

        ks.mnt = mnt;
        get_fs_root(current->fs, &ks.root);
        ret = do_statmount(&ks);
        path_put(&ks.root);
        up_read(&namespace_sem);

        if (!ret)
                ret = copy_statmount_to_user(&ks);
        kvfree(ks.seq.buf);
        if (retry_statmount(ret, &seq_size))
                goto retry;
        return ret;
}

static ssize_t do_listmount(struct mnt_namespace *ns, u64 mnt_parent_id,
                            u64 last_mnt_id, u64 *mnt_ids, size_t nr_mnt_ids,
                            bool reverse)
{
        struct path root __free(path_put) = {};
        struct path orig;
        struct mount *r, *first;
        ssize_t ret;

        rwsem_assert_held(&namespace_sem);

        ret = grab_requested_root(ns, &root);
        if (ret)
                return ret;

        if (mnt_parent_id == LSMT_ROOT) {
                orig = root;
        } else {
                orig.mnt = lookup_mnt_in_ns(mnt_parent_id, ns);
                if (!orig.mnt)
                        return -ENOENT;
                orig.dentry = orig.mnt->mnt_root;
        }

        /*
         * Don't trigger audit denials. We just want to determine what
         * mounts to show users.
         */
        if (!is_path_reachable(real_mount(orig.mnt), orig.dentry, &root) &&
            !ns_capable_noaudit(ns->user_ns, CAP_SYS_ADMIN))
                return -EPERM;

        ret = security_sb_statfs(orig.dentry);
        if (ret)
                return ret;

        if (!last_mnt_id) {
                if (reverse)
                        first = node_to_mount(rb_last(&ns->mounts));
                else
                        first = node_to_mount(rb_first(&ns->mounts));
        } else {
                if (reverse)
                        first = mnt_find_id_at_reverse(ns, last_mnt_id - 1);
                else
                        first = mnt_find_id_at(ns, last_mnt_id + 1);
        }

        for (ret = 0, r = first; r && nr_mnt_ids; r = listmnt_next(r, reverse)) {
                if (r->mnt_id_unique == mnt_parent_id)
                        continue;
                if (!is_path_reachable(r, r->mnt.mnt_root, &orig))
                        continue;
                *mnt_ids = r->mnt_id_unique;
                mnt_ids++;
                nr_mnt_ids--;
                ret++;
        }
        return ret;
}

SYSCALL_DEFINE4(listmount, const struct mnt_id_req __user *, req,
                u64 __user *, mnt_ids, size_t, nr_mnt_ids, unsigned int, flags)
{
        u64 *kmnt_ids __free(kvfree) = NULL;
        const size_t maxcount = 1000000;
        struct mnt_namespace *ns __free(mnt_ns_release) = NULL;
        struct mnt_id_req kreq;
        ssize_t ret;

        if (flags & ~LISTMOUNT_REVERSE)
                return -EINVAL;

        /*
         * If the mount namespace really has more than 1 million mounts the
         * caller must iterate over the mount namespace (and reconsider their
         * system design...).
         */
        if (unlikely(nr_mnt_ids > maxcount))
                return -EOVERFLOW;

        if (!access_ok(mnt_ids, nr_mnt_ids * sizeof(*mnt_ids)))
                return -EFAULT;

        ret = copy_mnt_id_req(req, &kreq);
        if (ret)
                return ret;

        kmnt_ids = kvmalloc_array(nr_mnt_ids, sizeof(*kmnt_ids),
                                  GFP_KERNEL_ACCOUNT);
        if (!kmnt_ids)
                return -ENOMEM;

        ns = grab_requested_mnt_ns(kreq.mnt_ns_id);
        if (!ns)
                return -ENOENT;

        if (kreq.mnt_ns_id && (ns != current->nsproxy->mnt_ns) &&
            !ns_capable_noaudit(ns->user_ns, CAP_SYS_ADMIN))
                return -ENOENT;

        scoped_guard(rwsem_read, &namespace_sem)
                ret = do_listmount(ns, kreq.mnt_id, kreq.param, kmnt_ids,
                                   nr_mnt_ids, (flags & LISTMOUNT_REVERSE));

        if (copy_to_user(mnt_ids, kmnt_ids, ret * sizeof(*mnt_ids)))
                return -EFAULT;

        return ret;
}

static void __init init_mount_tree(void)
{
        struct vfsmount *mnt;
        struct mount *m;
        struct mnt_namespace *ns;
        struct path root;

        mnt = vfs_kern_mount(&rootfs_fs_type, 0, "rootfs", NULL);
        if (IS_ERR(mnt))
                panic("Can't create rootfs");

        ns = alloc_mnt_ns(&init_user_ns, false);
        if (IS_ERR(ns))
                panic("Can't allocate initial namespace");
        m = real_mount(mnt);
        ns->root = m;
        ns->nr_mounts = 1;
        mnt_add_to_ns(ns, m);
        init_task.nsproxy->mnt_ns = ns;
        get_mnt_ns(ns);

        root.mnt = mnt;
        root.dentry = mnt->mnt_root;
        mnt->mnt_flags |= MNT_LOCKED;

        set_fs_pwd(current->fs, &root);
        set_fs_root(current->fs, &root);

        mnt_ns_tree_add(ns);
}

void __init mnt_init(void)
{
        int err;

        mnt_cache = kmem_cache_create("mnt_cache", sizeof(struct mount),
                        0, SLAB_HWCACHE_ALIGN|SLAB_PANIC|SLAB_ACCOUNT, NULL);

        mount_hashtable = alloc_large_system_hash("Mount-cache",
                                sizeof(struct hlist_head),
                                mhash_entries, 19,
                                HASH_ZERO,
                                &m_hash_shift, &m_hash_mask, 0, 0);
        mountpoint_hashtable = alloc_large_system_hash("Mountpoint-cache",
                                sizeof(struct hlist_head),
                                mphash_entries, 19,
                                HASH_ZERO,
                                &mp_hash_shift, &mp_hash_mask, 0, 0);

        if (!mount_hashtable || !mountpoint_hashtable)
                panic("Failed to allocate mount hash table\n");

        kernfs_init();

        err = sysfs_init();
        if (err)
                printk(KERN_WARNING "%s: sysfs_init error: %d\n",
                        __func__, err);
        fs_kobj = kobject_create_and_add("fs", NULL);
        if (!fs_kobj)
                printk(KERN_WARNING "%s: kobj create error\n", __func__);
        shmem_init();
        init_rootfs();
        init_mount_tree();
}

void put_mnt_ns(struct mnt_namespace *ns)
{
        if (!refcount_dec_and_test(&ns->ns.count))
                return;
        drop_collected_mounts(&ns->root->mnt);
        free_mnt_ns(ns);
}

struct vfsmount *kern_mount(struct file_system_type *type)
{
        struct vfsmount *mnt;
        mnt = vfs_kern_mount(type, SB_KERNMOUNT, type->name, NULL);
        if (!IS_ERR(mnt)) {
                /*
                 * it is a longterm mount, don't release mnt until
                 * we unmount before file sys is unregistered
                */
                real_mount(mnt)->mnt_ns = MNT_NS_INTERNAL;
        }
        return mnt;
}
EXPORT_SYMBOL_GPL(kern_mount);

void kern_unmount(struct vfsmount *mnt)
{
        /* release long term mount so mount point can be released */
        if (!IS_ERR(mnt)) {
                mnt_make_shortterm(mnt);
                synchronize_rcu();      /* yecchhh... */
                mntput(mnt);
        }
}
EXPORT_SYMBOL(kern_unmount);

void kern_unmount_array(struct vfsmount *mnt[], unsigned int num)
{
        unsigned int i;

        for (i = 0; i < num; i++)
                mnt_make_shortterm(mnt[i]);
        synchronize_rcu_expedited();
        for (i = 0; i < num; i++)
                mntput(mnt[i]);
}
EXPORT_SYMBOL(kern_unmount_array);

bool our_mnt(struct vfsmount *mnt)
{
        return check_mnt(real_mount(mnt));
}

bool current_chrooted(void)
{
        /* Does the current process have a non-standard root */
        struct path ns_root;
        struct path fs_root;
        bool chrooted;

        /* Find the namespace root */
        ns_root.mnt = &current->nsproxy->mnt_ns->root->mnt;
        ns_root.dentry = ns_root.mnt->mnt_root;
        path_get(&ns_root);
        while (d_mountpoint(ns_root.dentry) && follow_down_one(&ns_root))
                ;

        get_fs_root(current->fs, &fs_root);

        chrooted = !path_equal(&fs_root, &ns_root);

        path_put(&fs_root);
        path_put(&ns_root);

        return chrooted;
}

static bool mnt_already_visible(struct mnt_namespace *ns,
                                const struct super_block *sb,
                                int *new_mnt_flags)
{
        int new_flags = *new_mnt_flags;
        struct mount *mnt, *n;
        bool visible = false;

        down_read(&namespace_sem);
        rbtree_postorder_for_each_entry_safe(mnt, n, &ns->mounts, mnt_node) {
                struct mount *child;
                int mnt_flags;

                if (mnt->mnt.mnt_sb->s_type != sb->s_type)
                        continue;

                /* This mount is not fully visible if it's root directory
                 * is not the root directory of the filesystem.
                 */
                if (mnt->mnt.mnt_root != mnt->mnt.mnt_sb->s_root)
                        continue;

                /* A local view of the mount flags */
                mnt_flags = mnt->mnt.mnt_flags;

                /* Don't miss readonly hidden in the superblock flags */
                if (sb_rdonly(mnt->mnt.mnt_sb))
                        mnt_flags |= MNT_LOCK_READONLY;

                /* Verify the mount flags are equal to or more permissive
                 * than the proposed new mount.
                 */
                if ((mnt_flags & MNT_LOCK_READONLY) &&
                    !(new_flags & MNT_READONLY))
                        continue;
                if ((mnt_flags & MNT_LOCK_ATIME) &&
                    ((mnt_flags & MNT_ATIME_MASK) != (new_flags & MNT_ATIME_MASK)))
                        continue;

                /* This mount is not fully visible if there are any
                 * locked child mounts that cover anything except for
                 * empty directories.
                 */
                list_for_each_entry(child, &mnt->mnt_mounts, mnt_child) {
                        struct inode *inode = child->mnt_mountpoint->d_inode;
                        /* Only worry about locked mounts */
                        if (!(child->mnt.mnt_flags & MNT_LOCKED))
                                continue;
                        /* Is the directory permanetly empty? */
                        if (!is_empty_dir_inode(inode))
                                goto next;
                }
                /* Preserve the locked attributes */
                *new_mnt_flags |= mnt_flags & (MNT_LOCK_READONLY | \
                                               MNT_LOCK_ATIME);
                visible = true;
                goto found;
        next:   ;
        }
found:
        up_read(&namespace_sem);
        return visible;
}

static bool mount_too_revealing(const struct super_block *sb, int *new_mnt_flags)
{
        const unsigned long required_iflags = SB_I_NOEXEC | SB_I_NODEV;
        struct mnt_namespace *ns = current->nsproxy->mnt_ns;
        unsigned long s_iflags;

        if (ns->user_ns == &init_user_ns)
                return false;

        /* Can this filesystem be too revealing? */
        s_iflags = sb->s_iflags;
        if (!(s_iflags & SB_I_USERNS_VISIBLE))
                return false;

        if ((s_iflags & required_iflags) != required_iflags) {
                WARN_ONCE(1, "Expected s_iflags to contain 0x%lx\n",
                          required_iflags);
                return true;
        }

        return !mnt_already_visible(ns, sb, new_mnt_flags);
}

bool mnt_may_suid(struct vfsmount *mnt)
{
        /*
         * Foreign mounts (accessed via fchdir or through /proc
         * symlinks) are always treated as if they are nosuid.  This
         * prevents namespaces from trusting potentially unsafe
         * suid/sgid bits, file caps, or security labels that originate
         * in other namespaces.
         */
        return !(mnt->mnt_flags & MNT_NOSUID) && check_mnt(real_mount(mnt)) &&
               current_in_userns(mnt->mnt_sb->s_user_ns);
}

static struct ns_common *mntns_get(struct task_struct *task)
{
        struct ns_common *ns = NULL;
        struct nsproxy *nsproxy;

        task_lock(task);
        nsproxy = task->nsproxy;
        if (nsproxy) {
                ns = &nsproxy->mnt_ns->ns;
                get_mnt_ns(to_mnt_ns(ns));
        }
        task_unlock(task);

        return ns;
}

static void mntns_put(struct ns_common *ns)
{
        put_mnt_ns(to_mnt_ns(ns));
}

static int mntns_install(struct nsset *nsset, struct ns_common *ns)
{
        struct nsproxy *nsproxy = nsset->nsproxy;
        struct fs_struct *fs = nsset->fs;
        struct mnt_namespace *mnt_ns = to_mnt_ns(ns), *old_mnt_ns;
        struct user_namespace *user_ns = nsset->cred->user_ns;
        struct path root;
        int err;

        if (!ns_capable(mnt_ns->user_ns, CAP_SYS_ADMIN) ||
            !ns_capable(user_ns, CAP_SYS_CHROOT) ||
            !ns_capable(user_ns, CAP_SYS_ADMIN))
                return -EPERM;

        if (is_anon_ns(mnt_ns))
                return -EINVAL;

        if (fs->users != 1)
                return -EINVAL;

        get_mnt_ns(mnt_ns);
        old_mnt_ns = nsproxy->mnt_ns;
        nsproxy->mnt_ns = mnt_ns;

        /* Find the root */
        err = vfs_path_lookup(mnt_ns->root->mnt.mnt_root, &mnt_ns->root->mnt,
                                "/", LOOKUP_DOWN, &root);
        if (err) {
                /* revert to old namespace */
                nsproxy->mnt_ns = old_mnt_ns;
                put_mnt_ns(mnt_ns);
                return err;
        }

        put_mnt_ns(old_mnt_ns);

        /* Update the pwd and root */
        set_fs_pwd(fs, &root);
        set_fs_root(fs, &root);

        path_put(&root);
        return 0;
}

static struct user_namespace *mntns_owner(struct ns_common *ns)
{
        return to_mnt_ns(ns)->user_ns;
}

const struct proc_ns_operations mntns_operations = {
        .name           = "mnt",
        .type           = CLONE_NEWNS,
        .get            = mntns_get,
        .put            = mntns_put,
        .install        = mntns_install,
        .owner          = mntns_owner,
};

#ifdef CONFIG_SYSCTL
static struct ctl_table fs_namespace_sysctls[] = {
        {
                .procname       = "mount-max",
                .data           = &sysctl_mount_max,
                .maxlen         = sizeof(unsigned int),
                .mode           = 0644,
                .proc_handler   = proc_dointvec_minmax,
                .extra1         = SYSCTL_ONE,
        },
};

static int __init init_fs_namespace_sysctls(void)
{
        register_sysctl_init("fs", fs_namespace_sysctls);
        return 0;
}
fs_initcall(init_fs_namespace_sysctls);

#endif /* CONFIG_SYSCTL */