fs/procfs: add build ID fetching to PROCMAP_QUERY API

The need to get ELF build ID reliably is an important aspect when dealing
with profiling and stack trace symbolization, and /proc/<pid>/maps textual
representation doesn't help with this.

To get backing file's ELF build ID, application has to first resolve VMA,
then use it's start/end address range to follow a special
/proc/<pid>/map_files/<start>-<end> symlink to open the ELF file (this is
necessary because backing file might have been removed from the disk or
was already replaced with another binary in the same file path.

Such approach, beyond just adding complexity of having to do a bunch of
extra work, has extra security implications.  Because application opens
underlying ELF file and needs read access to its entire contents (as far
as kernel is concerned), kernel puts additional capable() checks on
following /proc/<pid>/map_files/<start>-<end> symlink.  And that makes
sense in general.

But in the case of build ID, profiler/symbolizer doesn't need the contents
of ELF file, per se.  It's only build ID that is of interest, and ELF
build ID itself doesn't provide any sensitive information.

So this patch adds a way to request backing file's ELF build ID along the
rest of VMA information in the same API.  User has control over whether
this piece of information is requested or not by either setting
build_id_size field to zero or non-zero maximum buffer size they provided
through build_id_addr field (which encodes user pointer as __u64 field).
This is a completely optional piece of information, and so has no
performance implications for user cases that don't care about build ID,
while improving performance and simplifying the setup for those
application that do need it.

Kernel already implements build ID fetching, which is used from BPF
subsystem.  We are reusing this code here, but plan a follow up changes to
make it work better under more relaxed assumption (compared to what
existing code assumes) of being called from user process context, in which
page faults are allowed.  BPF-specific implementation currently bails out
if necessary part of ELF file is not paged in, all due to extra
BPF-specific restrictions (like the need to fetch build ID in restrictive
contexts such as NMI handler).

[[email protected]: fix integer to pointer cast warning in do_procmap_query()]
Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Andrii Nakryiko <[email protected]>
Acked-by: Liam R. Howlett <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Mike Rapoport (IBM) <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Stephen Rothwell <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 75679b5de549076cc74603b8e6601b7f2676a4a0..728693ed00e648d76e8d866b569fd9cf66f53792 100644 (file)
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -22,6 +22,7 @@
 #include <linux/pkeys.h>
 #include <linux/minmax.h>
 #include <linux/overflow.h>
+#include <linux/buildid.h>
 
 #include <asm/elf.h>
 #include <asm/tlb.h>
@@ -445,6 +446,7 @@ skip_vma:
        addr = vma->vm_end;
        if (flags & PROCMAP_QUERY_COVERING_OR_NEXT_VMA)
                goto next_vma;
+
 no_vma:
        return ERR_PTR(-ENOENT);
 }
@@ -455,7 +457,7 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
        struct vm_area_struct *vma;
        struct mm_struct *mm;
        const char *name = NULL;
-       char *name_buf = NULL;
+       char build_id_buf[BUILD_ID_SIZE_MAX], *name_buf = NULL;
        __u64 usize;
        int err;
 
@@ -477,6 +479,8 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
        /* either both buffer address and size are set, or both should be zero */
        if (!!karg.vma_name_size != !!karg.vma_name_addr)
                return -EINVAL;
+       if (!!karg.build_id_size != !!karg.build_id_addr)
+               return -EINVAL;
 
        mm = priv->mm;
        if (!mm || !mmget_not_zero(mm))
@@ -539,6 +543,21 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
                }
        }
 
+       if (karg.build_id_size) {
+               __u32 build_id_sz;
+
+               err = build_id_parse(vma, build_id_buf, &build_id_sz);
+               if (err) {
+                       karg.build_id_size = 0;
+               } else {
+                       if (karg.build_id_size < build_id_sz) {
+                               err = -ENAMETOOLONG;
+                               goto out;
+                       }
+                       karg.build_id_size = build_id_sz;
+               }
+       }
+
        if (karg.vma_name_size) {
                size_t name_buf_sz = min_t(size_t, PATH_MAX, karg.vma_name_size);
                const struct path *path;
@@ -576,13 +595,17 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
        query_vma_teardown(mm, vma);
        mmput(mm);
 
-       if (karg.vma_name_size && copy_to_user((void __user *)karg.vma_name_addr,
+       if (karg.vma_name_size && copy_to_user(u64_to_user_ptr(karg.vma_name_addr),
                                               name, karg.vma_name_size)) {
                kfree(name_buf);
                return -EFAULT;
        }
        kfree(name_buf);
 
+       if (karg.build_id_size && copy_to_user(u64_to_user_ptr(karg.build_id_addr),
+                                              build_id_buf, karg.build_id_size))
+               return -EFAULT;
+
        if (copy_to_user(uarg, &karg, min_t(size_t, sizeof(karg), usize)))
                return -EFAULT;
 

diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index 5d440f9b5d92e0ee3d0debaf02f48e234dd0a2f9..2a4a5f50c98ecfddfa2551958d72719374ed30e3 100644 (file)
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -511,6 +511,26 @@ struct procmap_query {
         * If set to zero, vma_name_addr should be set to zero as well
         */
        __u32 vma_name_size;            /* in/out */
+       /*
+        * If set to non-zero value, signals the request to extract and return
+        * VMA's backing file's build ID, if the backing file is an ELF file
+        * and it contains embedded build ID.
+        *
+        * Kernel will set this field to zero, if VMA has no backing file,
+        * backing file is not an ELF file, or ELF file has no build ID
+        * embedded.
+        *
+        * Build ID is a binary value (not a string). Kernel will set
+        * build_id_size field to exact number of bytes used for build ID.
+        * If build ID is requested and present, but needs more bytes than
+        * user-supplied maximum buffer size (see build_id_addr field below),
+        * -E2BIG error will be returned.
+        *
+        * If this field is set to non-zero value, build_id_addr should point
+        * to valid user space memory buffer of at least build_id_size bytes.
+        * If set to zero, build_id_addr should be set to zero as well
+        */
+       __u32 build_id_size;            /* in/out */
        /*
         * User-supplied address of a buffer of at least vma_name_size bytes
         * for kernel to fill with matched VMA's name (see vma_name_size field
@@ -519,6 +539,14 @@ struct procmap_query {
         * Should be set to zero if VMA name should not be returned.
         */
        __u64 vma_name_addr;            /* in */
+       /*
+        * User-supplied address of a buffer of at least build_id_size bytes
+        * for kernel to fill with matched VMA's ELF build ID, if available
+        * (see build_id_size field description above for details).
+        *
+        * Should be set to zero if build ID should not be returned.
+        */
+       __u64 build_id_addr;            /* in */
 };
 
 #endif /* _UAPI_LINUX_FS_H */