block: drop BLK_PERM_GRAPH_MOD

First, this permission never protected a node from being changed, as
generic child-replacing functions don't check it.

Second, it's a strange thing: it presents a permission of parent node
to change its child. But generally, children are replaced by different
mechanisms, like jobs or qmp commands, not by nodes.

Graph-mod permission is hard to understand. All other permissions
describe operations which done by parent node on its child: read,
write, resize. Graph modification operations are something completely
different.

The only place where BLK_PERM_GRAPH_MOD is used as "perm" (not shared
perm) is mirror_start_job, for s->target. Still modern code should use
bdrv_freeze_backing_chain() to protect from graph modification, if we
don't do it somewhere it may be considered as a bug. So, it's a bit
risky to drop GRAPH_MOD, and analyzing of possible loss of protection
is hard. But one day we should do it, let's do it now.

One more bit of information is that locking the corresponding byte in
file-posix doesn't make sense at all.

Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Message-Id: <20210902093754[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>

diff --git a/block.c b/block.c
index 10346b501137c54f9633c4413741cdbc1d644fe4..7b3ce415d81bc52a5393326fa5408ae254ed4017 100644 (file)
--- a/block.c
+++ b/block.c
@@ -2485,7 +2485,6 @@ char *bdrv_perm_names(uint64_t perm)
         { BLK_PERM_WRITE,           "write" },
         { BLK_PERM_WRITE_UNCHANGED, "write unchanged" },
         { BLK_PERM_RESIZE,          "resize" },
-        { BLK_PERM_GRAPH_MOD,       "change children" },
         { 0, NULL }
     };
 
@@ -2601,8 +2600,7 @@ static void bdrv_default_perms_for_cow(BlockDriverState *bs, BdrvChild *c,
         shared = 0;
     }
 
-    shared |= BLK_PERM_CONSISTENT_READ | BLK_PERM_GRAPH_MOD |
-              BLK_PERM_WRITE_UNCHANGED;
+    shared |= BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE_UNCHANGED;
 
     if (bs->open_flags & BDRV_O_INACTIVE) {
         shared |= BLK_PERM_WRITE | BLK_PERM_RESIZE;
@@ -2720,7 +2718,6 @@ uint64_t bdrv_qapi_perm_to_blk_perm(BlockPermission qapi_perm)
         [BLOCK_PERMISSION_WRITE]            = BLK_PERM_WRITE,
         [BLOCK_PERMISSION_WRITE_UNCHANGED]  = BLK_PERM_WRITE_UNCHANGED,
         [BLOCK_PERMISSION_RESIZE]           = BLK_PERM_RESIZE,
-        [BLOCK_PERMISSION_GRAPH_MOD]        = BLK_PERM_GRAPH_MOD,
     };
 
     QEMU_BUILD_BUG_ON(ARRAY_SIZE(permissions) != BLOCK_PERMISSION__MAX);
@@ -5546,8 +5543,6 @@ int bdrv_drop_intermediate(BlockDriverState *top, BlockDriverState *base,
     update_inherits_from = bdrv_inherits_from_recursive(base, explicit_top);
 
     /* success - we can delete the intermediate states, and link top->base */
-    /* TODO Check graph modification op blockers (BLK_PERM_GRAPH_MOD) once
-     * we've figured out how they should work. */
     if (!backing_file_str) {
         bdrv_refresh_filename(base);
         backing_file_str = base->filename;

diff --git a/block/commit.c b/block/commit.c
index 10cc5ff4518a9a0c15fe5a0c379c32dec66c1677..b1fc7b908b292c22d0444c8c9a1fa6ad90bbcf02 100644 (file)
--- a/block/commit.c
+++ b/block/commit.c
@@ -370,7 +370,6 @@ void commit_start(const char *job_id, BlockDriverState *bs,
     s->base = blk_new(s->common.job.aio_context,
                       base_perms,
                       BLK_PERM_CONSISTENT_READ
-                      | BLK_PERM_GRAPH_MOD
                       | BLK_PERM_WRITE_UNCHANGED);
     ret = blk_insert_bs(s->base, base, errp);
     if (ret < 0) {

diff --git a/block/mirror.c b/block/mirror.c
index 959e3dfbd67e6dc2ef47520f4f8ea5f7c98335fa..69b2c1c697e121878476e584240c1d108b6e7016 100644 (file)
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -1139,10 +1139,7 @@ static void mirror_complete(Job *job, Error **errp)
         replace_aio_context = bdrv_get_aio_context(s->to_replace);
         aio_context_acquire(replace_aio_context);
 
-        /* TODO Translate this into permission system. Current definition of
-         * GRAPH_MOD would require to request it for the parents; they might
-         * not even be BlockDriverStates, however, so a BdrvChild can't address
-         * them. May need redefinition of GRAPH_MOD. */
+        /* TODO Translate this into child freeze system. */
         error_setg(&s->replace_blocker,
                    "block device is in use by block-job-complete");
         bdrv_op_block_all(s->to_replace, s->replace_blocker);
@@ -1666,7 +1663,7 @@ static BlockJob *mirror_start_job(
     s = block_job_create(job_id, driver, NULL, mirror_top_bs,
                          BLK_PERM_CONSISTENT_READ,
                          BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE_UNCHANGED |
-                         BLK_PERM_WRITE | BLK_PERM_GRAPH_MOD, speed,
+                         BLK_PERM_WRITE, speed,
                          creation_flags, cb, opaque, errp);
     if (!s) {
         goto fail;
@@ -1710,9 +1707,7 @@ static BlockJob *mirror_start_job(
             target_perms |= BLK_PERM_RESIZE;
         }
 
-        target_shared_perms |= BLK_PERM_CONSISTENT_READ
-                            |  BLK_PERM_WRITE
-                            |  BLK_PERM_GRAPH_MOD;
+        target_shared_perms |= BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE;
     } else if (bdrv_chain_contains(bs, bdrv_skip_filters(target))) {
         /*
          * We may want to allow this in the future, but it would
@@ -1723,10 +1718,6 @@ static BlockJob *mirror_start_job(
         goto fail;
     }
 
-    if (backing_mode != MIRROR_LEAVE_BACKING_CHAIN) {
-        target_perms |= BLK_PERM_GRAPH_MOD;
-    }
-
     s->target = blk_new(s->common.job.aio_context,
                         target_perms, target_shared_perms);
     ret = blk_insert_bs(s->target, target, errp);

diff --git a/hw/block/block.c b/hw/block/block.c
index d47ebf005adaaac2e3c5b5218e01c331b66a4335..25f45df7232d7b233b81d1a21d7fce3027c455f8 100644 (file)
--- a/hw/block/block.c
+++ b/hw/block/block.c
@@ -171,8 +171,7 @@ bool blkconf_apply_backend_options(BlockConf *conf, bool readonly,
         perm |= BLK_PERM_WRITE;
     }
 
-    shared_perm = BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE_UNCHANGED |
-                  BLK_PERM_GRAPH_MOD;
+    shared_perm = BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE_UNCHANGED;
     if (resizable) {
         shared_perm |= BLK_PERM_RESIZE;
     }

diff --git a/include/block/block.h b/include/block/block.h
index e5dd22b03435b6d7972fb8cf991c76681a2681cd..9d4050220b8a59663d7a8283147bee7e44478255 100644 (file)
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -269,12 +269,13 @@ enum {
     BLK_PERM_RESIZE             = 0x08,
 
     /**
-     * This permission is required to change the node that this BdrvChild
-     * points to.
+     * There was a now-removed bit BLK_PERM_GRAPH_MOD, with value of 0x10. QEMU
+     * 6.1 and earlier may still lock the corresponding byte in block/file-posix
+     * locking.  So, implementing some new permission should be very careful to
+     * not interfere with this old unused thing.
      */
-    BLK_PERM_GRAPH_MOD          = 0x10,
 
-    BLK_PERM_ALL                = 0x1f,
+    BLK_PERM_ALL                = 0x0f,
 
     DEFAULT_PERM_PASSTHROUGH    = BLK_PERM_CONSISTENT_READ
                                  | BLK_PERM_WRITE

diff --git a/qapi/block-core.json b/qapi/block-core.json
index bd0b2852459dd123e93e4c9ebd7a47ff496ece8e..9a5a3641d0b25776847533c45cedbdfee8721e6f 100644 (file)
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -1878,14 +1878,11 @@
 #
 # @resize: This permission is required to change the size of a block node.
 #
-# @graph-mod: This permission is required to change the node that this
-#             BdrvChild points to.
-#
 # Since: 4.0
 ##
 { 'enum': 'BlockPermission',
-  'data': [ 'consistent-read', 'write', 'write-unchanged', 'resize',
-            'graph-mod' ] }
+  'data': [ 'consistent-read', 'write', 'write-unchanged', 'resize' ] }
+
 ##
 # @XDbgBlockGraphEdge:
 #

diff --git a/scripts/render_block_graph.py b/scripts/render_block_graph.py
index da6acf050d12bf2c0ff8cd14fc712d48d6fd93a9..42288a3cfbe486b77fdb9d0778f652efa7079097 100755 (executable)
--- a/scripts/render_block_graph.py
+++ b/scripts/render_block_graph.py
@@ -35,7 +35,6 @@ def perm(arr):
     s = 'w' if 'write' in arr else '_'
     s += 'r' if 'consistent-read' in arr else '_'
     s += 'u' if 'write-unchanged' in arr else '_'
-    s += 'g' if 'graph-mod' in arr else '_'
     s += 's' if 'resize' in arr else '_'
     return s
 

diff --git a/tests/qemu-iotests/273.out b/tests/qemu-iotests/273.out
index 4e840b673031280034420a02c4e725075408391f..6a74a8138bad688aa9660b4ba96aaeb3f4f7a954 100644 (file)
--- a/tests/qemu-iotests/273.out
+++ b/tests/qemu-iotests/273.out
@@ -204,7 +204,6 @@ Testing: -blockdev file,node-name=base,filename=TEST_DIR/t.IMGFMT.base -blockdev
                 "name": "file",
                 "parent": 5,
                 "shared-perm": [
-                    "graph-mod",
                     "write-unchanged",
                     "consistent-read"
                 ],
@@ -219,7 +218,6 @@ Testing: -blockdev file,node-name=base,filename=TEST_DIR/t.IMGFMT.base -blockdev
                 "name": "backing",
                 "parent": 5,
                 "shared-perm": [
-                    "graph-mod",
                     "resize",
                     "write-unchanged",
                     "write",
@@ -233,7 +231,6 @@ Testing: -blockdev file,node-name=base,filename=TEST_DIR/t.IMGFMT.base -blockdev
                 "name": "file",
                 "parent": 3,
                 "shared-perm": [
-                    "graph-mod",
                     "write-unchanged",
                     "consistent-read"
                 ],
@@ -246,7 +243,6 @@ Testing: -blockdev file,node-name=base,filename=TEST_DIR/t.IMGFMT.base -blockdev
                 "name": "backing",
                 "parent": 3,
                 "shared-perm": [
-                    "graph-mod",
                     "resize",
                     "write-unchanged",
                     "write",