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 <vsementsov@virtuozzo.com>
Message-Id: <20210902093754.2352-1-vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 1 insertions, 6 deletions

diff --git a/block.c b/block.c
index 10346b5011..7b3ce415d8 100644
--- 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;