[Qemu-devel] [PATCH] block: fix permission update in bdrv_replace_node

[Qemu-devel] [PATCH] block: fix permission update in bdrv_replace_node

[Vladimir Sementsov-Ogievskiy]

It's wrong to OR shared permissions. It may lead to crash on further
permission updates.
Also, no needs to consider previously calculated permissions, as at
this point we already bind all new parents and bdrv_get_cumulative_perm
result is enough. So fix the bug by just set permissions by
bdrv_get_cumulative_perm result.

Bug was introduced in long ago 234ac1a9025, in 2.9.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---

Hi all!

I found this bug during my work around backup-top filter. It happens that
on filter removing, bdrv_replace_node() breaks permissions in graph which
lead to bdrv_set_backing_hd(new backing: NULL) on
assert(tighten_restrictions == false).

 block.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/block.c b/block.c
index 874a29a983..5944124845 100644
--- a/block.c
+++ b/block.c
@@ -4165,7 +4165,6 @@ void bdrv_replace_node(BlockDriverState *from, BlockDriverState *to,
 {
     BdrvChild *c, *next;
     GSList *list = NULL, *p;
-    uint64_t old_perm, old_shared;
     uint64_t perm = 0, shared = BLK_PERM_ALL;
     int ret;
 
@@ -4211,8 +4210,8 @@ void bdrv_replace_node(BlockDriverState *from, BlockDriverState *to,
         bdrv_unref(from);
     }
 
-    bdrv_get_cumulative_perm(to, &old_perm, &old_shared);
-    bdrv_set_perm(to, old_perm | perm, old_shared | shared);
+    bdrv_get_cumulative_perm(to, &perm, &shared);
+    bdrv_set_perm(to, perm, shared);
 
 out:
     g_slist_free(list);
-- 
2.18.0

Re: [Qemu-devel] [PATCH] block: fix permission update in bdrv_replace_node

[Max Reitz]

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On 24.08.19 12:07, Vladimir Sementsov-Ogievskiy wrote:
> It's wrong to OR shared permissions. It may lead to crash on further
> permission updates.
> Also, no needs to consider previously calculated permissions, as at
> this point we already bind all new parents and bdrv_get_cumulative_perm
> result is enough. So fix the bug by just set permissions by
> bdrv_get_cumulative_perm result.
> 
> Bug was introduced in long ago 234ac1a9025, in 2.9.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
> 
> Hi all!
> 
> I found this bug during my work around backup-top filter. It happens that
> on filter removing, bdrv_replace_node() breaks permissions in graph which
> lead to bdrv_set_backing_hd(new backing: NULL) on
> assert(tighten_restrictions == false).
> 
>  block.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)

Thanks, that makes sense.

Applied to my block branch:

https://git.xanclic.moe/XanClic/qemu/commits/branch/block

Max


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