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Subject[PATCH 3.16 224/305] btrfs: relocation: set trans to be NULL after ending transaction
3.16.63-rc1 review patch.  If anyone has any objections, please let me know.


From: Pan Bian <>

commit 42a657f57628402c73237547f0134e083e2f6764 upstream.

The function relocate_block_group calls btrfs_end_transaction to release
trans when update_backref_cache returns 1, and then continues the loop
body. If btrfs_block_rsv_refill fails this time, it will jump out the
loop and the freed trans will be accessed. This may result in a
use-after-free bug. The patch assigns NULL to trans after trans is
released so that it will not be accessed.

Fixes: 0647bf564f1 ("Btrfs: improve forever loop when doing balance relocation")
Reviewed-by: Qu Wenruo <>
Signed-off-by: Pan Bian <>
Reviewed-by: David Sterba <>
Signed-off-by: David Sterba <>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <>
fs/btrfs/relocation.c | 1 +
1 file changed, 1 insertion(+)

--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -3955,6 +3955,7 @@ static noinline_for_stack int relocate_b
if (update_backref_cache(trans, &rc->backref_cache)) {
btrfs_end_transaction(trans, rc->extent_root);
+ trans = NULL;

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