[PATCH 4.15 33/52] btrfs: add missing initialization in btrfs_check_shared

[Greg Kroah-Hartman]

4.15-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Edmund Nadolski <enadolski@xxxxxxxx>

commit 18bf591ba9753e3e5ba91f38f756a800693408f4 upstream.

This patch addresses an issue that causes fiemap to falsely
report a shared extent.  The test case is as follows:

xfs_io -f -d -c "pwrite -b 16k 0 64k" -c "fiemap -v" /media/scratch/file5
sync
xfs_io  -c "fiemap -v" /media/scratch/file5

which gives the resulting output:

wrote 65536/65536 bytes at offset 0
64 KiB, 4 ops; 0.0000 sec (121.359 MiB/sec and 7766.9903 ops/sec)
/media/scratch/file5:
 EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
   0: [0..127]:        24576..24703       128 0x2001
/media/scratch/file5:
 EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
   0: [0..127]:        24576..24703       128   0x1

This is because btrfs_check_shared calls find_parent_nodes
repeatedly in a loop, passing a share_check struct to report
the count of shared extent. But btrfs_check_shared does not
re-initialize the count value to zero for subsequent calls
from the loop, resulting in a false share count value. This
is a regressive behavior from 4.13.

With proper re-initialization the test result is as follows:

wrote 65536/65536 bytes at offset 0
64 KiB, 4 ops; 0.0000 sec (110.035 MiB/sec and 7042.2535 ops/sec)
/media/scratch/file5:
 EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
   0: [0..127]:        24576..24703       128   0x1
/media/scratch/file5:
 EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
   0: [0..127]:        24576..24703       128   0x1

which corrects the regression.

Fixes: 3ec4d3238ab ("btrfs: allow backref search checks for shared extents")
Signed-off-by: Edmund Nadolski <enadolski@xxxxxxxx>
[ add text from cover letter to changelog ]
Signed-off-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 fs/btrfs/backref.c |    1 +
 1 file changed, 1 insertion(+)

--- a/fs/btrfs/backref.c
+++ b/fs/btrfs/backref.c
@@ -1509,6 +1509,7 @@ int btrfs_check_shared(struct btrfs_root
 		if (!node)
 			break;
 		bytenr = node->val;
+		shared.share_count = 0;
 		cond_resched();
 	}