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[PATCH 4.19 069/101] btrfs: prop: fix vanished compression property after failed set

From: Anand Jain <anand.jain@xxxxxxxxxx>

commit 272e5326c7837697882ce3162029ba893059b616 upstream.

The compression property resets to NULL, instead of the old value if we
fail to set the new compression parameter.

  $ btrfs prop get /btrfs compression
  $ btrfs prop set /btrfs compression zli
    ERROR: failed to set compression for /btrfs: Invalid argument
  $ btrfs prop get /btrfs compression

This is because the compression property ->validate() is successful for
'zli' as the strncmp() used the length passed from the userspace.

Fix it by using the expected string length in strncmp().

Fixes: 63541927c8d1 ("Btrfs: add support for inode properties")
Fixes: 5c1aab1dd544 ("btrfs: Add zstd support")
CC: stable@xxxxxxxxxxxxxxx # 4.14+
Reviewed-by: Nikolay Borisov <nborisov@xxxxxxxx>
Signed-off-by: Anand Jain <anand.jain@xxxxxxxxxx>
Reviewed-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

 fs/btrfs/props.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/fs/btrfs/props.c
+++ b/fs/btrfs/props.c
@@ -366,11 +366,11 @@ int btrfs_subvol_inherit_props(struct bt
 static int prop_compression_validate(const char *value, size_t len)
-	if (!strncmp("lzo", value, len))
+	if (!strncmp("lzo", value, 3))
 		return 0;
-	else if (!strncmp("zlib", value, len))
+	else if (!strncmp("zlib", value, 4))
 		return 0;
-	else if (!strncmp("zstd", value, len))
+	else if (!strncmp("zstd", value, 4))
 		return 0;
 	return -EINVAL;