[PATCH 3.16 065/157] btrfs: prop: fix vanished compression property after failed set

[Ben Hutchings]

3.16.72-rc1 review patch.  If anyone has any objections, please let me know.

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

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
    compression=lzo
  $ 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")
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>
[bwh: Backported to 3.16: "zstd" is not supported]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/fs/btrfs/props.c
+++ b/fs/btrfs/props.c
@@ -378,9 +378,9 @@ 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;
 
 	return -EINVAL;