[PATCH 5.0 075/117] btrfs: prop: fix zstd compression parameter validation

[Greg Kroah-Hartman]

From: Anand Jain <anand.jain@xxxxxxxxxx>

commit 50398fde997f6be8faebdb5f38e9c9c467370f51 upstream.

We let pass zstd compression parameter even if it is not fully valid.
For example:

  $ btrfs prop set /btrfs compression zst
  $ btrfs prop get /btrfs compression
     compression=zst

zlib and lzo are fine.

Fix it by checking the correct prefix length.

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 |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/btrfs/props.c
+++ b/fs/btrfs/props.c
@@ -396,7 +396,7 @@ static int prop_compression_apply(struct
 		btrfs_set_fs_incompat(fs_info, COMPRESS_LZO);
 	} else if (!strncmp("zlib", value, 4)) {
 		type = BTRFS_COMPRESS_ZLIB;
-	} else if (!strncmp("zstd", value, len)) {
+	} else if (!strncmp("zstd", value, 4)) {
 		type = BTRFS_COMPRESS_ZSTD;
 		btrfs_set_fs_incompat(fs_info, COMPRESS_ZSTD);
 	} else {