[PATCH 3.16 037/136] btrfs: avoid null pointer dereference on fs_info when calling btrfs_crit

[Ben Hutchings]

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

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

From: Colin Ian King <colin.king@xxxxxxxxxxxxx>

commit 3993b112dac968612b0b213ed59cb30f50b0015b upstream.

There are checks on fs_info in __btrfs_panic to avoid dereferencing a
null fs_info, however, there is a call to btrfs_crit that may also
dereference a null fs_info. Fix this by adding a check to see if fs_info
is null and only print the s_id if fs_info is non-null.

Detected by CoverityScan CID#401973 ("Dereference after null check")

Fixes: efe120a067c8 ("Btrfs: convert printk to btrfs_ and fix BTRFS prefix")
Signed-off-by: Colin Ian King <colin.king@xxxxxxxxxxxxx>
Reviewed-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: David Sterba <dsterba@xxxxxxxx>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 fs/btrfs/super.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -185,7 +185,6 @@ static const char * const logtypes[] = {
 
 void btrfs_printk(const struct btrfs_fs_info *fs_info, const char *fmt, ...)
 {
-	struct super_block *sb = fs_info->sb;
 	char lvl[4];
 	struct va_format vaf;
 	va_list args;
@@ -207,7 +206,8 @@ void btrfs_printk(const struct btrfs_fs_
 	vaf.fmt = fmt;
 	vaf.va = &args;
 
-	printk("%sBTRFS %s (device %s): %pV\n", lvl, type, sb->s_id, &vaf);
+	printk("%sBTRFS %s (device %s): %pV\n", lvl, type,
+		fs_info ? fs_info->sb->s_id : "<unknown>", &vaf);
 
 	va_end(args);
 }