[PATCH] xfs: don't perform lookups on zero-height btrees

["Darrick J. Wong" <darrick.wong@xxxxxxxxxx>]

If the caller passes in a cursor to a zero-height btree (which is
impossible), we never set block to anything but NULL, which causes the
later dereference of it to crash.  Instead, just return -EFSCORRUPTED.

Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
---
 fs/xfs/libxfs/xfs_btree.c |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/xfs/libxfs/xfs_btree.c b/fs/xfs/libxfs/xfs_btree.c
index 64fd847..4bffea4 100644
--- a/fs/xfs/libxfs/xfs_btree.c
+++ b/fs/xfs/libxfs/xfs_btree.c
@@ -1814,6 +1814,10 @@ xfs_btree_lookup(
 
        XFS_BTREE_STATS_INC(cur, lookup);
 
+       /* No such thing as a zero-level tree. */
+       if (cur->bc_nlevels == 0)
+               return -EFSCORRUPTED;
+
        block = NULL;
        keyno = 0;