[PATCH v2] jfs: UBSAN: shift-out-of-bounds in dbFindBits

[Matt Jan <zoo868e@gmail.com>]

The starting bit number of free bits should not be >= 32 as it is
0-indexed.Assert that the number of bits is < 32, or if it is 32, the
input word must be 0.

#syz test

Reported-by: syzbot+9e90a1c5eedb9dc4c6cc@syzkaller.appspotmail.com
Signed-off-by: Matt Jan <zoo868e@gmail.com>
---
Changes in v2: Test if the patch resolve the issue through syzbot and
reference the reporter.

 fs/jfs/jfs_dmap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/jfs/jfs_dmap.c b/fs/jfs/jfs_dmap.c
index 974ecf5e0d95..b9eccf8900eb 100644
--- a/fs/jfs/jfs_dmap.c
+++ b/fs/jfs/jfs_dmap.c
@@ -3015,7 +3015,7 @@ static int dbFindBits(u32 word, int l2nb)
 	/* get the number of bits.
 	 */
 	nb = 1 << l2nb;
-	assert(nb <= DBWORD);
+	assert(nb < DBWORD || (nb == DBWORD && !word));
 
 	/* complement the word so we can use a mask (i.e. 0s represent
 	 * free bits) and compute the mask.
-- 
2.25.1