jfs: Fix shift-out-of-bounds in dbDiscardAG

When searching for the next smaller log2 block, BLKSTOL2() returned 0, causing shift exponent -1 to be negative.

This patch fixes the issue by exiting the loop directly when negative shift is found.

---
Syzbot reported the following error:

UBSAN: shift-out-of-bounds in fs/jfs/jfs_dmap.c:1629:18

shift exponent -1 is negative

If BLKSTOL2() returned 0, the shift exponent will be -1.

The solution is to check the exponent and if it is smaller than 0, exit the loop directly.

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

index cb3cda1390ad..5713994328cb 100644
@@ -1626,6 +1626,8 @@ s64 dbDiscardAG(struct inode *ip, int agno, s64 minlen)
 		} else if (rc == -ENOSPC) {
 			/* search for next smaller log2 block */
 			l2nb = BLKSTOL2(nblocks) - 1;
+			if (unlikely(l2nb < 0))
+				break;
 			nblocks = 1LL << l2nb;
 		} else {
 			/* Trim any already allocated blocks */

---
base-commit: 2ccbdf43d5e758f8493a95252073cf9078a5fea5
change-id: 20240625-bug0-11e890f449af

Best regards,

Pei Li <[email protected]>