[PATCH 5.15.y] jfs: Fix shift-out-of-bounds in dbDiscardAG

[Zhi Yang]

From: Pei Li <peili.dev@xxxxxxxxx>

commit 7063b80268e2593e58bee8a8d709c2f3ff93e2f2 upstream.

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.

Reported-by: syzbot+61be3359d2ee3467e7e4@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=61be3359d2ee3467e7e4
Signed-off-by: Pei Li <peili.dev@xxxxxxxxx>
Signed-off-by: Dave Kleikamp <dave.kleikamp@xxxxxxxxxx>
Signed-off-by: Zhi Yang <Zhi.Yang@xxxxxxxxxxxxx>
Signed-off-by: He Zhe <zhe.he@xxxxxxxxxxxxx>
---
Build test passed.
---
 fs/jfs/jfs_dmap.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/jfs/jfs_dmap.c b/fs/jfs/jfs_dmap.c
index e6cbe4c982c5..0d314fac32ca 100644
--- a/fs/jfs/jfs_dmap.c
+++ b/fs/jfs/jfs_dmap.c
@@ -1694,6 +1694,8 @@ 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 */
-- 
2.34.1