When reusing transaction locks for DTREE operations, the index field
may contain stale values from previous operations, causing assertion
failures in dtSplitRoot():

  ASSERT(dtlck->index == 0)

This results in kernel crashes like:

  kernel BUG at fs/jfs/jfs_dtree.c:1942!
  Call Trace:
   dtSplitRoot+0x1694/0x16c0 fs/jfs/jfs_dtree.c:1942
   dtSplitUp fs/jfs/jfs_dtree.c:1244 [inline]
   dtInsert+0x2525/0x5f40 fs/jfs/jfs_dtree.c:871
   jfs_create+0x6c8/0xa80 fs/jfs/namei.c:137

The bug occurs because txLock() has multiple code paths for lock
acquisition:

1. Fresh allocation (allocateLock) - correctly initializes index to 0
2. Lock reuse (same transaction) - skips initialization
3. Anonymous lock acquisition - skips initialization

Paths 2 and 3 jump directly to the grantLock label, bypassing the
index initialization. When dtSplitRoot() is called multiple times
within a batched transaction (which JFS uses for performance), it may
receive a reused lock with index=3 from a previous operation instead
of the expected index=0.

Example sequence:
  Transaction tid=1:
    - First dtSplitRoot: gets fresh lock, index=0 ✓
    - Modifies entries, index becomes 3
    - Lock returned to pool but not freed

  Transaction tid=1 (continues):
    - Second dtSplitRoot: reuses same lock
    - index still = 3 (stale value) ✗
    - ASSERT(index == 0) fails → crash

Fix by resetting dtlck->index to 0 at the grantLock label, but only
for operations with the tlckNEW flag set. This ensures:

- New pages (like dtSplitRoot) start with clean state (index=0)
- Existing pages preserve accumulated changes within a transaction
- No performance impact (only affects new page operations)

The tlckNEW flag is used by dtSplitRoot() when creating a new root
page, making this fix targeted to the exact scenario that requires
index=0.

Reported-by: syzbot+a099d674daa27a9272db@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=a099d674daa27a9272db
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
 fs/jfs/jfs_txnmgr.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/fs/jfs/jfs_txnmgr.c b/fs/jfs/jfs_txnmgr.c
index c16578af3a77..5ce2fc17d967 100644
--- a/fs/jfs/jfs_txnmgr.c
+++ b/fs/jfs/jfs_txnmgr.c
@@ -811,6 +811,17 @@ struct tlock *txLock(tid_t tid, struct inode *ip, struct metapage * mp,
 	 * update tlock vector
 	 */
       grantLock:
+	/*
+	 * Reset index for new DTREE locks to ensure clean state.
+	 * When locks are reused, index may contain stale values from
+	 * previous operations. Operations like dtSplitRoot() expect
+	 * index to be 0 when creating new pages (tlckNEW flag).
+	 */
+	if ((type & tlckDTREE) && (type & tlckNEW)) {
+		struct dt_lock *dtlck = (struct dt_lock *)&tlck->lock;
+
+		dtlck->index = 0;
+	}
 	tlck->type |= type;
 
 	return tlck;
-- 
2.43.0