Re: [syzbot] [overlayfs?] WARNING in fast_dput

[syzbot <syzbot+b74150fd2ef40e716ca2@syzkaller.appspotmail.com>]

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
WARNING in fast_dput

------------[ cut here ]------------
WARNING: fs/dcache.c:829 at fast_dput+0x334/0x430 fs/dcache.c:829, CPU#1: syz.0.17/6545
Modules linked in:
CPU: 1 UID: 0 PID: 6545 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
RIP: 0010:fast_dput+0x334/0x430 fs/dcache.c:829
Code: e3 81 ff 48 b8 00 00 00 00 00 fc ff df 41 0f b6 44 05 00 84 c0 0f 85 e2 00 00 00 41 80 0e 40 e9 fd fe ff ff e8 4d e3 81 ff 90 <0f> 0b 90 e9 ef fe ff ff 44 89 e6 81 e6 00 00 04 00 31 ff e8 74 e7
RSP: 0018:ffffc90003147cd8 EFLAGS: 00010293
RAX: ffffffff823fcfe3 RBX: ffff88805c212988 RCX: ffff8880779bbd00
RDX: 0000000000000000 RSI: 00000000ffffff80 RDI: 0000000000000001
RBP: 00000000ffffff80 R08: 0000000000000003 R09: 0000000000000004
R10: dffffc0000000000 R11: fffff52000628f8c R12: dffffc0000000000
R13: 1ffff1100b842539 R14: ffff88805c2128d0 R15: ffff88805c2129c8
FS:  000055558468e500(0000) GS:ffff888125f4f000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000001b30763fff CR3: 000000007d39a000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 dput+0xe8/0x1a0 fs/dcache.c:924
 __fput+0x68e/0xa70 fs/file_table.c:476
 task_work_run+0x1d4/0x260 kernel/task_work.c:233
 resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
 __exit_to_user_mode_loop kernel/entry/common.c:44 [inline]
 exit_to_user_mode_loop+0xff/0x4f0 kernel/entry/common.c:75
 __exit_to_user_mode_prepare include/linux/irq-entry-common.h:226 [inline]
 syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:256 [inline]
 syscall_exit_to_user_mode_work include/linux/entry-common.h:159 [inline]
 syscall_exit_to_user_mode include/linux/entry-common.h:194 [inline]
 do_syscall_64+0x2e3/0xf80 arch/x86/entry/syscall_64.c:100
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fb34398f749
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffd90d758e8 EFLAGS: 00000246 ORIG_RAX: 00000000000001b4
RAX: 0000000000000000 RBX: 0000000000021fa6 RCX: 00007fb34398f749
RDX: 0000000000000000 RSI: 000000000000001e RDI: 0000000000000003
RBP: 00007fb343be7da0 R08: 0000000000000001 R09: 0000000290d75bdf
R10: 0000001b2fb20000 R11: 0000000000000246 R12: 00007fb343be5fac
R13: 00007fb343be5fa0 R14: ffffffffffffffff R15: 00007ffd90d75a00
 </TASK>


Tested on:

commit:         7d31f578 Add linux-next specific files for 20251128
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=12b430c2580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=6336d8e94a7c517d
dashboard link: https://syzkaller.appspot.com/bug?extid=b74150fd2ef40e716ca2
compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
patch:          https://syzkaller.appspot.com/x/patch.diff?x=144dfe92580000