[syzbot ci] Re: exfat: unify FAT chain walking helpers

[syzbot ci]

syzbot ci has tested the following series

[v1] exfat: unify FAT chain walking helpers
https://lore.kernel.org/all/20260331091113.2...@163.com
* [PATCH v1 1/6] exfat: fix incorrect directory checksum after rename to shorter name
* [PATCH v1 2/6] exfat: introduce exfat_fat_walk helper
* [PATCH v1 3/6] exfat: use exfat_fat_walk helper to simplify fat entry walking
* [PATCH v1 4/6] exfat: remove NULL cache pointer case in exfat_ent_get
* [PATCH v1 5/6] exfat: introduce exfat_chain_advance helper
* [PATCH v1 6/6] exfat: use exfat_chain_advance helper

and found the following issue:
WARNING in exfat_find_empty_entry

Full report is available here:
https://ci.syzbot.org/series/d9d5a999-e11a-43e1-ac76-9c718e136f25

***

WARNING in exfat_find_empty_entry

tree: linux-next
URL: https://kernel.googlesource.com/pub/scm/linux/kernel/git/next/linux-next
base: d1d75eaf01abceb3d5cb50253375b5f254b6be54
arch: amd64
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
config: https://ci.syzbot.org/builds/177ab8e9-7eef-44cc-8b1d-6e8e2b44adaf/config
C repro: https://ci.syzbot.org/findings/9b0e35b9-b5be-440e-9461-2f19c03cb16a/c_repro
syz repro: https://ci.syzbot.org/findings/9b0e35b9-b5be-440e-9461-2f19c03cb16a/syz_repro

exFAT-fs (loop0): start_clu is invalid cluster(0xffffffff)
exFAT-fs (loop0): valid_size(150994954) is greater than size(10)
------------[ cut here ]------------
chain->dir != EXFAT_EOF_CLUSTER
WARNING: fs/exfat/exfat_fs.h:575 at exfat_chain_advance fs/exfat/exfat_fs.h:575 [inline], CPU#0: syz.0.17/5981
WARNING: fs/exfat/exfat_fs.h:575 at exfat_search_empty_slot fs/exfat/namei.c:249 [inline], CPU#0: syz.0.17/5981
WARNING: fs/exfat/exfat_fs.h:575 at exfat_find_empty_entry+0x754/0x13b0 fs/exfat/namei.c:318, CPU#0: syz.0.17/5981
Modules linked in:
CPU: 0 UID: 0 PID: 5981 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
RIP: 0010:exfat_chain_advance fs/exfat/exfat_fs.h:575 [inline]
RIP: 0010:exfat_search_empty_slot fs/exfat/namei.c:249 [inline]
RIP: 0010:exfat_find_empty_entry+0x754/0x13b0 fs/exfat/namei.c:318
Code: 00 bf ff ff ff ff 44 89 e6 e8 18 56 1f ff 41 83 fc ff 75 10 e8 cd 51 1f ff 45 29 ee 0f 8d ad fe ff ff eb 12 e8 bd 51 1f ff 90 <0f> 0b 90 45 29 ee 0f 8d 99 fe ff ff e8 ab 51 1f ff 41 01 df 48 8b
RSP: 0018:ffffc9000499f340 EFLAGS: 00010293
RAX: ffffffff82a639c3 RBX: 0000000000000020 RCX: ffff88816d588000
RDX: 0000000000000000 RSI: 000000000000000b RDI: 00000000ffffffff
RBP: ffffc9000499f570 R08: ffff888119e14a5b R09: 1ffff110233c294b
R10: dffffc0000000000 R11: ffffed10233c294c R12: 000000000000000b
R13: 0000000000000080 R14: 0000000000000000 R15: 0000000000000060
FS: 000055557e214500(0000) GS:ffff88818de66000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000200000006200 CR3: 00000001bcf7c000 CR4: 00000000000006f0
Call Trace:
<TASK>
exfat_rename_file fs/exfat/namei.c:1025 [inline]
__exfat_rename fs/exfat/namei.c:1177 [inline]
exfat_rename+0xb49/0x1f90 fs/exfat/namei.c:1251
vfs_rename+0xa96/0xeb0 fs/namei.c:6026
filename_renameat2+0x539/0x9c0 fs/namei.c:6144
__do_sys_rename fs/namei.c:6188 [inline]
__se_sys_rename+0x55/0x2c0 fs/namei.c:6184
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f66f1b9c819
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 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 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffcad04e2e8 EFLAGS: 00000246 ORIG_RAX: 0000000000000052
RAX: ffffffffffffffda RBX: 00007f66f1e15fa0 RCX: 00007f66f1b9c819
RDX: 0000000000000000 RSI: 0000200000000400 RDI: 0000200000006200
RBP: 00007f66f1c32c91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f66f1e15fac R14: 00007f66f1e15fa0 R15: 00007f66f1e15fa0
</TASK>


***

If these findings have caused you to resend the series or submit a
separate fix, please add the following tag to your commit message:
Tested-by: syz...@syzkaller.appspotmail.com

---
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[Chi Zhiling]

It looks like I should remove this WARN_ON_ONCE statement. This check is
not reasonable when accessing directories, and I will fix it in v2.

Thanks,