[syzbot] [exfat?] WARNING in fat_ent_write

[syzbot] [exfat?] WARNING in fat_ent_write

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    6a746cd265ae Add linux-next specific files for 20260819
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=13cdee79580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=4a1369648c551bbe
dashboard link: https://syzkaller.appspot.com/bug?extid=26461e903494e689c24f
compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=17760e79580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/32d71a199871/disk-6a746cd2.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/7bfb88f0043e/vmlinux-6a746cd2.xz
kernel image: https://storage.googleapis.com/syzbot-assets/4bc2ab35db01/bzImage-6a746cd2.xz
mounted in repro: https://storage.googleapis.com/syzbot-assets/500a74ccb5d4/mount_0.gz
  fsck result: failed (log: https://syzkaller.appspot.com/x/fsck.log?x=17e3d679580000)

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+26461e903494e689c24f@syzkaller.appspotmail.com

         and is ignored by this kernel. Remove the mand
         option from the mount to silence this warning.
=======================================================
Buffer I/O error on dev loop0, logical block 1, lost sync page write
------------[ cut here ]------------
!buffer_uptodate(bh)
WARNING: fs/buffer.c:991 at mark_buffer_dirty+0x299/0x410 fs/buffer.c:991, CPU#1: syz.0.17/5854
Modules linked in:
CPU: 1 UID: 0 PID: 5854 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
RIP: 0010:mark_buffer_dirty+0x299/0x410 fs/buffer.c:991
Code: 4c 89 f7 e8 09 0a d8 ff 49 8b 3e be 40 00 00 00 5b 41 5c 41 5e 41 5f 5d e9 74 52 fb ff e8 4f 40 68 ff eb 8c e8 48 40 68 ff 90 <0f> 0b 90 e9 a5 fd ff ff e8 3a 40 68 ff 90 0f 0b 90 e9 cf fd ff ff
RSP: 0018:ffffc9000345f2d0 EFLAGS: 00010293
RAX: ffffffff825f7f28 RBX: ffff8880714019a0 RCX: ffff8880317c5dc0
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000002
RBP: dffffc0000000001 R08: ffff8880714019a7 R09: 1ffff1100e280334
R10: dffffc0000000000 R11: ffffed100e280335 R12: ffffffff8c288c60
R13: ffff888032a9c248 R14: dffffc0000000000 R15: ffff8880717c46a8
FS:  00007fc05a3146c0(0000) GS:ffff888124dda000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f9293991000 CR3: 000000007992c000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 mmb_mark_buffer_dirty+0x2c/0x1f0 fs/buffer.c:624
 fat_ent_write+0xb8/0x190 fs/fat/fatent.c:422
 fat_chain_add+0x409/0x8e0 fs/fat/misc.c:140
 fat_add_cluster fs/fat/inode.c:113 [inline]
 __fat_get_block fs/fat/inode.c:155 [inline]
 fat_get_block+0x768/0xae0 fs/fat/inode.c:190
 __block_write_begin_int+0x6c2/0x1900 fs/buffer.c:2030
 block_write_begin fs/buffer.c:2144 [inline]
 cont_write_begin+0x71b/0xac0 fs/buffer.c:2506
 fat_write_begin+0x7a/0x150 fs/fat/inode.c:230
 cont_expand_zero fs/buffer.c:2434 [inline]
 cont_write_begin+0x2d6/0xac0 fs/buffer.c:2496
 fat_write_begin+0x7a/0x150 fs/fat/inode.c:230
 generic_perform_write+0x2d5/0x8f0 mm/filemap.c:4374
 generic_file_write_iter+0xae/0x330 mm/filemap.c:4517
 new_sync_write fs/read_write.c:595 [inline]
 vfs_write+0x612/0xba0 fs/read_write.c:687
 ksys_pwrite64 fs/read_write.c:794 [inline]
 __do_sys_pwrite64 fs/read_write.c:802 [inline]
 __se_sys_pwrite64 fs/read_write.c:799 [inline]
 __x64_sys_pwrite64+0x196/0x220 fs/read_write.c:799
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fc05939e0d9
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:00007fc05a314028 EFLAGS: 00000246 ORIG_RAX: 0000000000000012
RAX: ffffffffffffffda RBX: 00007fc059625fa0 RCX: 00007fc05939e0d9
RDX: 000000000000fdef RSI: 0000200000000140 RDI: 0000000000000004
RBP: 00007fc059435024 R08: 0000000000000000 R09: 0000000000000000
R10: 000000000000fecc R11: 0000000000000246 R12: 0000000000000000
R13: 00007fc059626038 R14: 00007fc059625fa0 R15: 00007fff03653f68
 </TASK>


---
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If the report is already addressed, let syzbot know by replying with:
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If you want to overwrite report's subsystems, reply with:
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Re: Please drop 64d9183203eeb in mm-nonmm-unstable ([syzbot] [exfat?] WARNING in fat_ent_write)

[OGAWA Hirofumi]

https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git/commit/fs/fat/misc.c?h=mm-nonmm-unstable&id=64d9183203eebe33de6188b70a8c1e91f52885db (fat: restore original value when fat_ent_write failed)

This commit was missing to consider about the path of error by sync
write.  Since current bh write clears uptodate flag if got I/O error, so
this patch doesn't work for now with wait==true.

Please drop this patch at least until stop clearing the uptodate by bh
sync write and buffer is reusable after the error.

Thanks.

syzbot <syzbot+26461e903494e689c24f@syzkaller.appspotmail.com> writes:

> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    6a746cd265ae Add linux-next specific files for 20260819
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=13cdee79580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=4a1369648c551bbe
> dashboard link: https://syzkaller.appspot.com/bug?extid=26461e903494e689c24f
> compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=17760e79580000
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/32d71a199871/disk-6a746cd2.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/7bfb88f0043e/vmlinux-6a746cd2.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/4bc2ab35db01/bzImage-6a746cd2.xz
> mounted in repro: https://storage.googleapis.com/syzbot-assets/500a74ccb5d4/mount_0.gz
>   fsck result: failed (log: https://syzkaller.appspot.com/x/fsck.log?x=17e3d679580000)
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+26461e903494e689c24f@syzkaller.appspotmail.com
>
>          and is ignored by this kernel. Remove the mand
>          option from the mount to silence this warning.
> =======================================================
> Buffer I/O error on dev loop0, logical block 1, lost sync page write
> ------------[ cut here ]------------
> !buffer_uptodate(bh)
> WARNING: fs/buffer.c:991 at mark_buffer_dirty+0x299/0x410 fs/buffer.c:991, CPU#1: syz.0.17/5854
> Modules linked in:
> CPU: 1 UID: 0 PID: 5854 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full) 
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
> RIP: 0010:mark_buffer_dirty+0x299/0x410 fs/buffer.c:991
> Code: 4c 89 f7 e8 09 0a d8 ff 49 8b 3e be 40 00 00 00 5b 41 5c 41 5e 41 5f 5d e9 74 52 fb ff e8 4f 40 68 ff eb 8c e8 48 40 68 ff 90 <0f> 0b 90 e9 a5 fd ff ff e8 3a 40 68 ff 90 0f 0b 90 e9 cf fd ff ff
> RSP: 0018:ffffc9000345f2d0 EFLAGS: 00010293
> RAX: ffffffff825f7f28 RBX: ffff8880714019a0 RCX: ffff8880317c5dc0
> RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000002
> RBP: dffffc0000000001 R08: ffff8880714019a7 R09: 1ffff1100e280334
> R10: dffffc0000000000 R11: ffffed100e280335 R12: ffffffff8c288c60
> R13: ffff888032a9c248 R14: dffffc0000000000 R15: ffff8880717c46a8
> FS:  00007fc05a3146c0(0000) GS:ffff888124dda000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f9293991000 CR3: 000000007992c000 CR4: 00000000003526f0
> Call Trace:
>  <TASK>
>  mmb_mark_buffer_dirty+0x2c/0x1f0 fs/buffer.c:624
>  fat_ent_write+0xb8/0x190 fs/fat/fatent.c:422
>  fat_chain_add+0x409/0x8e0 fs/fat/misc.c:140
>  fat_add_cluster fs/fat/inode.c:113 [inline]
>  __fat_get_block fs/fat/inode.c:155 [inline]
>  fat_get_block+0x768/0xae0 fs/fat/inode.c:190
>  __block_write_begin_int+0x6c2/0x1900 fs/buffer.c:2030
>  block_write_begin fs/buffer.c:2144 [inline]
>  cont_write_begin+0x71b/0xac0 fs/buffer.c:2506
>  fat_write_begin+0x7a/0x150 fs/fat/inode.c:230
>  cont_expand_zero fs/buffer.c:2434 [inline]
>  cont_write_begin+0x2d6/0xac0 fs/buffer.c:2496
>  fat_write_begin+0x7a/0x150 fs/fat/inode.c:230
>  generic_perform_write+0x2d5/0x8f0 mm/filemap.c:4374
>  generic_file_write_iter+0xae/0x330 mm/filemap.c:4517
>  new_sync_write fs/read_write.c:595 [inline]
>  vfs_write+0x612/0xba0 fs/read_write.c:687
>  ksys_pwrite64 fs/read_write.c:794 [inline]
>  __do_sys_pwrite64 fs/read_write.c:802 [inline]
>  __se_sys_pwrite64 fs/read_write.c:799 [inline]
>  __x64_sys_pwrite64+0x196/0x220 fs/read_write.c:799
>  do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
>  do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7fc05939e0d9
> 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:00007fc05a314028 EFLAGS: 00000246 ORIG_RAX: 0000000000000012
> RAX: ffffffffffffffda RBX: 00007fc059625fa0 RCX: 00007fc05939e0d9
> RDX: 000000000000fdef RSI: 0000200000000140 RDI: 0000000000000004
> RBP: 00007fc059435024 R08: 0000000000000000 R09: 0000000000000000
> R10: 000000000000fecc R11: 0000000000000246 R12: 0000000000000000
> R13: 00007fc059626038 R14: 00007fc059625fa0 R15: 00007fff03653f68
>  </TASK>
>
>
> ---
> This report is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
> syzbot engineers can be reached at syzkaller@googlegroups.com.
>
> syzbot will keep track of this issue. See:
> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
>
> If the report is already addressed, let syzbot know by replying with:
> #syz fix: exact-commit-title
>
> If you want syzbot to run the reproducer, reply with:
> #syz test: git://repo/address.git branch-or-commit-hash
> If you attach or paste a git patch, syzbot will apply it before testing.
>
> If you want to overwrite report's subsystems, reply with:
> #syz set subsystems: new-subsystem
> (See the list of subsystem names on the web dashboard)
>
> If the report is a duplicate of another one, reply with:
> #syz dup: exact-subject-of-another-report
>
> If you want to undo deduplication, reply with:
> #syz undup

-- 
OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>

Re: Please drop 64d9183203eeb in mm-nonmm-unstable ([syzbot] [exfat?] WARNING in fat_ent_write)

[Andrew Morton]

On Mon, 24 Aug 2026 18:31:29 +0900 OGAWA Hirofumi <hirofumi@mail.parknet.co.jp> wrote:

> https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git/commit/fs/fat/misc.c?h=mm-nonmm-unstable&id=64d9183203eebe33de6188b70a8c1e91f52885db (fat: restore original value when fat_ent_write failed)
> 
> This commit was missing to consider about the path of error by sync
> write.  Since current bh write clears uptodate flag if got I/O error, so
> this patch doesn't work for now with wait==true.
> 
> Please drop this patch at least until stop clearing the uptodate by bh
> sync write and buffer is reusable after the error.

This is now in Linus's tree, as 64d9183203ee.  Please send along a fix,
with 

Fixes: 64d9183203ee ("fat: restore original value when fat_ent_write failed")