[v6.1] WARNING in fat12_ent_put

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 1a8763b93150 Linux 6.1.188
git tree: linux-6.1.y
console output: https://syzkaller.appspot.com/x/log.txt?x=115c2035580000
kernel config: https://syzkaller.appspot.com/x/.config?x=31dfefc4a14efea3
dashboard link: https://syzkaller.appspot.com/bug?extid=06f803b4d15edad2e6da
compiler: Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/be5de8c5e41e/disk-1a8763b9.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/b803e8d05dd4/vmlinux-1a8763b9.xz
kernel image: https://storage.googleapis.com/syzbot-assets/723f53728c4f/bzImage-1a8763b9.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+06f803...@syzkaller.appspotmail.com

loop1: detected capacity change from 0 to 128
------------[ cut here ]------------
WARNING: CPU: 0 PID: 6168 at fs/buffer.c:1081 mark_buffer_dirty+0x383/0x7a0 fs/buffer.c:1081
Modules linked in:
CPU: 0 PID: 6168 Comm: syz.1.596 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/16/2026
RIP: 0010:mark_buffer_dirty+0x383/0x7a0 fs/buffer.c:1081
Code: 13 6b e2 ff 49 8b 3f be 04 00 00 00 5b 41 5c 41 5d 41 5e 41 5f 5d e9 ac a8 fc ff e8 37 26 91 ff e9 43 ff ff ff e8 2d 26 91 ff <0f> 0b e9 ba fc ff ff e8 21 26 91 ff 0f 0b e9 00 fd ff ff e8 15 26
RSP: 0018:ffffc900034073d8 EFLAGS: 00010283
RAX: ffffffff81f10cb3 RBX: ffff888054236d98 RCX: 0000000000080000
RDX: ffffc9000defb000 RSI: 00000000000128a1 RDI: 00000000000128a2
RBP: ffff888024c7bf01 R08: ffff888054236d9f R09: 1ffff1100a846db3
R10: dffffc0000000000 R11: ffffed100a846db4 R12: dffffc0000000000
R13: ffff888072a1a2f8 R14: ffffea0001c3bd18 R15: ffff888054236d98
FS: 00007fb0b9d396c0(0000) GS:ffff8880b8e00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fea0c3eb840 CR3: 0000000028191000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
mark_buffer_dirty_inode+0x82/0x2c0 fs/buffer.c:575
fat12_ent_put+0x1af/0x2e0 fs/fat/fatent.c:173
fat_ent_write+0xae/0x180 fs/fat/fatent.c:417
fat_chain_add+0x3e5/0x880 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+0x761/0xae0 fs/fat/inode.c:190
__block_write_begin_int+0x577/0x1ab0 fs/buffer.c:1991
__block_write_begin fs/buffer.c:2041 [inline]
block_write_begin+0x96/0x1e0 fs/buffer.c:2102
cont_write_begin+0x5f9/0x800 fs/buffer.c:2456
fat_write_begin+0x93/0x160 fs/fat/inode.c:235
generic_perform_write+0x2fa/0x5c0 mm/filemap.c:3862
__generic_file_write_iter+0x148/0x2a0 mm/filemap.c:3960
generic_file_write_iter+0xab/0x2e0 mm/filemap.c:3989
call_write_iter include/linux/fs.h:2271 [inline]
new_sync_write fs/read_write.c:491 [inline]
vfs_write+0x482/0x9a0 fs/read_write.c:584
ksys_write+0x14d/0x260 fs/read_write.c:637
do_syscall_x64 arch/x86/entry/common.c:46 [inline]
do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:76
entry_SYSCALL_64_after_hwframe+0x68/0xd2
RIP: 0033:0x7fb0b8d9e159
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:00007fb0b9d39028 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007fb0b9025fa0 RCX: 00007fb0b8d9e159
RDX: 00000000fffffceb RSI: 00002000000001c0 RDI: 0000000000000005
RBP: 00007fb0b8e3506b R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fb0b9026038 R14: 00007fb0b9025fa0 R15: 00007ffea4da7318
</TASK>


---
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[syzbot]

syzbot has found a reproducer for the following issue on:

HEAD commit: 1a8763b93150 Linux 6.1.188
git tree: linux-6.1.y

console output: https://syzkaller.appspot.com/x/log.txt?x=1644d3f5580000

kernel config: https://syzkaller.appspot.com/x/.config?x=31dfefc4a14efea3
dashboard link: https://syzkaller.appspot.com/bug?extid=06f803b4d15edad2e6da
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=1144d3f5580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=1544d3f5580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/be5de8c5e41e/disk-1a8763b9.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/b803e8d05dd4/vmlinux-1a8763b9.xz
kernel image: https://storage.googleapis.com/syzbot-assets/723f53728c4f/bzImage-1a8763b9.xz

mounted in repro: https://storage.googleapis.com/syzbot-assets/9d71ee9c1cf5/mount_0.gz
fsck result: failed (log: https://syzkaller.appspot.com/x/fsck.log?x=1696cbf5580000)

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+06f803...@syzkaller.appspotmail.com

------------[ cut here ]------------
WARNING: CPU: 1 PID: 4466 at fs/buffer.c:1081 mark_buffer_dirty+0x383/0x7a0 fs/buffer.c:1081
Modules linked in:
CPU: 1 PID: 4466 Comm: syz.1.18 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026

RIP: 0010:mark_buffer_dirty+0x383/0x7a0 fs/buffer.c:1081
Code: 13 6b e2 ff 49 8b 3f be 04 00 00 00 5b 41 5c 41 5d 41 5e 41 5f 5d e9 ac a8 fc ff e8 37 26 91 ff e9 43 ff ff ff e8 2d 26 91 ff <0f> 0b e9 ba fc ff ff e8 21 26 91 ff 0f 0b e9 00 fd ff ff e8 15 26

RSP: 0018:ffffc900035373d8 EFLAGS: 00010293
RAX: ffffffff81f10cb3 RBX: ffff88806f0dacb0 RCX: ffff888078ea3c00
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000002
RBP: ffff888024a5ca01 R08: ffff88806f0dacb7 R09: 1ffff1100de1b596
R10: dffffc0000000000 R11: ffffed100de1b597 R12: dffffc0000000000
R13: ffff888056f28368 R14: ffffea0001a32a98 R15: ffff88806f0dacb0
FS: 00007f9b3a34d6c0(0000) GS:ffff8880b8f00000(0000) knlGS:0000000000000000

CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033

CR2: 000055970d3e66f0 CR3: 0000000072c4a000 CR4: 00000000003506e0

RIP: 0033:0x7f9b3939e159

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:00007f9b3a34d028 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f9b39625fa0 RCX: 00007f9b3939e159

RDX: 00000000fffffceb RSI: 00002000000001c0 RDI: 0000000000000005

RBP: 00007f9b3943506b R08: 0000000000000000 R09: 0000000000000000

R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000

R13: 00007f9b39626038 R14: 00007f9b39625fa0 R15: 00007ffef2a78818
</TASK>


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