[moderation] [ext4?] KCSAN: data-race in data_push_tail / uuid_string (6)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 9d87d0fc8c8e Merge tag 'liveupdate-fixes-2026-05-30' of gi..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=123696ec580000
kernel config: https://syzkaller.appspot.com/x/.config?x=f571f22917457cd8
dashboard link: https://syzkaller.appspot.com/bug?extid=8a796968f6ec05e16d7a
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
CC: [adilger...@dilger.ca ja...@suse.cz liba...@linux.alibaba.com linux...@vger.kernel.org linux-...@vger.kernel.org oja...@linux.ibm.com rites...@gmail.com ty...@mit.edu yi.z...@huawei.com]

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/bb8529f4c85b/disk-9d87d0fc.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/1512d1408db5/vmlinux-9d87d0fc.xz
kernel image: https://storage.googleapis.com/syzbot-assets/87a8ecec5517/bzImage-9d87d0fc.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+8a7969...@syzkaller.appspotmail.com

loop4: lost filesystem error report for type 5 error -117
EXT4-fs (loop4): 1 truncate cleaned up
EXT4-fs (loop4): mounted filesystem 00000000-0000-0000-0000-000000000000 r/w without journal. Quota mode: writeback.
ext4 filesystem being mounted at /426/file0 supports timestamps until 2038-01-19 (0x7fffffff)
==================================================================
BUG: KCSAN: data-race in data_push_tail / uuid_string

write to 0xffffffff8936fab0 of 1 bytes by task 3302 on cpu 0:
string_nocheck lib/vsprintf.c:659 [inline]
uuid_string+0x3c4/0x440 lib/vsprintf.c:1772
pointer+0x3b5/0xcb0 lib/vsprintf.c:2567
vsnprintf+0x491/0x860 lib/vsprintf.c:2952
va_format lib/vsprintf.c:1723 [inline]
pointer+0x821/0xcb0 lib/vsprintf.c:2569
vsnprintf+0x491/0x860 lib/vsprintf.c:2952
vscnprintf+0x41/0x90 lib/vsprintf.c:3013
printk_sprint+0x30/0x2b0 kernel/printk/printk.c:2222
vprintk_store+0x57b/0x910 kernel/printk/printk.c:2364
vprintk_emit+0x1a4/0x600 kernel/printk/printk.c:2455
vprintk_default+0x26/0x30 kernel/printk/printk.c:2494
vprintk+0x1d/0x30 kernel/printk/printk_safe.c:82
_printk+0x79/0xa0 kernel/printk/printk.c:2504
__ext4_msg+0x1aa/0x1c0 fs/ext4/super.c:1001
ext4_put_super+0x90/0x7b0 fs/ext4/super.c:1302
generic_shutdown_super+0xee/0x220 fs/super.c:646
kill_block_super+0x2a/0x70 fs/super.c:1725
ext4_kill_sb+0x42/0x80 fs/ext4/super.c:7492
deactivate_locked_super+0x75/0x1c0 fs/super.c:476
deactivate_super+0x97/0xa0 fs/super.c:509
cleanup_mnt+0x2bb/0x330 fs/namespace.c:1312
__cleanup_mnt+0x19/0x20 fs/namespace.c:1319
task_work_run+0x130/0x1a0 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:67 [inline]
exit_to_user_mode_loop+0x211/0x820 kernel/entry/common.c:98
__exit_to_user_mode_prepare include/linux/irq-entry-common.h:207 [inline]
syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:230 [inline]
syscall_exit_to_user_mode include/linux/entry-common.h:318 [inline]
do_syscall_64+0x232/0x3b0 arch/x86/entry/syscall_64.c:100
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffffffff8936fab0 of 8 bytes by task 10707 on cpu 1:
data_make_reusable kernel/printk/printk_ringbuffer.c:608 [inline]
data_push_tail+0x100/0x470 kernel/printk/printk_ringbuffer.c:693
data_alloc+0x11b/0x390 kernel/printk/printk_ringbuffer.c:1089
prb_reserve+0x8d6/0xad0 kernel/printk/printk_ringbuffer.c:1727
vprintk_store+0x54a/0x910 kernel/printk/printk.c:2354
vprintk_emit+0x1a4/0x600 kernel/printk/printk.c:2455
vprintk_default+0x26/0x30 kernel/printk/printk.c:2494
vprintk+0x1d/0x30 kernel/printk/printk_safe.c:82
_printk+0x79/0xa0 kernel/printk/printk.c:2504
mnt_warn_timestamp_expiry+0x1c3/0x200 fs/namespace.c:3314
do_new_mount_fc fs/namespace.c:3774 [inline]
do_new_mount+0x354/0x8d0 fs/namespace.c:3834
path_mount+0x4d0/0xbc0 fs/namespace.c:4154
do_mount fs/namespace.c:4167 [inline]
__do_sys_mount fs/namespace.c:4383 [inline]
__se_sys_mount+0x28c/0x2e0 fs/namespace.c:4360
__x64_sys_mount+0x67/0x80 fs/namespace.c:4360
x64_sys_call+0x2d61/0x3020 arch/x86/include/generated/asm/syscalls_64.h:166
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x12c/0x3b0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x3ffffffffffff859 -> 0x303030303030302d

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 10707 Comm: syz.4.2087 Not tainted syzkaller #0 PREEMPT(lazy)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
==================================================================


---
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 syzk...@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 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