[moderation] [block?] KCSAN: data-race in memchr / vsnprintf

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: fd94619c4336 Merge tag 'zonefs-6.18-rc1' of git://git.kern..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=13d64a14580000
kernel config: https://syzkaller.appspot.com/x/.config?x=6f216ba0cc2d66e7
dashboard link: https://syzkaller.appspot.com/bug?extid=7bd4b5e87410b8f19c84
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
CC: [ax...@kernel.dk linux...@vger.kernel.org linux-...@vger.kernel.org]

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/2e4dd15782bf/disk-fd94619c.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/c9079bf2d397/vmlinux-fd94619c.xz
kernel image: https://storage.googleapis.com/syzbot-assets/fed533b7bf74/bzImage-fd94619c.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+7bd4b5...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in memchr / vsnprintf

write to 0xffffffff88e27527 of 32 bytes by task 31363 on cpu 0:
vsnprintf+0x2ce/0x890 lib/vsprintf.c:2878
vscnprintf+0x41/0x90 lib/vsprintf.c:2991
printk_sprint+0x30/0x2d0 kernel/printk/printk.c:2189
vprintk_store+0x599/0x860 kernel/printk/printk.c:2309
vprintk_emit+0x10d/0x580 kernel/printk/printk.c:2399
vprintk_default+0x26/0x30 kernel/printk/printk.c:2438
vprintk+0x1d/0x30 kernel/printk/printk_safe.c:82
_printk+0x79/0xa0 kernel/printk/printk.c:2448
set_capacity_and_notify+0x14c/0x1f0 block/genhd.c:93
loop_set_size+0x2e/0x70 drivers/block/loop.c:220
loop_configure+0x825/0x9c0 drivers/block/loop.c:1077
lo_ioctl+0x1e1/0x12b0 drivers/block/loop.c:1532
blkdev_ioctl+0x356/0x440 block/ioctl.c:705
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl+0xce/0x140 fs/ioctl.c:583
__x64_sys_ioctl+0x43/0x50 fs/ioctl.c:583
x64_sys_call+0x1816/0x3000 arch/x86/include/generated/asm/syscalls_64.h:17
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xd2/0x200 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffffffff88e27537 of 1 bytes by task 31361 on cpu 1:
memchr+0x28/0x50 lib/string.c:809
memchr include/linux/fortify-string.h:739 [inline]
count_lines kernel/printk/printk_ringbuffer.c:1815 [inline]
copy_data kernel/printk/printk_ringbuffer.c:1861 [inline]
prb_read kernel/printk/printk_ringbuffer.c:1954 [inline]
_prb_read_valid+0x65d/0x920 kernel/printk/printk_ringbuffer.c:2131
prb_read_valid_info+0x66/0x90 kernel/printk/printk_ringbuffer.c:2238
do_syslog+0x6e2/0x7e0 kernel/printk/printk.c:1826
__do_sys_syslog kernel/printk/printk.c:1850 [inline]
__se_sys_syslog kernel/printk/printk.c:1848 [inline]
__x64_sys_syslog+0x41/0x50 kernel/printk/printk.c:1848
x64_sys_call+0x2f45/0x3000 arch/x86/include/generated/asm/syscalls_64.h:104
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xd2/0x200 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x3a -> 0x79

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 31361 Comm: syz.5.9228 Not tainted syzkaller #0 PREEMPT(voluntary)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
==================================================================


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