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BUG: KCSAN: data-race in memchr / vsnprintf

write to 0xffffffff89393f16 of 53 bytes by task 26700 on cpu 1:
 vsnprintf+0x2ce/0x860 lib/vsprintf.c:2899
 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
 __nla_validate_parse+0x1735/0x1cf0 lib/nlattr.c:647
 __nla_parse+0x40/0x60 lib/nlattr.c:732
 __nlmsg_parse include/net/netlink.h:789 [inline]
 nlmsg_parse_deprecated include/net/netlink.h:830 [inline]
 xfrm_dump_sa+0x1ef/0x340 net/xfrm/xfrm_user.c:1519
 netlink_dump+0x455/0x870 net/netlink/af_netlink.c:2325
 __netlink_dump_start+0x43e/0x520 net/netlink/af_netlink.c:2440
 netlink_dump_start include/linux/netlink.h:341 [inline]
 xfrm_user_rcv_msg+0x2b4/0x670 net/xfrm/xfrm_user.c:3504
 netlink_rcv_skb+0x123/0x220 net/netlink/af_netlink.c:2550
 xfrm_netlink_rcv+0x48/0x60 net/xfrm/xfrm_user.c:3547
 netlink_unicast_kernel net/netlink/af_netlink.c:1318 [inline]
 netlink_unicast+0x5a8/0x680 net/netlink/af_netlink.c:1344
 netlink_sendmsg+0x5c8/0x6f0 net/netlink/af_netlink.c:1894
 sock_sendmsg_nosec net/socket.c:787 [inline]
 __sock_sendmsg net/socket.c:802 [inline]
 ____sys_sendmsg+0x563/0x5b0 net/socket.c:2698
 ___sys_sendmsg+0x195/0x1e0 net/socket.c:2752
 __sys_sendmsg net/socket.c:2784 [inline]
 __do_sys_sendmsg net/socket.c:2789 [inline]
 __se_sys_sendmsg net/socket.c:2787 [inline]
 __x64_sys_sendmsg+0xd4/0x160 net/socket.c:2787
 x64_sys_call+0x194c/0x3020 arch/x86/include/generated/asm/syscalls_64.h:47
 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

read to 0xffffffff89393f43 of 1 bytes by task 26698 on cpu 0:
 memchr+0x28/0x50 lib/string.c:809
 memchr include/linux/fortify-string.h:737 [inline]
 count_lines kernel/printk/printk_ringbuffer.c:1857 [inline]
 copy_data kernel/printk/printk_ringbuffer.c:1903 [inline]
 prb_read kernel/printk/printk_ringbuffer.c:1996 [inline]
 _prb_read_valid+0x7d6/0x950 kernel/printk/printk_ringbuffer.c:2173
 prb_read_valid_info+0x7a/0xa0 kernel/printk/printk_ringbuffer.c:2280
 find_first_fitting_seq+0x1cc/0x330 kernel/printk/printk.c:1567
 syslog_print_all+0x11b/0x3c0 kernel/printk/printk.c:1699
 do_syslog+0x2fd/0x7f0 kernel/printk/printk.c:1777
 __do_sys_syslog kernel/printk/printk.c:1855 [inline]
 __se_sys_syslog kernel/printk/printk.c:1853 [inline]
 __x64_sys_syslog+0x41/0x50 kernel/printk/printk.c:1853
 x64_sys_call+0x2b9e/0x3020 arch/x86/include/generated/asm/syscalls_64.h:104
 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: 0x3a -> 0x63

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 26698 Comm: syz.1.7493 Tainted: G        W           syzkaller #0 PREEMPT(full) 
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
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