[moderation] [fs?] KCSAN: data-race in __d_instantiate / link_path_walk

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 62085877ae65 Merge tag 'kbuild-fixes-6.19-2' of git://git...
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=101dcbfa580000
kernel config: https://syzkaller.appspot.com/x/.config?x=6c9e86480ac654d3
dashboard link: https://syzkaller.appspot.com/bug?extid=bcf34d9bb56be45d59b3
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
CC: [bra...@kernel.org ja...@suse.cz linux-...@vger.kernel.org linux-...@vger.kernel.org vi...@zeniv.linux.org.uk]

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/3cfa5ae35e8d/disk-62085877.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/5beaac31a028/vmlinux-62085877.xz
kernel image: https://storage.googleapis.com/syzbot-assets/d216fd89a57c/bzImage-62085877.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+bcf34d...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in __d_instantiate / link_path_walk

read-write to 0xffff88811afd2240 of 4 bytes by task 15879 on cpu 0:
fsnotify_update_flags include/linux/fsnotify_backend.h:-1 [inline]
__d_instantiate+0x2a8/0x390 fs/dcache.c:1977
d_make_persistent+0x78/0x110 fs/dcache.c:2793
shmem_symlink+0x3a4/0x3b0 mm/shmem.c:4142
vfs_symlink+0x1b4/0x1f0 fs/namei.c:5523
do_symlinkat+0xf0/0x460 fs/namei.c:5550
__do_sys_symlinkat fs/namei.c:5571 [inline]
__se_sys_symlinkat fs/namei.c:5568 [inline]
__x64_sys_symlinkat+0x5a/0x70 fs/namei.c:5568
x64_sys_call+0x2a36/0x3000 arch/x86/include/generated/asm/syscalls_64.h:267
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xc0/0x2a0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88811afd2240 of 4 bytes by task 15897 on cpu 1:
d_managed include/linux/dcache.h:403 [inline]
step_into fs/namei.c:2086 [inline]
walk_component fs/namei.c:2235 [inline]
link_path_walk+0x629/0xe30 fs/namei.c:2597
path_lookupat+0x8c/0x500 fs/namei.c:2753
filename_lookup+0x190/0x390 fs/namei.c:2783
user_path_at+0x3e/0x130 fs/namei.c:3576
__do_sys_chroot fs/open.c:601 [inline]
__se_sys_chroot+0x47/0x250 fs/open.c:595
__x64_sys_chroot+0x1f/0x30 fs/open.c:595
x64_sys_call+0x2238/0x3000 arch/x86/include/generated/asm/syscalls_64.h:162
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xc0/0x2a0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x00000080 -> 0x08300080

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 15897 Comm: syz.3.2482 Tainted: G W syzkaller #0 PREEMPT(voluntary)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
==================================================================


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