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

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 8f0b4cce4481 Linux 6.19-rc1
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=164b011a580000
kernel config: https://syzkaller.appspot.com/x/.config?x=b319ff1b6a2797ca
dashboard link: https://syzkaller.appspot.com/bug?extid=f87e35585919acb181af
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
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/0397a0339bfd/disk-8f0b4cce.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/1c266f6e8d6b/vmlinux-8f0b4cce.xz
kernel image: https://storage.googleapis.com/syzbot-assets/a748ab0188b2/bzImage-8f0b4cce.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+f87e35...@syzkaller.appspotmail.com

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

read-write to 0xffff888107a06cc0 of 4 bytes by task 3310 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:4146
vfs_symlink+0x1b4/0x1f0 fs/namei.c:5514
do_symlinkat+0xf0/0x470 fs/namei.c:5541
__do_sys_symlink fs/namei.c:5567 [inline]
__se_sys_symlink fs/namei.c:5565 [inline]
__x64_sys_symlink+0x50/0x60 fs/namei.c:5565
x64_sys_call+0x24a9/0x3000 arch/x86/include/generated/asm/syscalls_64.h:89
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xd8/0x2c0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888107a06cc0 of 4 bytes by task 3006 on cpu 1:
d_managed include/linux/dcache.h:403 [inline]
step_into fs/namei.c:2078 [inline]
walk_component fs/namei.c:2227 [inline]
lookup_last fs/namei.c:2722 [inline]
path_lookupat+0x1d8/0x500 fs/namei.c:2746
filename_lookup+0x147/0x340 fs/namei.c:2775
do_readlinkat+0x7d/0x320 fs/stat.c:576
__do_sys_readlink fs/stat.c:613 [inline]
__se_sys_readlink fs/stat.c:610 [inline]
__x64_sys_readlink+0x47/0x60 fs/stat.c:610
x64_sys_call+0x2af1/0x3000 arch/x86/include/generated/asm/syscalls_64.h:90
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xd8/0x2c0 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: 3006 Comm: udevd Not tainted syzkaller #0 PREEMPT(voluntary)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
==================================================================


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