[moderation] [fs?] KCSAN: data-race in __d_lookup / __d_rehash (12)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: f105f3631d51 Merge tag 'x86-urgent-2026-07-05' of git://gi..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=16074b39580000
kernel config: https://syzkaller.appspot.com/x/.config?x=84b3039e8461eef5
dashboard link: https://syzkaller.appspot.com/bug?extid=72bbfd57f17fc438a0a4
compiler: Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
CC: [da...@kernel.org drive...@lists.linux.dev gre...@linuxfoundation.org linux-...@vger.kernel.org linux-...@vger.kernel.org raf...@kernel.org]

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/4f431f8e6ae2/disk-f105f363.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/6fa69bae0583/vmlinux-f105f363.xz
kernel image: https://storage.googleapis.com/syzbot-assets/5e6879481290/bzImage-f105f363.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+72bbfd...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in __d_lookup / __d_rehash

write to 0xffff88811a845a90 of 8 bytes by task 15291 on cpu 0:
hlist_bl_add_head_rcu include/linux/rculist_bl.h:91 [inline]
__d_rehash+0xa3/0x1d0 fs/dcache.c:2709
__d_add+0x326/0x490 fs/dcache.c:2937
d_splice_alias_ops+0xe3/0x370 fs/dcache.c:3286
d_splice_alias+0x1f/0x30 fs/dcache.c:3315
simple_lookup+0xb7/0xd0 fs/libfs.c:81
lookup_one_qstr_excl+0xc7/0x240 fs/namei.c:1810
__start_dirop fs/namei.c:2920 [inline]
start_dirop+0x3e/0x70 fs/namei.c:2942
simple_start_creating+0xa2/0xd0 fs/libfs.c:2305
debugfs_start_creating+0xda/0x150 fs/debugfs/inode.c:394
debugfs_create_symlink+0x39/0x150 fs/debugfs/inode.c:672
nsim_dev_port_debugfs_init drivers/net/netdevsim/dev.c:437 [inline]
__nsim_dev_port_add+0x4fb/0x700 drivers/net/netdevsim/dev.c:1505
nsim_dev_port_add_all+0x33/0xc0 drivers/net/netdevsim/dev.c:1570
nsim_drv_probe+0x68f/0x7c0 drivers/net/netdevsim/dev.c:1731
nsim_bus_probe+0x15/0x20 drivers/net/netdevsim/bus.c:420
call_driver_probe drivers/base/dd.c:-1 [inline]
really_probe+0x1ba/0x600 drivers/base/dd.c:706
__driver_probe_device+0x14f/0x1c0 drivers/base/dd.c:868
driver_probe_device+0x37/0x2f0 drivers/base/dd.c:898
__device_attach_driver+0x227/0x330 drivers/base/dd.c:1026
bus_for_each_drv+0x1c0/0x230 drivers/base/bus.c:500
__device_attach+0x1d1/0x2a0 drivers/base/dd.c:1098
device_initial_probe+0x70/0x90 drivers/base/dd.c:1153
bus_probe_device+0x101/0x1b0 drivers/base/bus.c:620
device_add+0x5e8/0x790 drivers/base/core.c:3772
device_register+0x1d/0x30 drivers/base/core.c:3854
nsim_bus_dev_new drivers/net/netdevsim/bus.c:471 [inline]
new_device_store+0x276/0x470 drivers/net/netdevsim/bus.c:191
bus_attr_store+0x57/0x80 drivers/base/bus.c:172
sysfs_kf_write+0xfe/0x120 fs/sysfs/file.c:145
kernfs_fop_write_iter+0x1d2/0x2e0 fs/kernfs/file.c:345
new_sync_write fs/read_write.c:595 [inline]
vfs_write+0x57d/0x990 fs/read_write.c:687
ksys_write+0xdc/0x1a0 fs/read_write.c:739
__do_sys_write fs/read_write.c:750 [inline]
__se_sys_write fs/read_write.c:747 [inline]
__x64_sys_write+0x40/0x50 fs/read_write.c:747
x64_sys_call+0x27e1/0x3020 arch/x86/include/generated/asm/syscalls_64.h:2
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x136/0x3c0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88811a845a90 of 8 bytes by task 2981 on cpu 1:
hlist_bl_unhashed include/linux/list_bl.h:54 [inline]
d_unhashed include/linux/dcache.h:396 [inline]
__d_lookup+0xf8/0x340 fs/dcache.c:2620
lookup_fast+0x5e/0x320 fs/namei.c:1878
walk_component fs/namei.c:2278 [inline]
link_path_walk+0x523/0xdd0 fs/namei.c:2656
path_lookupat+0x53/0x4c0 fs/namei.c:2812
filename_lookup+0x18f/0x390 fs/namei.c:2842
do_readlinkat+0x74/0x2f0 fs/stat.c:571
__do_sys_readlink fs/stat.c:605 [inline]
__se_sys_readlink fs/stat.c:602 [inline]
__x64_sys_readlink+0x47/0x60 fs/stat.c:602
x64_sys_call+0x2b51/0x3020 arch/x86/include/generated/asm/syscalls_64.h:90
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x136/0x3c0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0xffff88823753f650 -> 0xffff888105f1a848

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 2981 Comm: udevd Tainted: G W syzkaller #0 PREEMPT(lazy)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
==================================================================


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