[syzbot] [trace?] [bpf?] possible deadlock in down_trylock (3)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 84b92a499e7e Add linux-next specific files for 20250731
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=11065aa2580000
kernel config: https://syzkaller.appspot.com/x/.config?x=b335f01a07f73eac
dashboard link: https://syzkaller.appspot.com/bug?extid=c3740bc819eb55460ec3
compiler: Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=14167834580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=16f27cf0580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/97d9ce461c85/disk-84b92a49.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/0ca812ed76e7/vmlinux-84b92a49.xz
kernel image: https://storage.googleapis.com/syzbot-assets/0959d28a047f/bzImage-84b92a49.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+c3740b...@syzkaller.appspotmail.com

FAULT_INJECTION: forcing a failure.
name fail_usercopy, interval 1, probability 0, space 0, times 0
============================================
WARNING: possible recursive locking detected
6.16.0-next-20250731-syzkaller #0 Not tainted
--------------------------------------------
syz.3.22/6137 is trying to acquire lock:
ffffffff8e12e278 ((console_sem).lock){-...}-{2:2}, at: down_trylock+0x20/0xb0 kernel/locking/semaphore.c:176

but task is already holding lock:
ffffffff8e12e278 ((console_sem).lock){-...}-{2:2}, at: down+0x39/0xd0 kernel/locking/semaphore.c:96

other info that might help us debug this:
Possible unsafe locking scenario:

CPU0
----
lock((console_sem).lock);
lock((console_sem).lock);

*** DEADLOCK ***

May be due to missing lock nesting notation

2 locks held by syz.3.22/6137:
#0: ffffffff8e12e278 ((console_sem).lock){-...}-{2:2}, at: down+0x39/0xd0 kernel/locking/semaphore.c:96
#1: ffffffff8e139f20 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:331 [inline]
#1: ffffffff8e139f20 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:841 [inline]
#1: ffffffff8e139f20 (rcu_read_lock){....}-{1:3}, at: __bpf_trace_run kernel/trace/bpf_trace.c:2256 [inline]
#1: ffffffff8e139f20 (rcu_read_lock){....}-{1:3}, at: bpf_trace_run2+0x186/0x4b0 kernel/trace/bpf_trace.c:2298

stack backtrace:
CPU: 0 UID: 0 PID: 6137 Comm: syz.3.22 Not tainted 6.16.0-next-20250731-syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
Call Trace:
<TASK>
dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
print_deadlock_bug+0x28b/0x2a0 kernel/locking/lockdep.c:3041
check_deadlock kernel/locking/lockdep.c:3093 [inline]
validate_chain+0x1a3f/0x2140 kernel/locking/lockdep.c:3895
__lock_acquire+0xab9/0xd20 kernel/locking/lockdep.c:5237
lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5868
__raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
_raw_spin_lock_irqsave+0xa7/0xf0 kernel/locking/spinlock.c:162
down_trylock+0x20/0xb0 kernel/locking/semaphore.c:176
__down_trylock_console_sem+0xd0/0x1e0 kernel/printk/printk.c:326
console_trylock kernel/printk/printk.c:2868 [inline]
console_trylock_spinning kernel/printk/printk.c:2009 [inline]
vprintk_emit+0x320/0x7a0 kernel/printk/printk.c:2449
_printk+0xcf/0x120 kernel/printk/printk.c:2475
fail_dump lib/fault-inject.c:66 [inline]
should_fail_ex+0x3f5/0x560 lib/fault-inject.c:174
strncpy_from_user+0x36/0x290 lib/strncpy_from_user.c:118
strncpy_from_user_nofault+0x72/0x150 mm/maccess.c:192
bpf_trace_copy_string kernel/bpf/helpers.c:755 [inline]
bpf_bprintf_prepare+0xbbc/0x13d0 kernel/bpf/helpers.c:976
____bpf_trace_printk kernel/trace/bpf_trace.c:373 [inline]
bpf_trace_printk+0xdb/0x190 kernel/trace/bpf_trace.c:363
bpf_prog_7c77c7e0f6645ad8+0x3e/0x44
bpf_dispatcher_nop_func include/linux/bpf.h:1322 [inline]
__bpf_prog_run include/linux/filter.h:718 [inline]
bpf_prog_run include/linux/filter.h:725 [inline]
__bpf_trace_run kernel/trace/bpf_trace.c:2257 [inline]
bpf_trace_run2+0x284/0x4b0 kernel/trace/bpf_trace.c:2298
__bpf_trace_contention_begin+0xdc/0x130 include/trace/events/lock.h:95
__do_trace_contention_begin include/trace/events/lock.h:95 [inline]
trace_contention_begin include/trace/events/lock.h:95 [inline]
__down_common+0x5ad/0x6a0 kernel/locking/semaphore.c:292
down+0x80/0xd0 kernel/locking/semaphore.c:100
console_lock+0x145/0x1b0 kernel/printk/printk.c:2849
do_fb_ioctl+0x509/0x750 drivers/video/fbdev/core/fb_chrdev.c:123
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:598 [inline]
__se_sys_ioctl+0xf9/0x170 fs/ioctl.c:584
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f92c678eb69
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffda68c54f8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f92c69b5fa0 RCX: 00007f92c678eb69
RDX: 0000200000000080 RSI: 0000000000004606 RDI: 0000000000000005
RBP: 00007ffda68c5550 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
R13: 00007f92c69b5fa0 R14: 00007f92c69b5fa0 R15: 0000000000000003
</TASK>


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[Hillf Danton]

> Date: Mon, 04 Aug 2025 09:50:30 -0700 [thread overview]

Because down_trylock can be used from interrupt context and the semaphore can
be released by any task or interrupt, testing with bpf enabled wastes minutes.

[Yonghong Song]

There is a similar discussion in the following old thread:
https://lore.kernel.org/bpf/345098dc-8cb4-4808...@I-love.SAKURA.ne.jp/

In that case, the recursive lock is rq lock. Looks like there is no good solution for
that thread.

Not sure how this semaphore deadlock could be resolved.