Subject: Re: [syzbot] [bpf?] WARNING in bpf_bprintf_prepare (3)

From: Yonghong Song <[email-masked]>
Date: Wed, 22 Oct 2025 09:57:22 -0700

On 10/20/25 2:08 PM, syzbot wrote:
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    a1e83d4c0361 selftests/bpf: Fix redefinition of 'off' as d..
> git tree:       bpf
> console output: https://syzkaller.appspot.com/x/log.txt?x=12d21de2580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=9ad7b090a18654a7
> dashboard link: https://syzkaller.appspot.com/bug?extid=b0cff308140f79a9c4cb
> compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=160cf542580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=128d5c58580000
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/2f6a7a0cd1b7/disk-a1e83d4c.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/873984cfc71e/vmlinux-a1e83d4c.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/16711d84070c/bzImage-a1e83d4c.xz
>
> The issue was bisected to:
>
> commit 7c33e97a6ef5d84e98b892c3e00c6d1678d20395
> Author: Sahil Chandna <[email-masked]>
> Date:   Tue Oct 14 18:56:35 2025 +0000
>
>      bpf: Do not disable preemption in bpf_test_run().
>
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=172fe492580000
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=14afe492580000
> console output: https://syzkaller.appspot.com/x/log.txt?x=10afe492580000
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: [email-masked]
> Fixes: 7c33e97a6ef5 ("bpf: Do not disable preemption in bpf_test_run().")
>
> ------------[ cut here ]------------
> WARNING: CPU: 1 PID: 6145 at kernel/bpf/helpers.c:781 bpf_try_get_buffers kernel/bpf/helpers.c:781 [inline]
> WARNING: CPU: 1 PID: 6145 at kernel/bpf/helpers.c:781 bpf_bprintf_prepare+0x12cf/0x13a0 kernel/bpf/helpers.c:834

Okay, the warning is due to the following WARN_ON_ONCE:

static DEFINE_PER_CPU(struct bpf_bprintf_buffers[MAX_BPRINTF_NEST_LEVEL], bpf_bprintf_bufs);
static DEFINE_PER_CPU(int, bpf_bprintf_nest_level);

int bpf_try_get_buffers(struct bpf_bprintf_buffers **bufs)
{
         int nest_level;

         nest_level = this_cpu_inc_return(bpf_bprintf_nest_level);
         if (WARN_ON_ONCE(nest_level > MAX_BPRINTF_NEST_LEVEL)) {
                 this_cpu_dec(bpf_bprintf_nest_level);
                 return -EBUSY;
         }
         *bufs = this_cpu_ptr(&bpf_bprintf_bufs[nest_level - 1]);

         return 0;
}

Basically without preempt disable, at process level, it is possible
more than one process may trying to take bpf_bprintf_buffers.
Adding softirq and nmi, it is totally likely to have more than 3
level for buffers. Also, more than one process with bpf_bprintf_buffers
will cause problem in releasing buffers, so we need to have
preempt_disable surrounding bpf_try_get_buffers() and
bpf_put_buffers().

There are some kfuncs/helpers need such preempt_disable
protection, e.g. bpf_stream_printk, bpf_snprintf,
bpf_trace_printk, bpf_trace_vprintk, bpf_seq_printf.
But please double check.

> Modules linked in:
> CPU: 1 UID: 0 PID: 6145 Comm: syz.4.53 Not tainted syzkaller #0 PREEMPT(full)
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
> RIP: 0010:bpf_try_get_buffers kernel/bpf/helpers.c:781 [inline]
> RIP: 0010:bpf_bprintf_prepare+0x12cf/0x13a0 kernel/bpf/helpers.c:834
> Code: ff e9 ce fe ff ff e8 10 ec e0 ff e9 be fe ff ff e8 06 ec e0 ff e9 b4 fe ff ff e8 fc eb e0 ff e9 aa fe ff ff e8 f2 eb e0 ff 90 <0f> 0b 90 65 ff 0d 27 fd b2 10 b8 f0 ff ff ff e9 17 ff ff ff e8 d8
> RSP: 0018:ffffc90003797840 EFLAGS: 00010293
> RAX: ffffffff81df57fe RBX: ffffc90003797a10 RCX: ffff888026493c80
> RDX: 0000000000000000 RSI: 0000000000000004 RDI: 0000000000000003
> RBP: ffffc90003797970 R08: 0000000000585870 R09: 0000000000000005
> R10: dffffc0000000000 R11: fffff520006f2f20 R12: dffffc0000000000
> R13: 0000000000000004 R14: 0000000000000003 R15: 1ffff920006f2f42
> FS:  00005555805f5500(0000) GS:ffff888125e0c000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000000 CR3: 000000007c04e000 CR4: 00000000003526f0
> Call Trace:
>   <TASK>
>   ____bpf_trace_printk kernel/trace/bpf_trace.c:372 [inline]
>   bpf_trace_printk+0xdb/0x190 kernel/trace/bpf_trace.c:362
>   bpf_prog_bfbd7bf4bf171090+0x41/0x5a
>   bpf_dispatcher_nop_func include/linux/bpf.h:1350 [inline]
>   __bpf_prog_run include/linux/filter.h:721 [inline]
>   bpf_prog_run include/linux/filter.h:728 [inline]
>   bpf_prog_run_pin_on_cpu include/linux/filter.h:745 [inline]
>   bpf_flow_dissect+0x225/0x720 net/core/flow_dissector.c:1024
>   bpf_prog_test_run_flow_dissector+0x37c/0x5c0 net/bpf/test_run.c:1414
>   bpf_prog_test_run+0x2c7/0x340 kernel/bpf/syscall.c:4688
>   __sys_bpf+0x562/0x860 kernel/bpf/syscall.c:6167
>   __do_sys_bpf kernel/bpf/syscall.c:6259 [inline]
>   __se_sys_bpf kernel/bpf/syscall.c:6257 [inline]
>   __x64_sys_bpf+0x7c/0x90 kernel/bpf/syscall.c:6257
>   do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>   do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
>   entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7f25b0f8efc9
> 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:00007ffe036cd5e8 EFLAGS: 00000246 ORIG_RAX: 0000000000000141
> RAX: ffffffffffffffda RBX: 00007f25b11e5fa0 RCX: 00007f25b0f8efc9
> RDX: 0000000000000050 RSI: 0000200000000180 RDI: 000000000000000a
> RBP: 00007f25b1011f91 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> R13: 00007f25b11e5fa0 R14: 00007f25b11e5fa0 R15: 0000000000000003
>   </TASK>
>
>
> ---
> This report is generated by a bot. It may contain errors.
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>
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