Message-ID: <31779ad7-1e95-4033-8de6-a9afa3b89b8c@linux.dev>
Date: Thu, 30 Oct 2025 08:52:13 -0700
From: Yonghong Song <yonghong.song@...ux.dev>
To: Tao Chen <chen.dylane@...il.com>, Sahil Chandna <chandna.sahil@...il.com>
Cc: syzbot+b0cff308140f79a9c4cb@...kaller.appspotmail.com, andrii@...nel.org,
 ast@...nel.org, bpf@...r.kernel.org, daniel@...earbox.net,
 eddyz87@...il.com, haoluo@...gle.com, john.fastabend@...il.com,
 jolsa@...nel.org, kpsingh@...nel.org, linux-kernel@...r.kernel.org,
 listout@...tout.xyz, martin.lau@...ux.dev, netdev@...r.kernel.org,
 sdf@...ichev.me, song@...nel.org, syzkaller-bugs@...glegroups.com,
 linux-rt-devel@...ts.linux.dev, bigeasy@...utronix.de
Subject: Re: [syzbot] [bpf?] WARNING in bpf_bprintf_prepare (3)



On 10/30/25 1:50 AM, Tao Chen wrote:
> 在 2025/10/29 23:26, Yonghong Song 写道:
>>
>>
>> On 10/29/25 4:22 AM, Sahil Chandna wrote:
>>> On Mon, Oct 27, 2025 at 08:45:25PM -0700, Yonghong Song wrote:
>>>>
>>>>
>>>> On 10/26/25 1:05 PM, Sahil Chandna wrote:
>>>>> On Wed, Oct 22, 2025 at 12:56:25PM -0700, Yonghong Song wrote:
>>>>>>
>>>>>>
>>>>>> On 10/22/25 11:40 AM, Sahil Chandna wrote:
>>>>>>> On Wed, Oct 22, 2025 at 09:57:22AM -0700, Yonghong Song wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>> 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 <chandna.sahil@...il.com>
>>>>>>>>> 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: 
>>>>>>>>> syzbot+b0cff308140f79a9c4cb@...kaller.appspotmail.com
>>>>>>>>> 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().
>>>>>>> Right, but using preempt_disable() may impact builds with
>>>>>>> CONFIG_PREEMPT_RT=y, similar to bug[1]? Do you think 
>>>>>>> local_lock() could be used here
>>>>>>
>>>>>> We should be okay. for all the kfuncs/helpers I mentioned below,
>>>>>> with the help of AI, I didn't find any spin_lock in the code path
>>>>>> and all these helpers although they try to *print* some contents,
>>>>>> but the kfuncs/helpers itself is only to deal with buffers and
>>>>>> actual print will happen asynchronously.
>>>>>>
>>>>>>> as nest level is per cpu variable and local lock semantics can work
>>>>>>> for both RT and non rt builds ?
>>>>>>
>>>>>> I am not sure about local_lock() in RT as for RT, local_lock() could
>>>>>> be nested and the release may not in proper order. See
>>>>>>  https://www.kernel.org/doc/html/v5.8/locking/locktypes.html
>>>>>>
>>>>>>  local_lock is not suitable to protect against preemption or 
>>>>>> interrupts on a
>>>>>>  PREEMPT_RT kernel due to the PREEMPT_RT specific spinlock_t 
>>>>>> semantics.
>>>>>>
>>>>>> So I suggest to stick to preempt_disable/enable approach.
>>>>>>
>>>>>>>>
>>>>>>>> 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.
>>>>>>>>
>>>>>>> Sure, thanks!
>>>>>
>>>>> Since these helpers eventually call bpf_bprintf_prepare(),
>>>>> I figured adding protection around bpf_try_get_buffers(),
>>>>> which triggers the original warning, should be sufficient.
>>>>> I tried a few approaches to address the warning as below :
>>>>>
>>>>> 1. preempt_disable() / preempt_enable() around 
>>>>> bpf_prog_run_pin_on_cpu()
>>>>> diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
>>>>> index 1b61bb25ba0e..6a128179a26f 100644
>>>>> --- a/net/core/flow_dissector.c
>>>>> +++ b/net/core/flow_dissector.c
>>>>> @@ -1021,7 +1021,9 @@ u32 bpf_flow_dissect(struct bpf_prog *prog, 
>>>>> struct bpf_flow_dissector *ctx,
>>>>>                (int)FLOW_DISSECTOR_F_STOP_AT_ENCAP);
>>>>>       flow_keys->flags = flags;
>>>>>
>>>>> +    preempt_disable();
>>>>>       result = bpf_prog_run_pin_on_cpu(prog, ctx);
>>>>> +    preempt_enable();
>>>>>
>>>>>       flow_keys->nhoff = clamp_t(u16, flow_keys->nhoff, nhoff, hlen);
>>>>>       flow_keys->thoff = clamp_t(u16, flow_keys->thoff,
>>>>> This fixes the original WARN_ON in both PREEMPT_FULL and RT builds.
>>>>> However, when tested with the syz reproducer of the original bug 
>>>>> [1], it
>>>>> still triggers the expected 
>>>>> DEBUG_LOCKS_WARN_ON(this_cpu_read(softirq_ctrl.cnt)) warning from 
>>>>> __local_bh_disable_ip(), due to the preempt_disable() interacting 
>>>>> with RT spinlock semantics.
>>>>> [1] [https://syzkaller.appspot.com/bug?extid=1f1fbecb9413cdbfbef8
>>>>> So this approach avoids the buffer nesting issue, but 
>>>>> re-introduces the following issue:
>>>>> [  363.968103][T21257] 
>>>>> DEBUG_LOCKS_WARN_ON(this_cpu_read(softirq_ctrl.cnt))
>>>>> [  363.968922][T21257] WARNING: CPU: 0 PID: 21257 at kernel/ 
>>>>> softirq.c:176 __local_bh_disable_ip+0x3d9/0x540
>>>>> [  363.969046][T21257] Modules linked in:
>>>>> [  363.969176][T21257] Call Trace:
>>>>> [  363.969181][T21257]  <TASK>
>>>>> [  363.969186][T21257]  ? __local_bh_disable_ip+0xa1/0x540
>>>>> [  363.969197][T21257]  ? sock_map_delete_elem+0xa2/0x170
>>>>> [  363.969209][T21257]  ? preempt_schedule_common+0x83/0xd0
>>>>> [  363.969252][T21257]  ? rt_spin_unlock+0x161/0x200
>>>>> [  363.969269][T21257]  sock_map_delete_elem+0xaf/0x170
>>>>> [  363.969280][T21257] bpf_prog_464bc2be3fc7c272+0x43/0x47
>>>>> [  363.969289][T21257]  bpf_flow_dissect+0x22b/0x750
>>>>> [  363.969299][T21257] bpf_prog_test_run_flow_dissector+0x37c/0x5c0
>>>>>
>>>>> 2. preempt_disable() inside bpf_try_get_buffers() and 
>>>>> bpf_put_buffers()
>>>>>
>>>>> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
>>>>> index 8eb117c52817..bc8630833a94 100644
>>>>> --- a/kernel/bpf/helpers.c
>>>>> +++ b/kernel/bpf/helpers.c
>>>>> @@ -777,12 +777,14 @@ int bpf_try_get_buffers(struct 
>>>>> bpf_bprintf_buffers **bufs)
>>>>>  {
>>>>>         int nest_level;
>>>>>
>>>>> +       preempt_disable();
>>>>>         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]);
>>>>> +       preempt_enable();
>>>>>
>>>>>         return 0;
>>>>>  }
>>>>> @@ -791,7 +793,10 @@ void bpf_put_buffers(void)
>>>>>  {
>>>>>         if (WARN_ON_ONCE(this_cpu_read(bpf_bprintf_nest_level) == 0))
>>>>>                 return;
>>>>> +
>>>>> +       preempt_disable();
>>>>>         this_cpu_dec(bpf_bprintf_nest_level);
>>>>> +       preempt_enable();
>>>>>  }
>>>>> This *still* reproduces the original syz issue, so the protection 
>>>>> needs to be placed around the entire program run, not inside the 
>>>>> helper itself as
>>>>> in above experiment.
>>>>
>>>> This does not work. See my earlier suggestions.
>>>>
>>>>> 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().
>>>>
>>>> That is,
>>>>  preempt_disable();
>>>>  ...
>>>>  bpf_try_get_buffers()
>>>>  ...
>>>>  bpf_put_buffers()
>>>>  ...
>>>>  preempt_enable();
>>>>
>>>>>
>>>>> 3. Using a per-CPU local_lock
>>>>> Finally, I tested with a per-CPU local_lock around 
>>>>> bpf_prog_run_pin_on_cpu():
>>>>> +struct bpf_cpu_lock {
>>>>> +    local_lock_t lock;
>>>>> +};
>>>>> +
>>>>> +static DEFINE_PER_CPU(struct bpf_cpu_lock, bpf_cpu_lock) = {
>>>>> +    .lock = INIT_LOCAL_LOCK(),
>>>>> +};
>>>>> @@ -1021,7 +1030,9 @@ u32 bpf_flow_dissect(struct bpf_prog *prog, 
>>>>> struct bpf_flow_dissector *ctx,
>>>>>                      (int)FLOW_DISSECTOR_F_STOP_AT_ENCAP);
>>>>>         flow_keys->flags = flags;
>>>>>
>>>>> +       local_lock(&bpf_cpu_lock.lock);
>>>>>         result = bpf_prog_run_pin_on_cpu(prog, ctx);
>>>>> +       local_unlock(&bpf_cpu_lock.lock);
>>>>>
>>>>> This approach avoid the warning on both RT and non-RT builds, with 
>>>>> both the syz reproducer. The intention of introducing the per-CPU 
>>>>> local_lock is to maintain consistent per-CPU execution semantics 
>>>>> between RT and non-RT kernels.
>>>>> On non-RT builds, local_lock maps to preempt_disable()/enable(),
>>>>> which provides the same semantics as before.
>>>>> On RT builds, it maps to an RT-safe per-CPU spinlock, avoiding the
>>>>> softirq_ctrl.cnt issue.
>>>>
>>>> This should work, but local lock disable interrupts which could have
>>>> negative side effects on the system. We don't want this.
>>>> That is the reason we have 3 nested level for bpf_bprintf_buffers.
>>>>
>>>> Please try my above preempt_disalbe/enable() solution.
>>>>
>>> I tried following patch with reproducer from both syzbot [1] and [2]
>>> and issue *did not reproduce* with them.
>>>
>>> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
>>> index 8eb117c52817..4be6dde89d39 100644
>>> --- a/kernel/bpf/helpers.c
>>> +++ b/kernel/bpf/helpers.c
>>> @@ -777,9 +777,11 @@ int bpf_try_get_buffers(struct 
>>> bpf_bprintf_buffers **bufs)
>>>  {
>>>         int nest_level;
>>>
>>> +       preempt_disable();
>>>         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);
>>> +               preempt_enable();
>>>                 return -EBUSY;
>>>         }
>>>         *bufs = this_cpu_ptr(&bpf_bprintf_bufs[nest_level - 1]);
>>> @@ -792,6 +794,7 @@ void bpf_put_buffers(void)
>>>         if (WARN_ON_ONCE(this_cpu_read(bpf_bprintf_nest_level) == 0))
>>
>> For completeness, we need to add preempt_enable() here as well.
>>
>>> return;
>>>         this_cpu_dec(bpf_bprintf_nest_level);
>>> +       preempt_enable();
>>>  }
>>>
>>> [1] https://syzkaller.appspot.com/bug?extid=1f1fbecb9413cdbfbef8
>>> [2] https://syzkaller.appspot.com/bug?extid=b0cff308140f79a9c4cb
>>>>>
>>>>> Let me know if you’d like me to run some more experiments on this.
>>>>
>>> Shall I submit a patch with your suggested changes ?
>>
>> Please. The change looks good to me.
>>
>>>
>>> Regards,
>>> Sahil
>>
>>
>
> Hi Yonghong, Sahil
>
> Previously, I removed preempt_disable from bpf_try_get_buffers,
> In my understanding, it is safe
> to access this_cpu_inc_return(bpf_bprintf_nest_level), can we just
> remove the WARN_ON_ONCE? It seems that BPF allows preemption after
> run under migration disabled. Is it right?

Yes, even with migration disabled, preemption can be disabled on
top of that.

Probably we can remove WARN_ON_ONCE esp. with preemption disabled.
But this should be a separate patch.

>
> https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/commit/?id=4223bf833c8495e40ae2886acbc0ecbe88fa6306 
>
>

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