From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-178.mta0.migadu.com (out-178.mta0.migadu.com [91.218.175.178]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B4064165F1A for ; Mon, 3 Nov 2025 04:40:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.178 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762144805; cv=none; b=dbRhivr1v08q3qc10GDjrwgIzkv16RSO1/f8YVNYX1EyUmPNam6t7/yucwl4u9fcRhB+NljHYAu30yzcqnECE+UYj5Sn2oEH/kqo0vNGT2a1dnBsL7UWru+lwo951mRgw0COMGTfMx1ecN7ZE9hN2famMi77/Q8MIp1enUe9b3c= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762144805; c=relaxed/simple; bh=NV65H0axuM1L3Gpnmyy+NACZl8BqexSnyfmA8CLRVbs=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=iPH9DFnd4Q8VzN20MwZgnGLr1k/0w6oEtkZy43pHDOYb3JcW6th3qX/dHzsIwa2tS5vlmpeWMJrZ2y9alfKp3aA6GgnqowSqDsyU638E0MavS5g4u1OyhF+W0Rn5H5BHSfx+HqBAcdzxWwZMY37OKYrrpHiejinebVrJ+ee7G4w= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=R4S7ha6F; arc=none smtp.client-ip=91.218.175.178 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="R4S7ha6F" Message-ID: <04a0f37c-e4ee-4fc9-9b8e-773a2065cc30@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1762144798; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Q2dlMWJbn4mOxIcKUF/auRNgxASC9MzVFXa3Dj2fufk=; b=R4S7ha6Ftsz0DaXOCCh8Uq+NVQJ26kQ+3sSDrAvtzAQmYap+AKZsrZLF2yr6DnC6rLlcSx WWZBf40CgDDm3iaCZaHaNNMp+6pXsJXH53uzej2vMRy2rV6Kx5QSJ5C3dKQDYoxmPudZxC dJdFQdZt2GrxUijSQPKa0XmbZi2aFEQ= Date: Sun, 2 Nov 2025 20:39:50 -0800 Precedence: bulk X-Mailing-List: linux-rt-devel@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [syzbot] [bpf?] WARNING in bpf_bprintf_prepare (3) Content-Language: en-GB To: Sahil Chandna Cc: Tao Chen , syzbot+b0cff308140f79a9c4cb@syzkaller.appspotmail.com, andrii@kernel.org, ast@kernel.org, bpf@vger.kernel.org, daniel@iogearbox.net, eddyz87@gmail.com, haoluo@google.com, john.fastabend@gmail.com, jolsa@kernel.org, kpsingh@kernel.org, linux-kernel@vger.kernel.org, listout@listout.xyz, martin.lau@linux.dev, netdev@vger.kernel.org, sdf@fomichev.me, song@kernel.org, syzkaller-bugs@googlegroups.com, linux-rt-devel@lists.linux.dev, bigeasy@linutronix.de References: <68f6a4c8.050a0220.1be48.0011.GAE@google.com> <14371cf8-e49a-4c68-b763-fa7563a9c764@linux.dev> <8dd359dd-b42f-4676-bb94-07288b38fac1@linux.dev> <95e1fd95-896f-4d33-956f-a0ef0e0f152c@linux.dev> <541b7765-28eb-4d1f-9409-863db6798395@linux.dev> <31779ad7-1e95-4033-8de6-a9afa3b89b8c@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Yonghong Song In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 11/2/25 5:49 PM, Sahil Chandna wrote: > On Thu, Oct 30, 2025 at 08:52:13AM -0700, Yonghong Song wrote: >> >> >> 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 >>>>>>>>>>> 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@syzkaller.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]  >>>>>>> [  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. >> > Hi Yonghong, Tao, > I printed nested level with the preempt_disable()/enable() patch and > found nested level remains 1 with this patch(below). I tried this with > original > syzbot reproducer and ran for couple of hours. It is not easy to reproduce it with preempt_disable() since the nested bpf_try_get_buffers(...) needs process context softirq context nmi context but it is hard to predict when nmi/softirq will be nested inside process context where the execution is within (bpf_try_get_buffers, bpf_put_buffers) > diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c > index 4be6dde89d39..657d2100f33c 100644 > --- a/kernel/bpf/helpers.c > +++ b/kernel/bpf/helpers.c > @@ -779,6 +779,7 @@ int bpf_try_get_buffers(struct bpf_bprintf_buffers > **bufs) > >         preempt_disable(); >         nest_level = this_cpu_inc_return(bpf_bprintf_nest_level); > +       pr_info("bpf nest inc cpu=%d level=%d\n", smp_processor_id(), > nest_level); >         if (WARN_ON_ONCE(nest_level > MAX_BPRINTF_NEST_LEVEL)) { >                 this_cpu_dec(bpf_bprintf_nest_level); >                 preempt_enable(); > > I am waiting for Sebastian review on this thread before sending out a > patch with > preempt_disable(), Shall I also > send out patch after that for removing the WARN_ON_ONCE ? If everything is correct, WARN_ON_ONCE indeed is not needed (when with preempt_disable()). But the point of WARN_ON_ONCE is to alert something may go wrong. Not sure whether it is worthwhile to remove them or not. I think this can be a separate patch if you want to do it. >>> >>> https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/commit/?id=4223bf833c8495e40ae2886acbc0ecbe88fa6306 >>> >>> >>> >>