From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-178.mta1.migadu.com (out-178.mta1.migadu.com [95.215.58.178]) (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 8F8692848B4 for ; Tue, 28 Oct 2025 03:45:37 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.178 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761623141; cv=none; b=esIxHqliGUJrYlbpyvUTJrW9zhDBq+OzP336dcx234MM0AZReIOlaHs+w51jFL7uAa8jVKMqNUQ22jG23sF8ttYi2VDOwavNJ9rOKBDihLHOfZlUyBOTSpF1ItdquHhE93QU0nKDyHMp4jlYVQCBElDWgVrTzPArKyWN9fe2Uw4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761623141; c=relaxed/simple; bh=8X7+djckz8qvZbXGeuo8MkGK5Q5O+A+menBRyrcAjwQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=eEPxO6kr4C1KD9glZpveSKoC8CQ/ByR7D7kV3G91HWteaR+bA0Pn0DRfSGMMztmsFYuLvqpimTAzNJc8mMtIulaa0hMpWMYhzF0pJxKGG8ou+mPE/sLFCVtH5OxBiMZleg8VcpiXk3wqwn1/rJLuIUA4g9U1TuDxPJdRVeXx57I= 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=Veshu2V1; arc=none smtp.client-ip=95.215.58.178 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="Veshu2V1" Message-ID: <95e1fd95-896f-4d33-956f-a0ef0e0f152c@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1761623134; 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=C5Pi/MoPJGYSa9PpvlvLW9NetB22nOP2t8J0Mto24cA=; b=Veshu2V13TCLK5s+B3ZXjCM8uceV/g1f/j+0l+NnK5T9gkf0TDbWnmudXhc3DuQgMNS35F DsmDAqzvIDt/1WNgT0qytf/+/DneTnS2hywHWwnrGkgfREqpfRg3hHLKlPgesCbKGvlYlJ iodJ8n/iByyGx3TY6FfVB4SOV2zlzNg= Date: Mon, 27 Oct 2025 20:45:25 -0700 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: 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> 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 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](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. > > Let me know if you’d like me to run some more experiments on this.