Message-ID: <aQH5EtKBbklfH0Wq@chandna.localdomain>
Date: Wed, 29 Oct 2025 16:52:58 +0530
From: Sahil Chandna <chandna.sahil@...il.com>
To: Yonghong Song <yonghong.song@...ux.dev>
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 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))
                 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 ?

Regards,
Sahil

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