linux-kernel - Re: [PATCH bpf-next] bpf: Disable migrate when kprobe_multi attach to access bpf_prog

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Message-ID: <3cf22505-e338-4cc1-ab76-896bfc336b40@linux.dev>
Date: Mon, 4 Aug 2025 21:05:00 -0700
From: Yonghong Song <yonghong.song@...ux.dev>
To: Jiri Olsa <olsajiri@...il.com>, Tao Chen <chen.dylane@...ux.dev>
Cc: ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
 martin.lau@...ux.dev, eddyz87@...il.com, song@...nel.org,
 john.fastabend@...il.com, kpsingh@...nel.org, sdf@...ichev.me,
 haoluo@...gle.com, mattbobrowski@...gle.com, rostedt@...dmis.org,
 mhiramat@...nel.org, mathieu.desnoyers@...icios.com, bpf@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next] bpf: Disable migrate when kprobe_multi attach to
 access bpf_prog_active



On 8/4/25 6:02 AM, Jiri Olsa wrote:
> On Mon, Aug 04, 2025 at 08:16:15PM +0800, Tao Chen wrote:
>> The syscall link_create not protected by bpf_disable_instrumentation,
>> accessing percpu data bpf_prog_active should use cpu local_lock when
>> kprobe_multi program attach.
>>
>> Fixes: 0dcac2725406 ("bpf: Add multi kprobe link")
>> Signed-off-by: Tao Chen <chen.dylane@...ux.dev>
>> ---
>>   kernel/trace/bpf_trace.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
>> index 3ae52978cae..f6762552e8e 100644
>> --- a/kernel/trace/bpf_trace.c
>> +++ b/kernel/trace/bpf_trace.c
>> @@ -2728,23 +2728,23 @@ kprobe_multi_link_prog_run(struct bpf_kprobe_multi_link *link,
>>   	struct pt_regs *regs;
>>   	int err;
>>   
>> +	migrate_disable();
>>   	if (unlikely(__this_cpu_inc_return(bpf_prog_active) != 1)) {
> this is called all the way from graph tracer, which disables preemption in
> function_graph_enter_regs, so I think we can safely use __this_cpu_inc_return

Agree. migrate_disable() is not needed here. But it would be great to add some
comments here since for most other prog_run, they typically have migrate_disable/enable.

>
>
>>   		bpf_prog_inc_misses_counter(link->link.prog);
>>   		err = 1;
>>   		goto out;
>>   	}
>>   
>> -	migrate_disable();
> hum, but now I'm not sure why we disable migration in here then

Probably a oversight.

>
> jirka
>
>>   	rcu_read_lock();
>>   	regs = ftrace_partial_regs(fregs, bpf_kprobe_multi_pt_regs_ptr());
>>   	old_run_ctx = bpf_set_run_ctx(&run_ctx.session_ctx.run_ctx);
>>   	err = bpf_prog_run(link->link.prog, regs);
>>   	bpf_reset_run_ctx(old_run_ctx);
>>   	rcu_read_unlock();
>> -	migrate_enable();
>>   
>>    out:
>>   	__this_cpu_dec(bpf_prog_active);
>> +	migrate_enable();
>>   	return err;
>>   }
>>   
>> -- 
>> 2.48.1
>>