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

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Message-ID: <aJCvY7G-gVR8taLh@krava>
Date: Mon, 4 Aug 2025 15:02:27 +0200
From: Jiri Olsa <olsajiri@...il.com>
To: 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,
	yonghong.song@...ux.dev, 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 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


>  		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

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
>