linux-kernel - Re: [PATCH 2/2] bpf: Implement kretprobe fallback for kprobe multi link

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Message-ID: <CAADnVQKC312JbOhjQZmMN-Me2V0GQ9qxoHeQkF+=PbYk0zc9KA@mail.gmail.com>
Date: Thu, 18 Dec 2025 09:53:16 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: liujing40 <liujing.root@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Masami Hiramatsu <mhiramat@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Eduard <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, bpf <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, 
	linux-trace-kernel <linux-trace-kernel@...r.kernel.org>, liujing40@...omi.com
Subject: Re: [PATCH 2/2] bpf: Implement kretprobe fallback for kprobe multi link

On Thu, Dec 18, 2025 at 5:07 AM liujing40 <liujing.root@...il.com> wrote:
>
> When fprobe is not available, provide a fallback implementation of
> kprobe_multi using the traditional kretprobe API.
>
> Uses kretprobe's entry_handler and handler callbacks to simulate fprobe's
> entry/exit functionality.
>
> Signed-off-by: Jing Liu <liujing40@...omi.com>
> ---
>  kernel/trace/bpf_trace.c | 307 +++++++++++++++++++++++++++++++++++++--
>  1 file changed, 295 insertions(+), 12 deletions(-)
>
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 1fd07c10378f..426a1c627508 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -2274,12 +2274,44 @@ struct bpf_session_run_ctx {
>         void *data;
>  };
>
> -#ifdef CONFIG_FPROBE
> +#if defined(CONFIG_FPROBE) || defined(CONFIG_KRETPROBES)
> +#ifndef CONFIG_FPROBE
> +struct bpf_kprobe {
> +       struct bpf_kprobe_multi_link *link;
> +       u64 cookie;
> +       struct kretprobe rp;
> +};
> +
> +static void bpf_kprobe_unregister(struct bpf_kprobe *kps, u32 cnt)
> +{
> +       for (int i = 0; i < cnt; i++)
> +               unregister_kretprobe(&kps[i].rp);
> +}

Nack.
This is not a good idea.
unregister_kretprobe() calls synchronize_rcu().
So the above loop will cause soft lockups for sure.

pw-bot: cr