linux-kernel - Re: [PATCH bpf-next v4 2/2] bpf: Add show_fdinfo for kprobe

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Message-ID: <CAADnVQ+SgYop50-1S7424ecRKWDM3R=8cHeizRCjeC_FXdkdLA@mail.gmail.com>
Date: Fri, 20 Jun 2025 11:25:06 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Tao Chen <chen.dylane@...ux.dev>
Cc: Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...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>, 
	Matt Bobrowski <mattbobrowski@...gle.com>, Steven Rostedt <rostedt@...dmis.org>, 
	Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 
	bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, 
	linux-trace-kernel <linux-trace-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v4 2/2] bpf: Add show_fdinfo for kprobe_multi

On Thu, Jun 19, 2025 at 8:31 PM Tao Chen <chen.dylane@...ux.dev> wrote:
>
> 在 2025/6/20 10:59, Alexei Starovoitov 写道:
> > On Thu, Jun 19, 2025 at 7:46 PM Tao Chen <chen.dylane@...ux.dev> wrote:
> >>
> >> 在 2025/6/20 01:17, Alexei Starovoitov 写道:
> >>> On Wed, Jun 18, 2025 at 8:44 PM Tao Chen <chen.dylane@...ux.dev> wrote:
> >>>>
> >>>> Show kprobe_multi link info with fdinfo, the info as follows:
> >>>>
> >>>> link_type:      kprobe_multi
> >>>> link_id:        1
> >>>> prog_tag:       a15b7646cb7f3322
> >>>> prog_id:        21
> >>>> type:   kprobe_multi
> >>>
> >>> ..
> >>>
> >>>> +       seq_printf(seq,
> >>>> +                  "type:\t%s\n"
> >>>> +                  "kprobe_cnt:\t%u\n"
> >>>> +                  "missed:\t%lu\n",
> >>>> +                  kmulti_link->flags == BPF_F_KPROBE_MULTI_RETURN ? "kretprobe_multi" :
> >>>> +                                        "kprobe_multi",
> >>>
> >>> why print the same info twice ?
> >>> seq_printf(m, "link_type:\t%s\n", bpf_link_type_strs[type]);
> >>> in bpf_link_show_fdinfo() already did it in a cleaner way.
> >>>
> >>
> >> link_type only shows 'kprobe_multi', maybe we can show the format like:
> >
> > Ohh. Especially so. It would be wrong and confusing to display:
> > link_type:      kprobe_multi
> > type: kretprobe_multi
> >
> > Let's fix 'link_type' to display it properly.
>
> What do you think show like this:
>
>      link_type:      kprobe_multi
>      link_id:        1
>      prog_tag:       33be53a4fd673e1d
>      prog_id:        21
>      retprobe:       false

It leaks implementation details.
For the kernel the link type is BPF_LINK_TYPE_KPROBE_MULTI for retprobe too,
but show_fdinfo is for humans.
'link_type:' field can be more precise and differentiate
what's effectively a subtype of the link.