netdev - Re: [RFC PATCH bpf-next 1/2] libbpf: Add support for dynamic tracepoint

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Message-ID: <CALOAHbAW2f-yupU6aedS=-YKGC58DEqcTx6W8nZH86UZQJYexA@mail.gmail.com>
Date: Tue, 7 Jan 2025 21:32:32 +0800
From: Yafang Shao <laoar.shao@...il.com>
To: Jiri Olsa <olsajiri@...il.com>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>, Andrii Nakryiko <andrii@...nel.org>, 
	Eddy Z <eddyz87@...il.com>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Martin KaFai Lau <martin.lau@...ux.dev>, 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>, 
	Eric Dumazet <edumazet@...gle.com>, bpf <bpf@...r.kernel.org>, 
	Network Development <netdev@...r.kernel.org>, Viktor Malik <vmalik@...hat.com>
Subject: Re: [RFC PATCH bpf-next 1/2] libbpf: Add support for dynamic tracepoint

On Tue, Jan 7, 2025 at 8:16 PM Jiri Olsa <olsajiri@...il.com> wrote:
>
> On Mon, Jan 06, 2025 at 10:32:15AM +0800, Yafang Shao wrote:
> > On Mon, Jan 6, 2025 at 8:16 AM Alexei Starovoitov
> > <alexei.starovoitov@...il.com> wrote:
> > >
> > > On Sun, Jan 5, 2025 at 4:44 AM Yafang Shao <laoar.shao@...il.com> wrote:
> > > >
> > > > Dynamic tracepoints can be created using debugfs. For example:
> > > >
> > > >    echo 'p:myprobe kernel_clone args' >> /sys/kernel/debug/tracing/kprobe_events
> > > >
> > > > This command creates a new tracepoint under debugfs:
> > > >
> > > >   $ ls /sys/kernel/debug/tracing/events/kprobes/myprobe/
> > > >   enable  filter  format  hist  id  trigger
> > > >
> > > > Although this dynamic tracepoint appears as a tracepoint, it is internally
> > > > implemented as a kprobe. However, it must be attached as a tracepoint to
> > > > function correctly in certain contexts.
> > >
> > > Nack.
> > > There are multiple mechanisms to create kprobe/tp via text interfaces.
> > > We're not going to mix them with the programmatic libbpf api.
> >
> > It appears that bpftrace still lacks support for adding a kprobe/tp
> > and then attaching to it directly. Is that correct?
> > What do you think about introducing this mechanism into bpftrace? With
> > such a feature, we could easily attach to inlined kernel functions
> > using bpftrace.
>
> so with the 'echo .. > kprobe_events' you create kprobe which will be
> exported through tracefs together with other tracepoints and bpftrace
> sees it as another tracepoint.. but it's a kprobe :-\

exactly.

>
> how about we add support for kprobe section like SEC("kprobe/SUBSYSTEM/PROBE"),
> so in your case above it'd be SEC("kprobe/kprobes/myprobe")

This is similar to what I'm currently proposing:

  SEC("dynamic_tp/kprobes/my_dynamic_tp")

My proposal requires only a 3-line change. In contrast, if we
implement it as you suggested, it may require significantly more code
changes. I prefer to introduce a new section, such as
SEC("dynamic_tracepoint/"), SEC("kprobe_tracepoint/"), or something
similar, for this special type of kprobe. However, if you believe
SEC("kprobe/SUBSYSTEM/PROBE") is a better approach, I’m happy to
implement it that way.

>
> then attach_kprobe would parse that out and use new new probe_attach_mode
> for bpf_program__attach_kprobe_opts to attach it correctly

Yes, that would be a great enhancement for tracing inlined kernel functions.

--
Regards
Yafang