linux-kernel - Re: [PATCH 0/8] bpf: Add fprobe link

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Date:   Fri, 4 Feb 2022 12:59:42 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, lkml <linux-kernel@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>, Jiri Olsa <olsajiri@...il.com>
Subject: Re: [PATCH 0/8] bpf: Add fprobe link

Hi Alexei,

On Thu, 3 Feb 2022 18:42:22 -0800
Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:

> On Thu, Feb 3, 2022 at 6:19 PM Steven Rostedt <rostedt@...dmis.org> wrote:
> >
> > On Thu, 3 Feb 2022 18:12:11 -0800
> > Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
> >
> > > > No, fprobe is NOT kprobe on ftrace, kprobe on ftrace is already implemented
> > > > transparently.
> > >
> > > Not true.
> > > fprobe is nothing but _explicit_ kprobe on ftrace.
> > > There was an implicit optimization for kprobe when ftrace
> > > could be used.
> > > All this new interface is doing is making it explicit.
> > > So a new name is not warranted here.
> > >
> > > > from that viewpoint, fprobe and kprobe interface are similar but different.
> > >
> > > What is the difference?
> > > I don't see it.
> >
> > IIUC, a kprobe on a function (or ftrace, aka fprobe) gives some extra
> > abilities that a normal kprobe does not. Namely, "what is the function
> > parameters?"
> >
> > You can only reliably get the parameters at function entry. Hence, by
> > having a probe that is unique to functions as supposed to the middle of a
> > function, makes sense to me.
> >
> > That is, the API can change. "Give me parameter X". That along with some
> > BTF reading, could figure out how to get parameter X, and record that.
> 
> This is more or less a description of kprobe on ftrace :)
> The bpf+kprobe users were relying on that for a long time.
> See PT_REGS_PARM1() macros in bpf_tracing.h
> They're meaningful only with kprobe on ftrace.
> So, no, fprobe is not inventing anything new here.

Hmm, you may be misleading why PT_REGS_PARAM1() macro works. You can use
it even if CONFIG_FUNCITON_TRACER=n if your kernel is built with
CONFIG_KPROBES=y. It is valid unless you put a probe out of function
entry.

> No one is using kprobe in the middle of the function.
> It's too difficult to make anything useful out of it,
> so no one bothers.
> When people say "kprobe" 99 out of 100 they mean
> kprobe on ftrace/fentry.

I see. But the kprobe is kprobe. It is not designed to support multiple
probe points. If I'm forced to say, I can rename the struct fprobe to
struct multi_kprobe, but that doesn't change the essence. You may need
to use both of kprobes and so-called multi_kprobe properly. (Someone
need to do that.)

Thank you,

-- 
Masami Hiramatsu <mhiramat@...nel.org>