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From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Thu, 3 Feb 2022 18:42:22 -0800
Message-ID: <CAADnVQKjNJjZDs+ZV7vcusEkKuDq+sWhSD3M5GtvNeZMx3Fcmg@mail.gmail.com>
Subject: Re: [PATCH 0/8] bpf: Add fprobe link
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Masami Hiramatsu <mhiramat@kernel.org>,
        Jiri Olsa <jolsa@redhat.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Network Development <netdev@vger.kernel.org>,
        bpf <bpf@vger.kernel.org>, lkml <linux-kernel@vger.kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>, Jiri Olsa <olsajiri@gmail.com>
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On Thu, Feb 3, 2022 at 6:19 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Thu, 3 Feb 2022 18:12:11 -0800
> Alexei Starovoitov <alexei.starovoitov@gmail.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.

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.