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From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Fri, 18 Feb 2022 18:10:08 -0800
Message-ID: <CAADnVQ+eojJ8KMwbieJrtOf7oWPqw7VDYV9EAAWpx3UoFHZFDQ@mail.gmail.com>
Subject: Re: [PATCH 0/8] bpf: Add fprobe link
To:     Masami Hiramatsu <mhiramat@kernel.org>
Cc:     Andrii Nakryiko <andrii.nakryiko@gmail.com>,
        Jiri Olsa <olsajiri@gmail.com>,
        Steven Rostedt <rostedt@goodmis.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>,
        Oleg Nesterov <oleg@redhat.com>
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On Thu, Feb 17, 2022 at 8:07 PM Masami Hiramatsu <mhiramat@kernel.org> wrote:
>
> On Thu, 17 Feb 2022 14:01:30 -0800
> Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
>
>
> > > > Is there any chance to support this fast multi-attach for uprobe? If
> > > > yes, we might want to reuse the same link for both (so should we name
> > > > it more generically?
> > >
> > > There is no interface to do that but also there is no limitation to
> > > expand uprobes. For the kprobes, there are some limitations for the
> > > function entry because it needs to share the space with ftrace. So
> > > I introduced fprobe for easier to use.
> > >
> > > > on the other hand BPF program type for uprobe is
> > > > BPF_PROG_TYPE_KPROBE anyway, so keeping it as "kprobe" also would be
> > > > consistent with what we have today).
> > >
> > > Hmm, I'm not sure why BPF made such design choice... (Uprobe needs
> > > the target program.)
> > >
> >
> > We've been talking about sleepable uprobe programs, so we might need
> > to add uprobe-specific program type, probably. But historically, from
> > BPF point of view there was no difference between kprobe and uprobe
> > programs (in terms of how they are run and what's available to them).
> > From BPF point of view, it was just attaching BPF program to a
> > perf_event.
>
> Got it, so that will reuse the uprobe_events in ftrace. But I think
> the uprobe requires a "path" to the attached binary, how is it
> specified?
>
> > > > But yeah, the main question is whether there is something preventing
> > > > us from supporting multi-attach uprobe as well? It would be really
> > > > great for USDT use case.
> > >
> > > Ah, for the USDT, it will be useful. But since now we will have "user-event"
> > > which is faster than uprobes, we may be better to consider to use it.
> >
> > Any pointers? I'm not sure what "user-event" refers to.
>
> Here is the user-events series, which allows user program to define
> raw dynamic events and it can write raw event data directly from
> user space.
>
> https://lore.kernel.org/all/20220118204326.2169-1-beaub@linux.microsoft.com/

Is this a way for user space to inject user bytes into kernel events?
What is the use case?