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From:   Song Liu <song@kernel.org>
Date:   Tue, 31 May 2022 09:47:33 -0700
Message-ID: <CAPhsuW7ah1BPBbVsOH4Q4Bd1ZAFisnwFKc768+vqTMZOHee40w@mail.gmail.com>
Subject: Re: [PATCH bpf-next 0/2] Add PROG_TEST_RUN support to BPF_PROG_TYPE_KPROBE
To:     Daniel Xu <dxu@dxuuu.xyz>
Cc:     bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        open list <linux-kernel@vger.kernel.org>
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On Mon, May 30, 2022 at 7:56 AM Daniel Xu <dxu@dxuuu.xyz> wrote:
>
> Hi Song,
>
> On Sun, May 29, 2022 at 11:00:48PM -0700, Song Liu wrote:
> > On Sun, May 29, 2022 at 3:06 PM Daniel Xu <dxu@dxuuu.xyz> wrote:
> > >
> > > This patchset adds PROG_TEST_RUN support to BPF_PROG_TYPE_KPROBE progs.
> > > On top of being generally useful for unit testing kprobe progs, this
> > > feature more specifically helps solve a relability problem with bpftrace
> > > BEGIN and END probes.
> > >
> > > BEGIN and END probes are run exactly once at the beginning and end of a
> > > bpftrace tracing session, respectively. bpftrace currently implements
> > > the probes by creating two dummy functions and attaching the BEGIN and
> > > END progs, if defined, to those functions and calling the dummy
> > > functions as appropriate. This works pretty well most of the time except
> > > for when distros strip symbols from bpftrace. Every now and then this
> > > happens and users get confused. Having PROG_TEST_RUN support will help
> > > solve this issue by allowing us to directly trigger uprobes from
> > > userspace.
> > >
> > > Admittedly, this is a pretty specific problem and could probably be
> > > solved other ways. However, PROG_TEST_RUN also makes unit testing more
> > > convenient, especially as users start building more complex tracing
> > > applications. So I see this as killing two birds with one stone.
> >
> > We have BPF_PROG_RUN which is an alias of BPF_PROG_TEST_RUN. I guess
> > that's a better name for the BEGIN/END use case.
>
> Right, sorry. Was getting names mixed up.
>
> >
> > Have you checked out bpf_prog_test_run_raw_tp()? AFAICT, it works as good as
> > kprobe for this use case.
>
> I just took a look -- I think it'll work for BEGIN/END use case. But
> also like I mentioned, BPF_PROG_RUN/BPF_PROG_TEST_RUN support for
> kprobes is probably still useful. For example if kprobe accesses 13th
> register. I suppose the raw_tp 12 arg limit could be lifted but it might
> be tricky to capture that logic in the absence of something like `struct
> pt_regs` to check the ctx_size_in against.

Agreed that unit tests support for kprobe programs is great.

Thanks,
Song