linux-kernel - Re: [PATCH bpf-next 0/2] Add PROG_TEST_RUN support to BPF_PROG_TYPE

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Message-ID: <20220530145639.slbwvbwewonj6im2@kashmir.localdomain>
Date:   Mon, 30 May 2022 09:56:39 -0500
From:   Daniel Xu <dxu@...uu.xyz>
To:     Song Liu <song@...nel.org>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next 0/2] Add PROG_TEST_RUN support to
 BPF_PROG_TYPE_KPROBE

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@...uu.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.

Thanks,
Daniel