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From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Tue, 31 May 2022 11:07:31 -0700
Message-ID: <CAADnVQL6Duc5qdwkqf+DWqYhngE3Dj-J37=7QoVA3ycFoWBU2w@mail.gmail.com>
Subject: Re: [PATCH bpf-next 1/2] bpf, test_run: Add PROG_TEST_RUN support to 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>,
        LKML <linux-kernel@vger.kernel.org>
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On Sun, May 29, 2022 at 3:06 PM Daniel Xu <dxu@dxuuu.xyz> wrote:
>
> This commit adds PROG_TEST_RUN support to BPF_PROG_TYPE_KPROBE progs. On
> top of being generally useful for unit testing kprobe progs, this commit
> 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.

bpftrace approach of uprobe-ing into BEGIN_trigger can
be solved with raw_tp prog.
"BEGIN" bpftrace's program doesn't have to be uprobe.
I can be raw_tp and prog_test_run command is
already implemented for raw_tp.

kprobe prog has pt_regs as arguments,
raw_tp has tracepoint args.
Both progs expect kernel addresses in args.
Passing 'struct pt_regs' filled with integers and non-kernel addresses
won't help to unit test bpf-kprobe programs.
There is little use in creating a dummy kprobe-bpf prog
that expects RDI to be 1 and RSI to be 2
(like selftest from patch 2 does) and running it.
We already have raw_tp with similar args and such
progs can be executed already.
Whether SEC() part says kprobe/ or raw_tp/ doesn't change
much in the prog itself.
More so raw_tp prog will work on all architectures,
whereas kprobe's pt_regs are arch specific.
So even if kprobe progs were runnable, bpftrace
should probably be using raw_tp to be arch independent.