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From:   Yafang Shao <laoar.shao@gmail.com>
Date:   Thu, 27 Apr 2023 23:23:31 +0800
Message-ID: <CALOAHbDRSNvihv5n0jJpAsK3onezRRipO78RG3rAf3LGgkzYHg@mail.gmail.com>
Subject: Re: [PATCH bpf-next 5/6] bpf: Improve tracing recursion prevention mechanism
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org,
        kafai@fb.com, songliubraving@fb.com, yhs@fb.com,
        john.fastabend@gmail.com, kpsingh@kernel.org, sdf@google.com,
        haoluo@google.com, jolsa@kernel.org, mhiramat@kernel.org,
        bpf@vger.kernel.org, linux-trace-kernel@vger.kernel.org,
        linux-kernel@vger.kernel.org
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On Thu, Apr 27, 2023 at 11:18=E2=80=AFPM Steven Rostedt <rostedt@goodmis.or=
g> wrote:
>
> On Thu, 27 Apr 2023 22:22:22 +0800
> Yafang Shao <laoar.shao@gmail.com> wrote:
>
> > IIUC, the acquire/release pair works as follows,
> >
> >    test_recursion_try_acquire
> >      [ protection area ]
> >    test_recursion_release
> >
> > After release, there will be no protection, and thus it will fail the
> > tools/testing/selftests/bpf/progs/recursion.c[1] test case, because
> > the recursion occurs in the bpf_prog_run() itself,
>
> But bpf programs are allowed to recurs. Hence, you need separate logic to
> detect that. The test_recursion_*() code is for cases that are not allowe=
d
> to recurs.
>

Agreed.

> >
> >   __bpf_prog_enter
> >      test_recursion_try_acquire
> >      [...]
> >      test_recursion_release
> >   // no protection after the release
> >   bpf_prog_run()
> >     bpf_prog_run() // the recursion can't be prevented.
>
> But I thought you can run a bpf_prog from another bpf_prog. So you don't
> want to prevent it. You need other logic to detect if it was not suppose =
to
> recurs.
>

If so, we have to keep the prog->active to prevent it, then I'm not
sure if it is worth adding test_recursion_*().

--=20
Regards
Yafang