Re: [PATCH bpf-next 5/6] bpf: Improve tracing recursion prevention mechanism

[Yafang Shao]

On Thu, Apr 27, 2023 at 11:18 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> On Thu, 27 Apr 2023 22:22:22 +0800
> Yafang Shao <laoar.shao@xxxxxxxxx> 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 allowed
> 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_*().

-- 
Regards
Yafang