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

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Date:   Thu, 27 Apr 2023 11:18:42 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Yafang Shao <laoar.shao@...il.com>
Cc:     ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        kafai@...com, songliubraving@...com, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org, sdf@...gle.com,
        haoluo@...gle.com, jolsa@...nel.org, mhiramat@...nel.org,
        bpf@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next 5/6] bpf: Improve tracing recursion prevention
 mechanism

On Thu, 27 Apr 2023 22:22:22 +0800
Yafang Shao <laoar.shao@...il.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 allowed
to recurs.

> 
>   __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.

-- Steve


>         __bpf_prog_enter
>             test_recursion_try_acquire
>             [...]
>             test_recursion_release
>        bpf_prog_run()
>            bpf_prog_run()
>                __bpf_prog_enter
>                   test_recursion_try_acquire
>                   [...]
>                   test_recursion_release
>               bpf_prog_run()
>               [ And so on ... ]
> 
> [1]. https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/tools/testing/selftests/bpf/progs/recursion.c#n38