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

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