Re: [PATCH bpf-next v5 0/4] Relax tracing prog recursive attach rules

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On Fri, Dec 1, 2023 at 7:51 AM Dmitrii Dolgov <9erthalion6@xxxxxxxxx> wrote:
>
> Currently, it's not allowed to attach an fentry/fexit prog to another
> fentry/fexit. At the same time it's not uncommon to see a tracing
> program with lots of logic in use, and the attachment limitation
> prevents usage of fentry/fexit for performance analysis (e.g. with
> "bpftool prog profile" command) in this case. An example could be
> falcosecurity libs project that uses tp_btf tracing programs for
> offloading certain part of logic into tail-called programs, but the
> use-case is still generic enough -- a tracing program could be
> complicated and heavy enough to warrant its profiling, yet frustratingly
> it's not possible to do so use best tooling for that.
>
> Following the corresponding discussion [1], the reason for that is to
> avoid tracing progs call cycles without introducing more complex
> solutions. But currently it seems impossible to load and attach tracing
> programs in a way that will form such a cycle. Replace "no same type"
> requirement with verification that no more than one level of attachment
> nesting is allowed. In this way only one fentry/fexit program could be
> attached to another fentry/fexit to cover profiling use case, and still
> no cycle could be formed.
>
> The series contains a test for recursive attachment, as well as a fix +
> test for an issue in re-attachment branch of bpf_tracing_prog_attach.
> When preparing the test for the main change set, I've stumbled upon the
> possibility to construct a sequence of events when attach_btf would be
> NULL while computing a trampoline key. It doesn't look like this issue
> is triggered by the main change, because the reproduces doesn't actually
> need to have an fentry attachment chain.

It appears this set breaks test_progs/trace_ext:

https://github.com/kernel-patches/bpf/actions/runs/7062243664/job/19225827450

Thanks,
Song

>
> [1]: https://lore.kernel.org/bpf/20191108064039.2041889-16-ast@xxxxxxxxxx/
>
> Dmitrii Dolgov (3):
>   bpf: Relax tracing prog recursive attach rules
>   selftests/bpf: Add test for recursive attachment of tracing progs
>   selftests/bpf: Test re-attachment fix for bpf_tracing_prog_attach
>
> Jiri Olsa (1):
>   bpf: Fix re-attachment branch in bpf_tracing_prog_attach
>
>  include/linux/bpf.h                           |   1 +
>  include/uapi/linux/bpf.h                      |   1 +
>  kernel/bpf/syscall.c                          |  16 +++
>  kernel/bpf/verifier.c                         |  33 ++---
>  tools/include/uapi/linux/bpf.h                |   1 +
>  .../bpf/prog_tests/recursive_attach.c         | 117 ++++++++++++++++++
>  .../selftests/bpf/progs/fentry_recursive.c    |  19 +++
>  .../bpf/progs/fentry_recursive_target.c       |  31 +++++
>  8 files changed, 205 insertions(+), 14 deletions(-)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/recursive_attach.c
>  create mode 100644 tools/testing/selftests/bpf/progs/fentry_recursive.c
>  create mode 100644 tools/testing/selftests/bpf/progs/fentry_recursive_target.c
>
>
> base-commit: 40d0eb0259ae77ace3e81d7454d1068c38bc95c2
> --
> 2.41.0
>





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