netdev - Re: [PATCH bpf-next v2] libbpf: support weak typed ksyms.

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Date:   Wed, 11 Aug 2021 14:47:33 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Hao Luo <haoluo@...gle.com>
Cc:     Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andriin@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>
Subject: Re: [PATCH bpf-next v2] libbpf: support weak typed ksyms.

On Tue, Aug 10, 2021 at 11:22 AM Hao Luo <haoluo@...gle.com> wrote:
>
> Currently weak typeless ksyms have default value zero, when they don't
> exist in the kernel. However, weak typed ksyms are rejected by libbpf
> if they can not be resolved. This means that if a bpf object contains
> the declaration of a nonexistent weak typed ksym, it will be rejected
> even if there is no program that references the symbol.
>
> Nonexistent weak typed ksyms can also default to zero just like
> typeless ones. This allows programs that access weak typed ksyms to be
> accepted by verifier, if the accesses are guarded. For example,
>
> extern const int bpf_link_fops3 __ksym __weak;
>
> /* then in BPF program */
>
> if (&bpf_link_fops3) {
>    /* use bpf_link_fops3 */
> }
>
> If actual use of nonexistent typed ksym is not guarded properly,
> verifier would see that register is not PTR_TO_BTF_ID and wouldn't
> allow to use it for direct memory reads or passing it to BPF helpers.
>
> Signed-off-by: Hao Luo <haoluo@...gle.com>
> ---

Selftests are failing ([0]). Did they work locally?..

  test_weak_syms:PASS:test_ksyms_weak__open_and_load 0 nsec
  test_weak_syms:FAIL:existing typed ksym unexpected existing typed
ksym: actual -1 != expected 0
  test_weak_syms:FAIL:existing typeless ksym unexpected existing
typeless ksym: actual -1 == expected -1
  test_weak_syms:FAIL:nonexistent typeless ksym unexpected nonexistent
typeless ksym: actual -1 != expected 0
  test_weak_syms:FAIL:nonexistent typed ksym unexpected nonexistent
typed ksym: actual -1 != expected 0
  #59/3 weak_ksyms:FAIL
  #59 ksyms_btf:FAIL


  [0] https://github.com/kernel-patches/bpf/pull/1611/checks?check_run_id=3305288074

> Changes since v1:
>  - Weak typed symbols default to zero, as suggested by Andrii.
>  - Use ASSERT_XXX() for tets.
>
>  tools/lib/bpf/libbpf.c                        | 17 ++++--
>  .../selftests/bpf/prog_tests/ksyms_btf.c      | 25 ++++++++
>  .../selftests/bpf/progs/test_ksyms_weak.c     | 57 +++++++++++++++++++
>  3 files changed, 94 insertions(+), 5 deletions(-)
>  create mode 100644 tools/testing/selftests/bpf/progs/test_ksyms_weak.c
>

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