netdev - Re: [PATCH bpf-next v3] libbpf: support weak typed ksyms.

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Message-ID: <CA+khW7i6KpzwB_U7KkYGnRoXfoYH9k5xU7qFmXcmymu+gmf8AQ@mail.gmail.com>
Date:   Tue, 17 Aug 2021 10:25:35 -0700
From:   Hao Luo <haoluo@...gle.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.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 v3] libbpf: support weak typed ksyms.

On Fri, Aug 13, 2021 at 4:01 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Wed, Aug 11, 2021 at 5:40 PM 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>
> > ---
>
> Looks good, applied to bpf-next. For the future, please split libbpf
> and selftests changes into separate patches, it's nicer to have those
> logically separate.
>
> At some point we should probably also improve libbpf error reporting
> for such situations, for better user experience. We have a similar
> problem with CO-RE relocation, verifier doesn't know about those
> concepts, so verifier log is not very helpful, but libbpf can make
> sense out of it with some extra BPF verifier log parsing.
>

Thanks. Will split the tests in future.


> > Changes since v2:
> >  - Move special handling and warning from find_ksym_btf_id() to
> >    bpf_object__resolve_ksym_var_btf_id().
> >  - Removed bpf_link_fops3 from tests since it's not used.
> >  - Separated variable declaration and statements.
> >
> > Changes since v1:
> >  - Weak typed symbols default to zero, as suggested by Andrii.
> >  - Use ASSERT_XXX() for tests.
> >
> >  tools/lib/bpf/libbpf.c                        | 16 +++---
> >  .../selftests/bpf/prog_tests/ksyms_btf.c      | 31 ++++++++++
> >  .../selftests/bpf/progs/test_ksyms_weak.c     | 56 +++++++++++++++++++
> >  3 files changed, 96 insertions(+), 7 deletions(-)
> >  create mode 100644 tools/testing/selftests/bpf/progs/test_ksyms_weak.c
> >
>
> [...]