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

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Date:   Wed, 11 Aug 2021 17:20:00 -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 v2] libbpf: support weak typed ksyms.

On Wed, Aug 11, 2021 at 4:41 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Wed, Aug 11, 2021 at 3:40 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>
> > ---
Thanks for taking a look! All the comments seem reasonable. Let me fix
them, test and send a new version.

> >  Changes since v1:
> >   - Weak typed symbols default to zero, as suggested by Andrii.
> >   - Use ASSERT_XXX() for tests.
> >
> >  tools/lib/bpf/libbpf.c                        | 17 ++++--
> >  .../selftests/bpf/prog_tests/ksyms_btf.c      | 31 ++++++++++
> >  .../selftests/bpf/progs/test_ksyms_weak.c     | 57 +++++++++++++++++++
> >  3 files changed, 100 insertions(+), 5 deletions(-)
> >  create mode 100644 tools/testing/selftests/bpf/progs/test_ksyms_weak.c
> >
[...]