Date:   Fri, 6 Aug 2021 17:48:48 -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] libbpf: support weak typed ksyms.

Thanks for taking a look.

On Fri, Aug 6, 2021 at 3:40 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Mon, Aug 2, 2021 at 2:29 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.
> > This means that if a bpf object contains the declaration of a
> > non-existing weak typed ksym, it will be rejected even if there is
> > no program that references the symbol.
> >
> > In fact, we could let them to pass the checks in libbpf and leave the
> > object to be rejected by the bpf verifier. More specifically, upon
> > seeing a weak typed symbol, libbpf can assign it a zero btf_id, which
> > is associated to the type 'void'. The verifier expects the symbol to
> > be BTF_VAR_KIND instead, therefore will reject loading.
> >
> > In practice, we often add new kernel symbols and roll out the kernel
> > changes to fleet. And we want to release a single bpf object that can
> > be loaded on both the new and the old kernels. Passing weak typed ksyms
> > in libbpf allows us to do so as long as the programs that reference the
> > new symbols are disabled on the old kernel.
>
> How do you detect whether a given ksym is present or not? You check
> that from user-space and then use .rodata to turn off pieces of BPF
> logic? That's quite inconvenient. It would be great if these typed
> ksyms worked the same way as typeless ones:
>
It's not by detect. In my use case, I can add a flag to the
application to disable/enable loading a BPF program. Because we know
at which kernel version a new symbol was introduced, we can coordinate
the application flag with the kernel version to avoid the faulting
code being loaded on an old kernel.

> extern const int bpf_link_fops3 __ksym __weak;
>
> /* then in BPF program */
>
> if (&bpf_link_fops3) {
>    /* use bpf_link_fops3 */
> }
>
>
> I haven't tried, but I suspect it could be made to work if libbpf
> replaces corresponding ldimm64 instruction (with BTF ID) into a plain
> ldimm64 instruction loading 0 directly. That would allow the above
> check (and it would be known false to the verifier) to succeed without
> the verifier rejecting the BPF program. If actual use of non-existing
> typed symbol 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.
>
> Have you considered such an approach?
>
I haven't thought about this approach. I just grabbed the quickest
solution I can think of. Will follow your suggestion and see if it
works.

>
> Separately, please use ASSERT_XXX() macros for tests, not plain
> CHECK()s. Thanks.
>
ACK.

> >
> > Signed-off-by: Hao Luo <haoluo@...gle.com>
> > ---
> >  tools/lib/bpf/libbpf.c                        | 17 +++++-
> >  .../selftests/bpf/prog_tests/ksyms_btf.c      | 42 +++++++++++++
> >  .../selftests/bpf/progs/test_ksyms_weak.c     | 60 +++++++++++++++++++
> >  3 files changed, 116 insertions(+), 3 deletions(-)
> >  create mode 100644 tools/testing/selftests/bpf/progs/test_ksyms_weak.c
> >
>
> [...]

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