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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Mon, Aug 2, 2021 at 2:29 PM Hao Luo <haoluo@google.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:

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?


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

>
> Signed-off-by: Hao Luo <haoluo@google.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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