netdev - Re: [PATCH bpf-next v1 2/6] libbpf: Add typeless and weak ksym support to gen

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Message-ID: <CAPhsuW6nCQK71aeyR1YthvMWGNgH--RwbLnA0_rhi071juTsYg@mail.gmail.com>
Date:   Thu, 7 Oct 2021 14:45:10 -0700
From:   Song Liu <song@...nel.org>
To:     Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v1 2/6] libbpf: Add typeless and weak ksym
 support to gen_loader

On Tue, Oct 5, 2021 at 5:29 PM Kumar Kartikeya Dwivedi <memxor@...il.com> wrote:
>
> This patch adds typeless and weak ksym support to BTF_KIND_VAR
> relocation code in gen_loader. For typeless ksym, we use the newly added
> bpf_kallsyms_lookup_name helper.
>
> For weak ksym, we simply skip error check, and fix up the srg_reg for
> the insn, as keeping it as BPF_PSEUDO_BTF_ID for weak ksym with its
> insn[0].imm and insn[1].imm set as 0 will cause a failure.  This is
> consistent with how libbpf relocates these two cases of BTF_KIND_VAR.
>
> We also modify cleanup_relos to check for typeless ksyms in fd closing
> loop, since those have no fd associated with the ksym. For this we can
> reuse the unused 'off' member of ksym_desc.
>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@...il.com>
[...]

Everything above (trimmed) makes sense to me.

> +/* Expects:
> + * BPF_REG_8 - pointer to instruction
> + */
> +static void emit_relo_ksym_btf(struct bpf_gen *gen, struct ksym_relo_desc *relo, int insn)
> +{

But I don't quite follow why we need these changes to emit_relo_ksym_btf.
Maybe we should have these changes in a separate patch and add some
more explanations?

Thanks,
Song

[...]