netdev - Re: [PATCH bpf-next 01/10] libbpf: add .BTF.ext offset relocation section loading

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Message-ID: <CAEf4Bzbrm0mrteuK=jFs6gdfLceMkjLDJ=UhOg1x8JeRfai07A@mail.gmail.com>
Date:   Wed, 24 Jul 2019 14:42:25 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Andrii Nakryiko <andriin@...com>
Cc:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Yonghong Song <yhs@...com>, Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next 01/10] libbpf: add .BTF.ext offset relocation
 section loading

On Wed, Jul 24, 2019 at 12:28 PM Andrii Nakryiko <andriin@...com> wrote:
>
> Add support for BPF CO-RE offset relocations. Add section/record
> iteration macros for .BTF.ext. These macro are useful for iterating over
> each .BTF.ext record, either for dumping out contents or later for BPF
> CO-RE relocation handling.
>
> To enable other parts of libbpf to work with .BTF.ext contents, moved
> a bunch of type definitions into libbpf_internal.h.
>
> Signed-off-by: Andrii Nakryiko <andriin@...com>
> ---

[...]

> + *
> + * Example to provide a better feel.
> + *
> + *   struct sample {
> + *       int a;
> + *       struct {
> + *           int b[10];
> + *       };
> + *   };
> + *
> + *   struct sample *s = ...;
> + *   int x = &s->a;     // encoded as "0:0" (a is field #0)
> + *   int y = &s->b[5];  // encoded as "0:1:5" (b is field #1, arr elem #5)

This should be "0:1:0:5", actually. Anon struct is field #1 in BTF, b
is field #0 inside that anon struct + array index 5.
Will update it locally and incorporate into next version once the rest
of patch set is reviewed.

> + *   int z = &s[10]->b; // encoded as "10:1" (ptr is used as an array)
> + *
> + * type_id for all relocs in this example  will capture BTF type id of
> + * `struct sample`.
> + *
> + *   [0] https://llvm.org/docs/LangRef.html#getelementptr-instruction
> + */
> +struct bpf_offset_reloc {
> +       __u32   insn_off;
> +       __u32   type_id;
> +       __u32   access_str_off;
> +};
> +
>  #endif /* __LIBBPF_LIBBPF_INTERNAL_H */
> --
> 2.17.1
>