Re: [PATCH v4 bpf-next 08/16] libbpf: Use CO-RE in the kernel in light skeleton.

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

On Tue, Nov 23, 2021 at 10:02 PM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> From: Alexei Starovoitov <ast@kernel.org>
>
> Without lskel the CO-RE relocations are processed by libbpf before any other
> work is done. Instead, when lskel is needed, remember relocation as RELO_CORE
> kind. Then when loader prog is generated for a given bpf program pass CO-RE
> relos of that program to gen loader via bpf_gen__record_relo_core(). The gen
> loader will remember them as-is and pass it later as-is into the kernel.
>
> The normal libbpf flow is to process CO-RE early before call relos happen. In
> case of gen_loader the core relos have to be added to other relos to be copied
> together when bpf static function is appended in different places to other main
> bpf progs. During the copy the append_subprog_relos() will adjust insn_idx for
> normal relos and for RELO_CORE kind too. When that is done each struct
> reloc_desc has good relos for specific main prog.
>
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> ---
>  tools/lib/bpf/bpf_gen_internal.h |   3 +
>  tools/lib/bpf/gen_loader.c       |  41 +++++++++++-
>  tools/lib/bpf/libbpf.c           | 108 ++++++++++++++++++++++---------
>  3 files changed, 119 insertions(+), 33 deletions(-)
>

LGTM, minor styling nit, please address if/when resubmitting.

Acked-by: Andrii Nakryiko <andrii@kernel.org>

[...]

> @@ -6600,6 +6638,16 @@ static int bpf_program__record_externs(struct bpf_program *prog)
>                                                ext->is_weak, false, BTF_KIND_FUNC,
>                                                relo->insn_idx);
>                         break;
> +               case RELO_CORE: {
> +                       struct bpf_core_relo cr = {
> +                               .insn_off = relo->insn_idx * 8,
> +                               .type_id = relo->core_relo->type_id,
> +                               .access_str_off = relo->core_relo->access_str_off,
> +                               .kind = relo->core_relo->kind,
> +                       };

nit: empty line between variable and statements

> +                       bpf_gen__record_relo_core(obj->gen_loader, &cr);
> +                       break;
> +               }
>                 default:
>                         continue;
>                 }
> @@ -6639,7 +6687,7 @@ static int bpf_object_load_prog(struct bpf_object *obj, struct bpf_program *prog
>                                 prog->name, prog->instances.nr);
>                 }
>                 if (obj->gen_loader)
> -                       bpf_program__record_externs(prog);
> +                       bpf_program_record_relos(prog);
>                 err = bpf_object_load_prog_instance(obj, prog,
>                                                     prog->insns, prog->insns_cnt,
>                                                     license, kern_ver, &fd);
> --
> 2.30.2
>