Re: [PATCH bpf-next v4 1/3] libbpf: Introduce 'btf_custom_path' to 'bpf_obj_open_opts'

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

On Tue, Jul 13, 2021 at 5:43 AM Shuyi Cheng
<chengshuyi@linux.alibaba.com> wrote:
>
> btf_custom_path allows developers to load custom BTF, and subsequent
> CO-RE will use custom BTF for relocation.
>
> Learn from Andrii's comments in [0], add the btf_custom_path parameter
> to bpf_obj_open_opts, you can directly use the skeleton's
> <objname>_bpf__open_opts function to pass in the btf_custom_path
> parameter.
>
> Prior to this, there was also a developer who provided a patch with
> similar functions. It is a pity that the follow-up did not continue to
> advance. See [1].
>
>         [0]https://lore.kernel.org/bpf/CAEf4BzbJZLjNoiK8_VfeVg_Vrg=9iYFv+po-38SMe=UzwDKJ=Q@mail.gmail.com/#t
>         [1]https://yhbt.net/lore/all/CAEf4Bzbgw49w2PtowsrzKQNcxD4fZRE6AKByX-5-dMo-+oWHHA@mail.gmail.com/
>
> Signed-off-by: Shuyi Cheng <chengshuyi@linux.alibaba.com>
> ---
>  tools/lib/bpf/libbpf.c | 36 ++++++++++++++++++++++++++++++------
>  tools/lib/bpf/libbpf.h |  9 ++++++++-
>  2 files changed, 38 insertions(+), 7 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 1e04ce7..6e11a7b 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -498,6 +498,13 @@ struct bpf_object {
>          * it at load time.
>          */
>         struct btf *btf_vmlinux;
> +       /* Path to the custom BTF to be used for BPF CO-RE relocations.
> +        * This custom BTF completely replaces the use of vmlinux BTF
> +        * for the purpose of CO-RE relocations.
> +        * NOTE: any other BPF feature (e.g., fentry/fexit programs,
> +        * struct_ops, etc) will need actual kernel BTF at /sys/kernel/btf/vmlinux.
> +        */

this comment completely duplicates the one from bpf_object_open_opts,
I'll remove or shorten it

> +       char *btf_custom_path;
>         /* vmlinux BTF override for CO-RE relocations */
>         struct btf *btf_vmlinux_override;
>         /* Lazily initialized kernel module BTFs */
> @@ -2645,10 +2652,6 @@ static bool obj_needs_vmlinux_btf(const struct bpf_object *obj)
>         struct bpf_program *prog;
>         int i;
>
> -       /* CO-RE relocations need kernel BTF */
> -       if (obj->btf_ext && obj->btf_ext->core_relo_info.len)
> -               return true;
> -
>         /* Support for typed ksyms needs kernel BTF */
>         for (i = 0; i < obj->nr_extern; i++) {
>                 const struct extern_desc *ext;
> @@ -2665,6 +2668,13 @@ static bool obj_needs_vmlinux_btf(const struct bpf_object *obj)
>                         return true;
>         }
>
> +       /* CO-RE relocations need kernel BTF, only when btf_custom_path
> +        * is not specified
> +        */
> +       if (obj->btf_ext && obj->btf_ext->core_relo_info.len
> +               && !obj->btf_custom_path)
> +               return true;

not sure why you moved it, I'll move it back to minimize code churn

> +
>         return false;
>  }
>

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