netdev - Re: [PATCH v2 bpf-next 6/7] libbpf: support kernel module ksym externs

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Date:   Mon, 11 Jan 2021 11:00:39 -0800
From:   Hao Luo <haoluo@...gle.com>
To:     Andrii Nakryiko <andrii@...nel.org>
Cc:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next 6/7] libbpf: support kernel module ksym externs

Acked-by: Hao Luo <haoluo@...gle.com>, with a couple of nits.

On Fri, Jan 8, 2021 at 2:09 PM Andrii Nakryiko <andrii@...nel.org> wrote:
>
> Add support for searching for ksym externs not just in vmlinux BTF, but across
> all module BTFs, similarly to how it's done for CO-RE relocations. Kernels
> that expose module BTFs through sysfs are assumed to support new ldimm64
> instruction extension with BTF FD provided in insn[1].imm field, so no extra
> feature detection is performed.
>
> Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
> ---
>  tools/lib/bpf/libbpf.c | 47 +++++++++++++++++++++++++++---------------
>  1 file changed, 30 insertions(+), 17 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 6ae748f6ea11..57559a71e4de 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
[...]
> @@ -7319,7 +7321,8 @@ static int bpf_object__read_kallsyms_file(struct bpf_object *obj)
>  static int bpf_object__resolve_ksyms_btf_id(struct bpf_object *obj)
>  {
>         struct extern_desc *ext;
> -       int i, id;
> +       struct btf *btf;
> +       int i, j, id, btf_fd, err;
>
>         for (i = 0; i < obj->nr_extern; i++) {
>                 const struct btf_type *targ_var, *targ_type;
> @@ -7331,8 +7334,22 @@ static int bpf_object__resolve_ksyms_btf_id(struct bpf_object *obj)
>                 if (ext->type != EXT_KSYM || !ext->ksym.type_id)
>                         continue;
>
> -               id = btf__find_by_name_kind(obj->btf_vmlinux, ext->name,
> -                                           BTF_KIND_VAR);
> +               btf = obj->btf_vmlinux;
> +               btf_fd = 0;
> +               id = btf__find_by_name_kind(btf, ext->name, BTF_KIND_VAR);

Is "if (id <= 0)" better? Just in case, more error code is introduced in future.

> +               if (id == -ENOENT) {
> +                       err = load_module_btfs(obj);
> +                       if (err)
> +                               return err;
> +
> +                       for (j = 0; j < obj->btf_module_cnt; j++) {
> +                               btf = obj->btf_modules[j].btf;
> +                               btf_fd = obj->btf_modules[j].fd;
> +                               id = btf__find_by_name_kind(btf, ext->name, BTF_KIND_VAR);
> +                               if (id != -ENOENT)
> +                                       break;
> +                       }
> +               }
>                 if (id <= 0) {

Nit: the warning message isn't accurate any more, right? We also
searched kernel modules' BTF.

>                         pr_warn("extern (ksym) '%s': failed to find BTF ID in vmlinux BTF.\n",
>                                 ext->name);
> @@ -7343,24 +7360,19 @@ static int bpf_object__resolve_ksyms_btf_id(struct bpf_object *obj)
[...]

> --
> 2.24.1
>