netdev - Re: [PATCH bpf-next RFC v1 4/8] libbpf: Resolve invalid kfunc calls with imm = 0, off = 0

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Date:   Tue, 31 Aug 2021 17:35:05 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
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 RFC v1 4/8] libbpf: Resolve invalid kfunc calls
 with imm = 0, off = 0

On Mon, Aug 30, 2021 at 10:34 AM Kumar Kartikeya Dwivedi
<memxor@...il.com> wrote:
>
> Preserve these calls as it allows verifier to succeed in loading the
> program if they are determined to be unreachable after dead code
> elimination during program load. If not, the verifier will fail at
> runtime.
>

This should be controlled by whether extern for func is weak or not,
just like we do for variables (see
bpf_object__resolve_ksym_var_btf_id()).

> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@...il.com>
> ---
>  tools/lib/bpf/libbpf.c | 20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index c4677ef97caa..9df90098f111 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -6736,9 +6736,14 @@ static int bpf_object__resolve_ksym_func_btf_id(struct bpf_object *obj,
>         kfunc_id = find_ksym_btf_id(obj, ext->name, BTF_KIND_FUNC,
>                                     &kern_btf, &kern_btf_fd);
>         if (kfunc_id < 0) {
> -               pr_warn("extern (func ksym) '%s': not found in kernel BTF\n",
> +               pr_warn("extern (func ksym) '%s': not found in kernel BTF, encoding btf_id as 0\n",
>                         ext->name);
> -               return kfunc_id;
> +               /* keep invalid kfuncs, so that verifier can load the program if
> +                * they get removed during DCE pass in the verifier.
> +                * The encoding must be insn->imm = 0, insn->off = 0.
> +                */
> +               kfunc_id = kern_btf_fd = 0;
> +               goto resolve;
>         }
>
>         if (kern_btf != obj->btf_vmlinux) {
> @@ -6798,11 +6803,18 @@ static int bpf_object__resolve_ksym_func_btf_id(struct bpf_object *obj,
>                 return -EINVAL;
>         }
>
> +resolve:
>         ext->is_set = true;
>         ext->ksym.kernel_btf_obj_fd = kern_btf_fd;
>         ext->ksym.kernel_btf_id = kfunc_id;
> -       pr_debug("extern (func ksym) '%s': resolved to kernel [%d]\n",
> -                ext->name, kfunc_id);
> +       if (kfunc_id) {
> +               pr_debug("extern (func ksym) '%s': resolved to kernel [%d]\n",
> +                        ext->name, kfunc_id);
> +       } else {
> +               ext->ksym.offset = 0;
> +               pr_debug("extern (func ksym) '%s': added special invalid kfunc with imm = 0\n",
> +                        ext->name);
> +       }
>
>         return 0;
>  }
> --
> 2.33.0
>