netdev - Re: [PATCH bpf-next v2 1/2] libbpf: do not resolve size on duplicate FUNCs

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Message-ID: <CAADnVQLdmmvJRyf+br=CtJaDw6PowqKGTXb3z-q7LpbYiYFpHQ@mail.gmail.com>
Date: Sun, 29 Sep 2024 09:32:34 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: i@...k3r.moe
Cc: bpf <bpf@...r.kernel.org>, Andrii Nakryiko <andrii@...nel.org>, 
	Alexei Starovoitov <ast@...nel.org>, Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 1/2] libbpf: do not resolve size on duplicate FUNCs

On Sun, Sep 29, 2024 at 2:31 AM Eric Long via B4 Relay
<devnull+i.hack3r.moe@...nel.org> wrote:
>
> From: Eric Long <i@...k3r.moe>
>
> FUNCs do not have sizes, thus currently btf__resolve_size will fail
> with -EINVAL. Add conditions so that we only update size when the BTF
> object is not function or function prototype.
>
> Signed-off-by: Eric Long <i@...k3r.moe>
> ---
>  tools/lib/bpf/linker.c | 23 +++++++++++++----------
>  1 file changed, 13 insertions(+), 10 deletions(-)
>
> diff --git a/tools/lib/bpf/linker.c b/tools/lib/bpf/linker.c
> index 81dbbdd79a7c65a4b048b85e1dba99cb5f7cb56b..cffb388fa40ef054c2661b8363120f8a4d3c3784 100644
> --- a/tools/lib/bpf/linker.c
> +++ b/tools/lib/bpf/linker.c
> @@ -2452,17 +2452,20 @@ static int linker_append_btf(struct bpf_linker *linker, struct src_obj *obj)
>                                 __s64 sz;
>
>                                 dst_var = &dst_sec->sec_vars[glob_sym->var_idx];
> -                               /* Because underlying BTF type might have
> -                                * changed, so might its size have changed, so
> -                                * re-calculate and update it in sec_var.
> -                                */
> -                               sz = btf__resolve_size(linker->btf, glob_sym->underlying_btf_id);
> -                               if (sz < 0) {
> -                                       pr_warn("global '%s': failed to resolve size of underlying type: %d\n",
> -                                               name, (int)sz);
> -                                       return -EINVAL;
> +                               t = btf__type_by_id(linker->btf, glob_sym->underlying_btf_id);
> +                               if (btf_kind(t) != BTF_KIND_FUNC && btf_kind(t) != BTF_KIND_FUNC_PROTO) {
> +                                       /* Because underlying BTF type might have
> +                                        * changed, so might its size have changed, so
> +                                        * re-calculate and update it in sec_var.
> +                                        */
> +                                       sz = btf__resolve_size(linker->btf, glob_sym->underlying_btf_id);
> +                                       if (sz < 0) {
> +                                               pr_warn("global '%s': failed to resolve size of underlying type: %d\n",
> +                                                       name, (int)sz);
> +                                               return -EINVAL;
> +                                       }
> +                                       dst_var->size = sz;

Looks like a hack to me.

In the test you're using:
void *bpf_cast_to_kern_ctx(void *obj) __ksym;

but __weak is missing.
Is that the reason you're hitting this issue?