Re: [PATCH bpf-next v1 2/4] libbpf: Render decl_tags in btf_dump

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Wed, Sep 16, 2026 at 06:20 PM Ihor Solodrai <ihor.solodrai@linux.dev> wrote:
> BTF_KIND_DECL_TAG is an attribute on a declaration: with kind_flag=0
> the tag name is the argument of a btf_decl_tag(), and with kind_flag=1
> it encodes a complete __attribute__.
>
> When doing btf_dump, render both forms at every declaration btf_dump
> emits - a record, a record member and a typedef:

[...]

> Every decl tag in vmlinux BTF is a bpf_kfunc or bpf_fastcall tag on a
> FUNC, which has no declaration in the C output, so generated vmlinux.h
> does not change.

iirc the goal of this series is "bpftool btf dump file prog.bpf.o
format c" and skeletons growing
__attribute__((btf_decl_tag("contains:..."))) on members.
Pls say so in the commit log.
Otherwise it's 200 lines of code for "generated vmlinux.h doesn't
change".

> +static int btf_dump_cmp_decl_tags(const void *a, const void *b)
> +{
> +	const struct decl_tag_desc *x = a, *y = b;
> +
> +	if (x->target_id != y->tar
get_id)
> +		return x->target_id < y->target_id ? -1 : 1;
> +	return x->tag_id < y->tag_id ? -1 : 1;
> +}

nit: return 0 when equal. cmp(x, x) != 0 is asking for trouble.

> @@ -220,8 +258,18 @@ static int btf_dump_resize(struct btf_dump *d)
>  		err = btf_dump_mark_referenced(d, t);
>  		if (err)
>  			return err;
> +
> +		if (btf_is_decl_tag(t)) {
> +			err = btf_dump_push_decl_tag(d, i, t);
> +			if (err)
> +				return err;
> +		}
>  	}

\edited robot voice

On either error d->last_id is not advanced but d->decl_tags_cnt keeps
whatever was pushed so far, so the next btf_dump__dump_type() walks
the same ids again and pushes the same tags twice -> duplicated
__attribute__ in the output ?

pw-bot: cr