Re: [PAHOLE v4 2/3] dwarf_loader: Add support for DW_TAG_GNU_annotation

[Yonghong Song <yonghong.song@linux.dev>]

On 6/2/26 12:55 PM, Vineet Gupta wrote:
> gcc 16 was the first release to support DW_TAG_GNU_annotations and this
> patch enables the same in pahole. Bulk of changes are dwarf_loader but
> btf_encoder also gains support with minimal changes.
>
> GCC encodes btf_type_tag and btf_decl_tag annotations differently from
> LLVM. While LLVM uses DW_TAG_LLVM_annotation (0x6000) as child DIEs,
> GCC uses DW_TAG_GNU_annotation (0x6001) as standalone sibling DIEs
> referenced via DW_AT_GNU_annotation (0x2139) attributes, with chaining
> through the same attribute on annotation DIEs themselves.
>
> Handle both encoding styles:
>
> For btf_type_tag (pointer annotations):
> - Recognize DW_TAG_GNU_annotation alongside DW_TAG_LLVM_annotation in
>    child annotation scanning.
> - Follow DW_AT_GNU_annotation attribute chains on pointer types for
>    GCC-style btf_type_tag resolution.
> - Normalize DW_TAG_GNU_annotation to DW_TAG_LLVM_annotation in the
>    internal representation so downstream code works unchanged.
>
> For btf_decl_tag (function/struct/member annotations):
> - Add add_gnu_annotation_chain() to follow DW_AT_GNU_annotation
>    attribute chains on function, struct, and member DIEs.
> - GCC puts DW_AT_GNU_annotation on the function/struct DIE itself
>    (not as child DIEs), referencing sibling annotation DIEs that chain
>    via the same attribute.
>
> Also:
> - Silently skip standalone DW_TAG_GNU_annotation DIEs at CU level.
> - Add tag__is_annotation() helper macro for annotation tag checks.
> - Rename add_llvm_annotation -> add_tag_annotation,
>    skip_llvm_annotations -> skip_tag_annotations since these now
>    handle both LLVM and GNU annotation formats.
>
> Signed-off-by: Vineet Gupta <vineet.gupta@linux.dev>

LGTM except a nit below.

Acked-by: Yonghong Song <yonghong.song@linux.dev>

[...]

> @@ -943,16 +948,40 @@ static int add_child_llvm_annotations(Dwarf_Die *die, int component_idx,
>   
>   	die = &child;
>   	do {
> -		if (dwarf_tag(die) == DW_TAG_LLVM_annotation) {
> -			ret = add_llvm_annotation(die, component_idx, conf, head);
> -			if (ret)
> -				return ret;
> -		}
> +		if (!die__tag_is_annotation(die))
> +			continue;
> +
> +		ret = add_tag_annotation(die, component_idx, conf, head);
> +		if (ret)
> +			return ret;
>   	} while (dwarf_siblingof(die, die) == 0);
>   
>   	return 0;
>   }
>   
> +/*
> + * Handle gcc style btf_decl_tag annotations for functions/struct/member tags.

For 'struct', gcc is not supported yet.

> + * Pointers are handled separately, inline in die__create_new_pointer_tag()
> + */
> +static int add_gnu_annotation_chain(Dwarf_Die *die, int component_idx,
> +				    struct conf_load *conf, struct list_head *head)
> +{
> +	Dwarf_Attribute attr;
> +	Dwarf_Die annot_die;
> +
> +	while (dwarf_attr(die, DW_AT_GNU_annotation, &attr) != NULL &&
> +	       dwarf_formref_die(&attr, &annot_die) != NULL &&
> +	       dwarf_tag(&annot_die) == DW_TAG_GNU_annotation) {
> +		int ret = add_tag_annotation(&annot_die, component_idx, conf, head);
> +		if (ret)
> +			return ret;
> +
> +		die = &annot_die;
> +	}
> +
> +	return 0;
> +}
> +
>
[...]