Re: [PAHOLE v3 2/3] dwarf_loader: Add support for DW_TAG_GNU_annotation

[Arnaldo Carvalho de Melo <acme@kernel.org>]

On Mon, Jun 01, 2026 at 11:35:10AM -0700, Vineet Gupta wrote:
> gcc 16 was first release to support DW_TAG_GNU_annotation and this patch
> enables the same in pahole. Bulk of changes are in 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, with cycle detection.
> - 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.
> - Update pfunct-btf-decl-tags.sh test to use GCC 16+ when available and
>   now passes.
> 
> Signed-off-by: Vineet Gupta <vineet.gupta@linux.dev>
> ---
> Changes since v2 [2]
>  - Removed loop detection logic
>  - Move test changes to different patch
> 
> Changes since v1 [1]
>  - NFC Reduce indentation with early exits (Alexei offlist)
> 
> [2] https://lore.kernel.org/bpf/20260528223616.2035618-2-vineet.gupta@linux.dev/
> [1] https://lore.kernel.org/bpf/20260526181818.4159927-2-vineet.gupta@linux.dev/
> ---
> 
>  btf_encoder.c     |   1 +
>  dutil.h           |   8 ++++
>  dwarf_loader.c    | 102 ++++++++++++++++++++++++++++++++++++++++++----
>  dwarves.h         |   3 +-
>  dwarves_fprintf.c |   8 +++-
>  5 files changed, 110 insertions(+), 12 deletions(-)
> 
> diff --git a/btf_encoder.c b/btf_encoder.c
> index 633bc6162ce0..d5af706d7638 100644
> --- a/btf_encoder.c
> +++ b/btf_encoder.c
> @@ -1831,6 +1831,7 @@ static int btf_encoder__encode_tag(struct btf_encoder *encoder, struct tag *tag,
>  		name = namespace__name(tag__namespace(tag));
>  		return btf_encoder__add_ref_type(encoder, BTF_KIND_TYPEDEF, ref_type_id, name, false);
>  	case DW_TAG_LLVM_annotation:
> +	case DW_TAG_GNU_annotation:
>  		name = tag__btf_type_tag(tag)->value;
>  		return btf_encoder__add_ref_type(encoder, BTF_KIND_TYPE_TAG, ref_type_id, name, false);
>  	case DW_TAG_structure_type:
> diff --git a/dutil.h b/dutil.h
> index ff78aa6dfd10..abe0e62b412f 100644
> --- a/dutil.h
> +++ b/dutil.h
> @@ -35,6 +35,14 @@
>  #define DW_TAG_LLVM_annotation 0x6000
>  #endif
>  
> +#ifndef DW_TAG_GNU_annotation
> +#define DW_TAG_GNU_annotation 0x6001
> +#endif
> +
> +#ifndef DW_AT_GNU_annotation
> +#define DW_AT_GNU_annotation 0x2139
> +#endif
> +
>  static inline __attribute__((const)) bool is_power_of_2(unsigned long n)
>  {
>          return (n != 0 && ((n & (n - 1)) == 0));
> diff --git a/dwarf_loader.c b/dwarf_loader.c
> index 8b5b526299b5..878565884f85 100644
> --- a/dwarf_loader.c
> +++ b/dwarf_loader.c
> @@ -908,6 +908,12 @@ static int tag__recode_dwarf_bitfield(struct tag *tag, struct cu *cu, uint16_t b
>  	return -ENOMEM;
>  }
>  
> +static bool die__tag_is_annotation(Dwarf_Die *die)
> +{
> +	unsigned int tag = dwarf_tag(die);
> +	return tag == DW_TAG_LLVM_annotation || tag == DW_TAG_GNU_annotation;
> +}
> +
>  static int add_llvm_annotation(Dwarf_Die *die, int component_idx, struct conf_load *conf,
>  			       struct list_head *head)
>  {
> @@ -943,7 +949,7 @@ static int add_child_llvm_annotations(Dwarf_Die *die, int component_idx,
>  
>  	die = &child;
>  	do {
> -		if (dwarf_tag(die) == DW_TAG_LLVM_annotation) {
> +		if (die__tag_is_annotation(die)) {
>  			ret = add_llvm_annotation(die, component_idx, conf, head);
>  			if (ret)
>  				return ret;
> @@ -953,6 +959,35 @@ static int add_child_llvm_annotations(Dwarf_Die *die, int component_idx,
>  	return 0;
>  }
>  
> +/* Handle gcc sytle btf_decl_tag annotations for functions/struct/member tags
> + * Pointers are handled seperately, 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;
> +
> +	if (dwarf_attr(die, DW_AT_GNU_annotation, &attr) == NULL ||
> +	    dwarf_formref_die(&attr, &annot_die) == NULL)
> +		return 0;
> +
> +	for (;;) {
> +		if (dwarf_tag(&annot_die) != DW_TAG_GNU_annotation)
> +			break;
> +
> +		int ret = add_llvm_annotation(&annot_die, component_idx, conf, head);
> +		if (ret)
> +			return ret;
> +
> +		if (dwarf_attr(&annot_die, DW_AT_GNU_annotation, &attr) == NULL ||
> +		    dwarf_formref_die(&attr, &annot_die) == NULL)
> +			break;

Make this more compact, no need to have these two if blocks, one
suffices:

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;
	int ret = 0;

	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) {
		if ((ret = add_llvm_annotation(&annot_die, component_idx, conf, head)) != 0)
			break;
	}

	return ret;
}


> +	}
> +
> +	return 0;
> +}
> +
>  int class_member__dwarf_recode_bitfield(struct class_member *member,
>  					struct cu *cu)
>  {
> @@ -1596,6 +1631,8 @@ static struct btf_type_tag_type *die__create_new_btf_type_tag_type(Dwarf_Die *di
>  		return NULL;
>  
>  	tag__init(&tag->tag, cu, die);
> +	/* Normalize DW_TAG_GNU_annotation to DW_TAG_LLVM_annotation internally */
> +	tag->tag.tag = DW_TAG_LLVM_annotation;
>  	tag->value = attr_string(die, DW_AT_const_value, conf);
>  	return tag;
>  }
> @@ -1636,19 +1673,21 @@ static struct tag *die__create_new_pointer_tag(Dwarf_Die *die, struct cu *cu,
>  {
>  	struct btf_type_tag_ptr_type *tag = NULL;
>  	Dwarf_Die *cdie, child;
> +	Dwarf_Attribute attr;
> +	Dwarf_Die annot_die;
>  	const char *name;
>  
> -	/* If no child tags or skipping btf_type_tag encoding, just create a new tag
> -	 * and return
> -	 */
> -	if (!dwarf_haschildren(die) || dwarf_child(die, &child) != 0 ||
> -	    conf->skip_encoding_btf_type_tag)
> +	/* If skipping btf_type_tag encoding, just create a new tag, return */
> +	if (conf->skip_encoding_btf_type_tag)
>  		return tag__new(die, cu);

>  
> -	/* Otherwise, check DW_TAG_LLVM_annotation child tags */
> +	/* Handle LLVM style annotation tags if present */
> +	if (!dwarf_haschildren(die) || dwarf_child(die, &child) != 0)
> +		goto check_gnu_attr;
> +
>  	cdie = &child;
>  	do {
> -		if (dwarf_tag(cdie) != DW_TAG_LLVM_annotation)
> +		if (!die__tag_is_annotation(cdie))
>  			continue;
>  
>  		/* Only check btf_type_tag annotations */
> @@ -1661,6 +1700,31 @@ static struct tag *die__create_new_pointer_tag(Dwarf_Die *die, struct cu *cu,
>  			return NULL;
>  	} while (dwarf_siblingof(cdie, cdie) == 0);
>  
> +check_gnu_attr:
> +	/* Check for GCC-style DW_AT_GNU_annotation attribute */
> +	if (tag != NULL ||
> +	    dwarf_attr(die, DW_AT_GNU_annotation, &attr) == NULL ||
> +	    dwarf_formref_die(&attr, &annot_die) == NULL)
> +		goto out;
> +
> +	for (;;) {
> +		if (dwarf_tag(&annot_die) != DW_TAG_GNU_annotation)
> +			break;
> +
> +		name = attr_string(&annot_die, DW_AT_name, conf);
> +		if (strcmp(name, "btf_type_tag") != 0)
> +			break;
> +
> +		tag = die__add_btf_type_tag(tag, die, &annot_die, cu, conf);
> +		if (tag == NULL)
> +			return NULL;
> +
> +		if (dwarf_attr(&annot_die, DW_AT_GNU_annotation, &attr) == NULL ||
> +		    dwarf_formref_die(&attr, &annot_die) == NULL)
> +			break;
> +	}

The loop above probably can get some simplification?

> +
> +out:
>  	return tag ? &tag->tag : tag__new(die, cu);
>  }
>  
> @@ -1689,6 +1753,12 @@ static struct tag *die__create_new_class(Dwarf_Die *die, struct cu *cu, struct c
>  		}
>  	}
>  
> +	if (class != NULL &&
> +	    add_gnu_annotation_chain(die, -1, conf, &class->type.namespace.annots) != 0) {
> +		class__delete(class, cu);
> +		class = NULL;
> +	}
> +
>  	return class ? &class->type.namespace.tag : NULL;
>  }
>  
> @@ -2050,10 +2120,13 @@ static int die__process_class(Dwarf_Die *die, struct type *class,
>  			cu__hash(cu, &member->tag);
>  			if (add_child_llvm_annotations(die, member_idx, conf, &class->namespace.annots))
>  				return -ENOMEM;
> +			if (add_gnu_annotation_chain(die, member_idx, conf, &class->namespace.annots))
> +				return -ENOMEM;
>  			member_idx++;
>  		}
>  			continue;
>  		case DW_TAG_LLVM_annotation:
> +		case DW_TAG_GNU_annotation:
>  			if (add_llvm_annotation(die, -1, conf, &class->namespace.annots))
>  				return -ENOMEM;
>  			continue;
> @@ -2359,6 +2432,7 @@ static int die__process_function(Dwarf_Die *die, struct ftype *ftype,
>  				goto out_enomem;
>  			continue;
>  		case DW_TAG_LLVM_annotation:
> +		case DW_TAG_GNU_annotation:
>  			if (add_llvm_annotation(die, -1, conf, &(tag__function(&ftype->tag)->annots)))
>  				goto out_enomem;
>  			continue;
> @@ -2407,6 +2481,12 @@ static struct tag *die__create_new_function(Dwarf_Die *die, struct cu *cu, struc
>  		function = NULL;
>  	}
>  
> +	if (function != NULL &&
> +	    add_gnu_annotation_chain(die, -1, conf, &function->annots) != 0) {
> +		function__delete(function, cu);
> +		function = NULL;
> +	}
> +
>  	return function ? &function->proto.tag : NULL;
>  }
>  
> @@ -2468,6 +2548,9 @@ static struct tag *__die__process_tag(Dwarf_Die *die, struct cu *cu,
>  		 */
>  		tag = &unsupported_tag;
>  		break;
> +	case DW_TAG_GNU_annotation:
> +		tag = &unsupported_tag;
> +		break;
>  	case DW_TAG_label:
>  		if (conf->ignore_labels)
>  			tag = &unsupported_tag; // callers will assume conf->ignore_labels is true
> @@ -2493,7 +2576,8 @@ static int die__process_unit(Dwarf_Die *die, struct cu *cu, struct conf_load *co
>  			// XXX special case DW_TAG_dwarf_procedure, appears when looking at a recent ~/bin/perf
>  			// Investigate later how to properly support this...
>  			if (dwarf_tag(die) != DW_TAG_dwarf_procedure &&
> -			    dwarf_tag(die) != DW_TAG_label) // conf->ignore_labels == true, see die__process_tag()
> +			    dwarf_tag(die) != DW_TAG_label && // conf->ignore_labels == true, see die__process_tag()
> +			    dwarf_tag(die) != DW_TAG_GNU_annotation)
>  				tag__print_not_supported(die);
>  			continue;
>  		}
> diff --git a/dwarves.h b/dwarves.h
> index 5ec16e750e83..42b8e39aa2dd 100644
> --- a/dwarves.h
> +++ b/dwarves.h
> @@ -670,7 +670,8 @@ static inline int tag__is_tag_type(const struct tag *tag)
>  	       tag->tag == DW_TAG_volatile_type ||
>  	       tag->tag == DW_TAG_atomic_type ||
>  	       tag->tag == DW_TAG_unspecified_type ||
> -	       tag->tag == DW_TAG_LLVM_annotation;
> +	       tag->tag == DW_TAG_LLVM_annotation ||
> +	       tag->tag == DW_TAG_GNU_annotation;
>  }
>  
>  static inline const char *tag__decl_file(const struct tag *tag,
> diff --git a/dwarves_fprintf.c b/dwarves_fprintf.c
> index 1ec478c2a027..a514d7e98923 100644
> --- a/dwarves_fprintf.c
> +++ b/dwarves_fprintf.c
> @@ -140,6 +140,8 @@ const char *dwarf_tag_name(const uint32_t tag)
>  		return dwarf_gnu_tag_names[tag - DW_TAG_MIPS_loop];
>  	else if (tag == DW_TAG_LLVM_annotation)
>  		return "LLVM_annotation";
> +	else if (tag == DW_TAG_GNU_annotation)
> +		return "GNU_annotation";
>  	return "INVALID";
>  }
>  
> @@ -658,6 +660,7 @@ static const char *__tag__name(const struct tag *tag, const struct cu *cu,
>  		snprintf(bf, len, "%s", variable__name(tag__variable(tag)));
>  		break;
>  	case DW_TAG_LLVM_annotation:
> +	case DW_TAG_GNU_annotation:
>  		type = cu__type(cu, tag->type);
>  		if (type == NULL && tag->type != 0)
>  			tag__id_not_found_snprintf(bf, len, tag->type);
> @@ -731,7 +734,7 @@ static type_id_t skip_llvm_annotations(const struct cu *cu, type_id_t id)
>  		if (id == 0)
>  			break;
>  		type = cu__type(cu, id);
> -		if (type == NULL || type->tag != DW_TAG_LLVM_annotation || type->type == id)
> +		if (type == NULL || (type->tag != DW_TAG_LLVM_annotation && type->tag != DW_TAG_GNU_annotation) || type->type == id)
>  			break;
>  		id = type->type;
>  	}
> @@ -936,7 +939,8 @@ print_modifier: {
>  		else
>  			printed += enumeration__fprintf(type, &tconf, fp);
>  		break;
> -	case DW_TAG_LLVM_annotation: {
> +	case DW_TAG_LLVM_annotation:
> +	case DW_TAG_GNU_annotation: {
>  		struct tag *ttype = cu__type(cu, type->type);
>  		if (ttype) {
>  			type = ttype;
> -- 
> 2.54.0