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

[Eduard Zingerman <eddyz87@gmail.com>]

On Wed, 2026-09-16 at 18:20 -0700, Ihor Solodrai 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:
> 
> 	struct foo { ... } __attribute__((bar));
> 	struct foo { int a __attribute__((bar)); };
> 	typedef struct { ... } __attribute__((bar)) foo_t;
> 
> A decl tag is a standalone type pointing at its target.  Build an
> index of decl tags in btf_dump_resize().  Keep it as a flat array
> sorted by (target ID, tag ID). This allows for a stable emission order
> in btf_dump_emit_decl_tags(). Tag IDs are unique, so the comparison is
> a total order and qsort not being stable does not matter.
> 
> Only composite and typedef targets are indexed. Valid BTF allows for
> decl_tags on many types, however btf_dump only supports records,
> record members and typedefs and ignores datasec, var and func.
> 
> btf_dump_emit_decl_tags() binary searches for where the target's
> entries would begin and walks tags while the target matches. A record
> shares its target with its members, so the component_idx is matched
> there.
> 
> A record attribute goes after the closing brace, where
> __attribute__((packed)) already goes. A member attribute goes after
> the bit-field width: clang rejects one between the declarator and the
> ':'. A typedef takes it after the declarator, so a typedef of an
> anonymous record can carry two groups at once, one binding to the
> record and one to the 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.
> 
> Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
> ---

Acked-by: Eduard Zingerman <eddyz87@gmail.com>

> @@ -962,6 +1011,47 @@ static void btf_dump_emit_struct_fwd(struct btf_dump *d, __u32 id,
>  			btf_dump_type_name(d, id));
>  }
>  
> +static void btf_dump_emit_decl_tag(struct btf_dump *d, const struct btf_type *t)
> +{
> +	const char *name = btf_name_of(d, t->name_off);
> +
> +	if (btf_kflag(t))
> +		btf_dump_printf(d, " __attribute__((%s))", name);
> +	else
> +		btf_dump_printf(d, " __attribute__((btf_decl_tag(\"%s\")))", name);
> +}
> +
> +/*
> + * btf_dump_resize() keeps d->decl_tags sorted by (target ID, tag ID), so the
> + * tags of one type form a run that binary search finds the start of, in a
> + * fixed order so that the same BTF always renders the same C.
> + *

--- 8< ---

> + * component_idx is not stored in d->decl_tags: a record and its members share
> + * a target ID, so it is read from each tag.

Nit: useless detail, obvious from the code.

--- >8 ---

...

> @@ -1007,6 +1097,8 @@ static void btf_dump_emit_struct_def(struct btf_dump *d,
>  			prev_bitfield = false;
>  		}
>  
> +		/* after the bit-field width; an attribute cannot precede it */

Nit: /* after the bit-field width */ ?

> +		btf_dump_emit_decl_tags(d, id, i);
>  		btf_dump_printf(d, ";");
>  	}
>  

...