Re: [PATCH v2 dwarves 2/4] btf_encoder: emit type tags for bpf_arena pointers

[Eduard Zingerman <eddyz87@gmail.com>]

On Wed, 2025-02-12 at 12:15 -0800, Ihor Solodrai wrote:

[...]

> diff --git a/btf_encoder.c b/btf_encoder.c
> index 965e8f0..3cec106 100644
> --- a/btf_encoder.c
> +++ b/btf_encoder.c

[...]

> +static int btf__tag_bpf_arena_ptr(struct btf *btf, int ptr_id)
> +{
> +	const struct btf_type *ptr;
> +	int tagged_type_id;
> +
> +	ptr = btf__type_by_id(btf, ptr_id);
> +	if (!btf_is_ptr(ptr))
> +		return -EINVAL;
> +
> +	tagged_type_id = btf__add_type_attr(btf, BPF_ARENA_ATTR, ptr->type);
> +	if (tagged_type_id < 0)
> +		return tagged_type_id;
> +
> +	return btf__add_ptr(btf, tagged_type_id);
> +}

I might be confused, but this is a bit strange.
The type constructed here is: ptr -> type_tag -> t.
However, address_space is an attribute of a pointer, not a pointed type.
I think that correct sequence should be: type_tag -> ptr -> t.
This would make libbpf emit C declaration as follows:

  void * __attribute__((address_space(1))) ptr;

Instead of current:

  void __attribute__((address_space(1))) * ptr;

clang generates identical IR for both declarations:

  @ptr = dso_local global ptr addrspace(1) null, align 8

Thus, imo, this function should be simplified as below:

  static int btf__tag_bpf_arena_ptr(struct btf *btf, int ptr_id)
  {
	const struct btf_type *ptr;

	ptr = btf__type_by_id(btf, ptr_id);
	if (!btf_is_ptr(ptr))
		return -EINVAL;

	return btf__add_type_attr(btf, BPF_ARENA_ATTR, ptr_id);
  }

[...]