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

[Eduard Zingerman <eddyz87@gmail.com>]

On Tue, 2025-02-18 at 20:36 -0800, Eduard Zingerman wrote:
> 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);
>   }
> 
> [...]
>

Ok, this comment can be ignored.
The following C code:

int foo(void * __attribute__((address_space(1))) ptr) {
  return ptr != 0;
}

does not compile, with the following error reported:

test3.c:1:49: error: parameter may not be qualified with an address space
    1 | int foo(void *__attribute__((address_space(1))) ptr) {
      |

While the following works:

int foo(void __attribute__((address_space(1))) *ptr) {
  return ptr != 0;
}

With the following IR generated:

define dso_local i32 @foo(ptr addrspace(1) noundef %0) #0 { ... }

Strange.