Re: [PATCH v2 bpf-next 02/13] libbpf: add btf__distill_base() creating split BTF with distilled base BTF

[Alan Maguire <alan.maguire@oracle.com>]

On 01/05/2024 18:43, Eduard Zingerman wrote:
> On Wed, 2024-05-01 at 18:29 +0100, Alan Maguire wrote:
> 
> [...]
> 
>>>> +/* Check if a member of a split BTF struct/union refers to a base BTF
>>>> + * struct/union.  Members can be const/restrict/volatile/typedef
>>>> + * reference types, but if a pointer is encountered, type is no longer
>>>> + * considered embedded.
>>>> + */
>>>> +static int btf_find_embedded_composite_type_ids(__u32 *id, void *ctx)
>>>> +{
>>>> +	struct btf_distill *dist = ctx;
>>>> +	const struct btf_type *t;
>>>> +	__u32 next_id = *id;
>>>> +
>>>> +	do {
>>>> +		if (next_id == 0)
>>>> +			return 0;
>>>> +		t = btf_type_by_id(dist->pipe.src, next_id);
>>>> +		switch (btf_kind(t)) {
>>>> +		case BTF_KIND_CONST:
>>>> +		case BTF_KIND_RESTRICT:
>>>> +		case BTF_KIND_VOLATILE:
>>>> +		case BTF_KIND_TYPEDEF:
>>>
>>> I think BTF_KIND_TYPE_TAG is missing.
>>>
>>
>> It's implicit in the default clause; I can't see a case for having a
>> split BTF type tag base BTF types, but I might be missing something
>> there. I can make all the unexpected types explicit if that would be
>> clearer?
> 
> I mean, this skips a series of modifiers, e.g.:
> 
> struct buz {
>   // next_id will get to 'struct bar' eventually
>   const volatile struct bar foo;
> }
> 
> Now, it is legal to have this chain like below:
> 
> struct buz {
>   const volatile __type_tag("quux") struct bar foo;
> }
> 
> In which case the traversal does not have to stop.
> Am I confused?
>

no, sorry, I was! You're absolutely right, BTF_KIND_TYPE_TAG needs to be
accounted for here. Thanks for catching this!

> (Note: at the moment type tags are only applied to pointers but that
>  would change in the future, I have a stalled LLVM change for this).
> 
> [...]