[Bug debug/106263] New: BTF_KIND_FUNC type does not encode linkage

["ibhagat at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106263

            Bug ID: 106263
           Summary: BTF_KIND_FUNC type does not encode linkage
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: debug
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ibhagat at gcc dot gnu.org
  Target Milestone: ---

GCC generates BTF_KIND_FUNC with vlen set to 0.  libbpf expects the vlen field
to include the linkage information.

The issue was reported here:

https://lore.kernel.org/bpf/8735fbcv3x.fsf@oracle.com/

Although GCC is currently generating the BTF_KIND_FUNC as per the BTF format
documentation (see excerpt below), this needs to be changed to allow the tools
to work well with each other.

[Source: https://www.kernel.org/doc/Documentation/bpf/btf.rst]

...
2.2.12 BTF_KIND_FUNC
~~~~~~~~~~~~~~~~~~~~

``struct btf_type`` encoding requirement:
  * ``name_off``: offset to a valid C identifier
  * ``info.kind_flag``: 0
  * ``info.kind``: BTF_KIND_FUNC
  * ``info.vlen``: 0
  * ``type``: a BTF_KIND_FUNC_PROTO type
...