Re: [PATCH bpf-next 0/5] BTF: arbitrary __attribute__ encoding

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

> This patch series extends BPF Type Format (BTF) to support arbitrary
> __attribute__ encoding.
>
> Setting the kind_flag to 1 in BTF type tags and decl tags now changes
> the meaning for the encoded tag, in particular with respect to
> btf_dump in libbpf.
>
> If the kflag is set, then the string encoded by the tag represents the
> full attribute-list of an attribute specifier [1].

Why is extending BTF necessary for this?  Type and declaration tags
contain arbitrary strings, and AFAIK you can have more than one type tag
associated with a single type or declaration.  Why coupling the
interpretation of the contents of the string with the transport format?

Something like "cattribute:always_inline".

> This feature will allow extending tools such as pahole and bpftool to
> capture and use more granular type information, and make it easier to
> manage compatibility between clang and gcc BPF compilers.
>
> [1] https://gcc.gnu.org/onlinedocs/gcc-13.2.0/gcc/Attribute-Syntax.html
>
> Ihor Solodrai (5):
>   libbpf: introduce kflag for type_tags and decl_tags in BTF
>   libbpf: check the kflag of type tags in btf_dump
>   selftests/bpf: add a btf_dump test for type_tags
>   bpf: allow kind_flag for BTF type and decl tags
>   selftests/bpf: add a BTF verification test for kflagged type_tag
>
>  Documentation/bpf/btf.rst                     |  27 +++-
>  kernel/bpf/btf.c                              |   7 +-
>  tools/include/uapi/linux/btf.h                |   3 +-
>  tools/lib/bpf/btf.c                           |  87 +++++++---
>  tools/lib/bpf/btf.h                           |   3 +
>  tools/lib/bpf/btf_dump.c                      |   5 +-
>  tools/lib/bpf/libbpf.map                      |   2 +
>  tools/testing/selftests/bpf/prog_tests/btf.c  |  23 ++-
>  .../selftests/bpf/prog_tests/btf_dump.c       | 148 +++++++++++++-----
>  tools/testing/selftests/bpf/test_btf.h        |   6 +
>  10 files changed, 234 insertions(+), 77 deletions(-)