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

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

On 22/01/2025 02:53, Ihor Solodrai wrote:
> 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].
> 
> 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.
>

sounds good! So presumably pahole will then have a "full_attribute" or
similar BTF feature that will only do full attribute encoding for
kernels that expect the kind flag to be set? Otherwise we'll run the
risk of generating invalid BTF for older kernels with newer pahole
(since those older kernels will fail to verify tags with a kind flag set).

Thanks!

Alan

> [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(-)
>