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

[Ihor Solodrai <ihor.solodrai@linux.dev>]

On 1/22/25 2:13 AM, Alan Maguire wrote:
 > 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).

Hi Alan. Sorry I missed this message on the first pass.

Yes, this BTF extension enables pahole to encode any __attribute__ in
BTF. There is already a use case for that, which led me to this patch
series: expressing bpf_arena tags in vmlinux.h.

As to "full_attribute"-like feature in pahole, my intuition is that
it's a significant chunk of work, and it might not be necessary in the
near future. But with these changes in BTF it at least becomes a
possibility. And you're right, it will have to be an optional feature.

 >
 > 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(-)
 >>
 >