On Wed, Jan 22, 2025 at 3:44 AM Jose E. Marchesi
<jose.marchesi@xxxxxxxxxx> 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].
>
> Why is extending BTF necessary for this? Type and declaration tags
> contain arbitrary strings, and AFAIK you can have more than one type tag
Because currently TYPE_TAG(some_string) is
__attribute__((btf_type_tag("some_string"))).
That btf_type_tag() attribute name is hard-coded in the semantics of
current TYPE_TAG (and DECL_TAG as well). So here Ihor is generalizing
this to be __attribute__((some_string)).
> 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".
I think that ship has sailed. We didn't define any extra semantics for
any sort of "prefix:" part of TYPE_TAG's string, and I'm not sure we
want to retroactively define anything like that at this point.
What is exactly the problem with using kflag=1? Keep in mind, at least
initially, this is meant for tools like pahole and bpftool, not
GCC/Clang itself, to augment BTF with extra annotations (like
preserve_access_index attribute that is added when generating
vmlinux.h).
>
> > 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(-)
