Re: [PATCH bpf-next v3 0/8] libbpf: rename btf__get_from_id() and btf__load() APIs, support split BTF

On Fri, 30 Jul 2021 at 18:24, Andrii Nakryiko [off-list ref] wrote:

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The right approach will be to define
LIBBPF_MAJOR_VERSION/LIBBPF_MINOR_VERSION in some sort of
auto-generated header, included from libbpf_common.h and installed as
part of libbpf package.

So generating this header is easy. Installing it with the other headers
is simple too. It becomes a bit trickier when we build outside of the
directory (it seems I need to pass -I$(OUTPUT) to build libbpf).

Not sure why using the header is tricky. We auto-generate
bpf_helper_defs.h, which is included from bpf_helpers.h, which is
included in every single libbpf-using application. Works good with no
extra magic.

bpf_helper_defs.h is the first thing I looked at, and I processed
libbpf_version.h just like it. But there is a difference:
bpf_helper_defs.h is _not_ included in libbpf itself, nor is it needed
in bpftool at the bootstrap stage (it is only included from the eBPF
skeletons for profiling or showing PIDs etc., which are compiled after
libbpf). The version header is needed in both cases.

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The step I'm most struggling with at the moment is bpftool, which
bootstraps a first version of itself before building libbpf, by looking
at the headers directly in libbpf's directory. It means that the
generated header with the version number has not yet been generated. Do
you think it is worth changing bpftool's build steps to implement this
deprecation helper?

If it doesn't do that already, bpftool should do `make install` for
libbpf, not just build. Install will put all the headers, generated or
otherwise, into a designated destination folder, which should be
passed as -I parameter. But that should be already happening due to
bpf_helper_defs.h.

bpftool does not run "make install". It compiles libbpf passing
"OUTPUT=$(LIBBPF_OUTPUT)", sets LIBBPF_PATH to the same directory, and
then adds "-I$(LIBBPF_PATH)" for accessing bpf_helper_defs.h and compile
its eBPF programs. It is possible to include libbpf_version.h the same
way, but only after libbpf has been compiled, after the bootstrap.

I'll look into updating the Makefile to compile and install libbpf
before the bootstrap, when I have some time.