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

Andrii Nakryiko
2021-07-30T00:32:02Z

On Thu, Jul 29, 2021 at 9:20 AM Quentin Monnet <quentin@isovalent.com> wrote:
>
> As part of the effort to move towards a v1.0 for libbpf [0], this set
> improves some confusing function names related to BTF loading from and to
> the kernel:
>
> - btf__load() becomes btf__load_into_kernel().
> - btf__get_from_id becomes btf__load_from_kernel_by_id().
> - A new version btf__load_from_kernel_by_id_split() extends the former to
> add support for split BTF.
>
> The old functions are marked for deprecation for the next minor version
> (0.6) of libbpf.
>
> The last patch is a trivial change to bpftool to add support for dumping
> split BTF objects by referencing them by their id (and not only by their
> BTF path).
>
> [0] https://github.com/libbpf/libbpf/wiki/Libbpf:-the-road-to-v1.0#btfh-apis
>
> v3:
> - Use libbpf_err_ptr() in btf__load_from_kernel_by_id(), ERR_PTR() in
> bpftool's get_map_kv_btf().
> - Move the definition of btf__load_from_kernel_by_id() closer to the
> btf__parse() group in btf.h (move the legacy function with it).
> - Fix a bug on the return value in libbpf_find_prog_btf_id(), as a new
> patch.
> - Move the btf__free() fixes to their own patch.
> - Add "Fixes:" tags to relevant patches.
> - Re-introduce deprecation (removed in v2) for the legacy functions, as a
> new macro LIBBPF_DEPRECATED_SINCE(major, minor, message).
>
> v2:
> - Remove deprecation marking of legacy functions (patch 4/6 from v1).
> - Make btf__load_from_kernel_by_id{,_split}() return the btf struct, adjust
> surrounding code and call btf__free() when missing.
> - Add new functions to v0.5.0 API (and not v0.6.0).
>
> Quentin Monnet (8):
> libbpf: return non-null error on failures in libbpf_find_prog_btf_id()
> libbpf: rename btf__load() as btf__load_into_kernel()
> libbpf: rename btf__get_from_id() as btf__load_from_kernel_by_id()
> tools: free BTF objects at various locations
> tools: replace btf__get_from_id() with btf__load_from_kernel_by_id()
> libbpf: prepare deprecation of btf__get_from_id(), btf__load()
> libbpf: add split BTF support for btf__load_from_kernel_by_id()
> tools: bpftool: support dumping split BTF by id
>
> tools/bpf/bpftool/btf.c | 8 ++---
> tools/bpf/bpftool/btf_dumper.c | 6 ++--
> tools/bpf/bpftool/map.c | 14 ++++-----
> tools/bpf/bpftool/prog.c | 29 +++++++++++------
> tools/lib/bpf/Makefile | 3 ++
> tools/lib/bpf/btf.c | 33 ++++++++++++++------
> tools/lib/bpf/btf.h | 7 ++++-
> tools/lib/bpf/libbpf.c | 11 ++++---
> tools/lib/bpf/libbpf.map | 3 ++
> tools/lib/bpf/libbpf_common.h | 19 +++++++++++
> tools/perf/util/bpf-event.c | 11 ++++---
> tools/perf/util/bpf_counter.c | 12 +++++--
> tools/testing/selftests/bpf/prog_tests/btf.c | 4 ++-
> 13 files changed, 113 insertions(+), 47 deletions(-)
>
> --
> 2.30.2
>

I dropped patch #7 with deprecations and LIBBPF_DEPRECATED_SINCE and
applied to bpf-next.

Current LIBBPF_DEPRECATED_SINCE approach doesn't work (and you should
have caught this when you built selftests/bpf, what happened there?).
bpftool build generates warnings like this:

In file included from /data/users/andriin/linux/tools/lib/bpf/libbpf.h:20,
from xlated_dumper.c:10:
/data/users/andriin/linux/tools/lib/bpf/libbpf_common.h:22:23:
warning: "LIBBPF_MAJOR_VERSION" is not defined, evaluates to 0
[-Wundef]
__LIBBPF_GET_VERSION(LIBBPF_MAJOR_VERSION, LIBBPF_MINOR_VERSION)
^~~~~~~~~~~~~~~~~~~~

And it makes total sense. LIBBPF_DEPRECATED_SINCE() assumes
LIBBPF_MAJOR_VERSION/LIBBPF_MINOR_VERSION is defined at compilation
time of the *application that is using libbpf*, not just libbpf's
compilation time. And that's clearly a bogus assumption which we can't
and shouldn't make. 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.

Anyways, I've removed all the LIBBPF_DEPRECATED_SINCE stuff and
applied all the rest, as it looks good and is a useful addition. We
should work some more on deprecation helpers, though.