Re: [PATCH bpf-next 0/6] Improve libbpf support of old kernels

[Daniel Borkmann <daniel@iogearbox.net>]

On 7/8/20 3:53 AM, Andrii Nakryiko wrote:
> This patch set improves libbpf's support of old kernels, missing features like
> BTF support, global variables support, etc.
> 
> Most critical one is a silent drop of CO-RE relocations if libbpf fails to
> load BTF (despite sanitization efforts). This is frequently the case for
> kernels that have no BTF support whatsoever. There are still useful BPF
> applications that could work on such kernels and do rely on CO-RE. To that
> end, this series revamps the way BTF is handled in libbpf. Failure to load BTF
> into kernel doesn't prevent libbpf from using BTF in its full capability
> (e.g., for CO-RE relocations) internally.
> 
> Another issue that was identified was reliance of perf_buffer__new() on
> BPF_OBJ_GET_INFO_BY_FD command, which is more recent that perf_buffer support
> itself. Furthermore, BPF_OBJ_GET_INFO_BY_FD is needed just for some sanity
> checks to provide better user errors, so could be safely omitted if kernel
> doesn't provide it.
> 
> Perf_buffer selftest was adjusted to use skeleton, instead of bpf_prog_load().
> The latter uses BPF_F_TEST_RND_HI32 flag, which is a relatively recent
> addition and unnecessary fails selftest in libbpf's Travis CI tests. By using
> skeleton we both get a shorter selftest and it work on pretty ancient kernels,
> giving better libbpf test coverage.
> 
> One new selftest was added that relies on basic CO-RE features, but otherwise
> doesn't expect any recent features (like global variables) from kernel. Again,
> it's good to have better coverage of old kernels in libbpf testing.
> 
> Cc: Matthew Lim <matthewlim@fb.com>
> 
> Andrii Nakryiko (6):
>    libbpf: make BTF finalization strict
>    libbpf: add btf__set_fd() for more control over loaded BTF FD
>    libbpf: improve BTF sanitization handling
>    selftests/bpf: add test relying only on CO-RE and no recent kernel
>      features
>    libbpf: handle missing BPF_OBJ_GET_INFO_BY_FD gracefully in
>      perf_buffer
>    selftests/bpf: switch perf_buffer test to tracepoint and skeleton
> 
>   tools/lib/bpf/btf.c                           |   7 +-
>   tools/lib/bpf/btf.h                           |   1 +
>   tools/lib/bpf/libbpf.c                        | 150 ++++++++++--------
>   tools/lib/bpf/libbpf.map                      |   1 +
>   .../selftests/bpf/prog_tests/core_retro.c     |  33 ++++
>   .../selftests/bpf/prog_tests/perf_buffer.c    |  42 ++---
>   .../selftests/bpf/progs/test_core_retro.c     |  30 ++++
>   .../selftests/bpf/progs/test_perf_buffer.c    |   4 +-
>   8 files changed, 167 insertions(+), 101 deletions(-)
>   create mode 100644 tools/testing/selftests/bpf/prog_tests/core_retro.c
>   create mode 100644 tools/testing/selftests/bpf/progs/test_core_retro.c
> 

Applied, thanks!