selftests/bpf: Fix spurious test failures in core_retro selftest

core_retro selftest uses BPF program that's triggered on sys_enter
system-wide, but has no protection from some unrelated process doing syscall
while selftest is running. This leads to occasional test failures with
unexpected PIDs being returned. Fix that by filtering out all processes that
are not test_progs process.

Fixes: fcda189a5133 ("selftests/bpf: Add test relying only on CO-RE and no recent kernel features")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200731204957.2047119-1-andriin@fb.com

selftests/bpf: Add test relying only on CO-RE and no recent kernel features

Add a test that relies on CO-RE, but doesn't expect any of the recent
features, not available on old kernels. This is useful for Travis CI tests
running against very old kernels (e.g., libbpf has 4.9 kernel testing now), to
verify that CO-RE still works, even if kernel itself doesn't support BTF yet,
as long as there is .BTF embedded into vmlinux image by pahole. Given most of
CO-RE doesn't require any kernel awareness of BTF, it is a useful test to
validate that libbpf's BTF sanitization is working well even with ancient
kernels.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200708015318.3827358-5-andriin@fb.com