Merge branch 'bpf-libbpf-link-trace'

Andrii Nakryiko says:

====================
This patchset adds the following APIs to allow attaching BPF programs to
tracing entities:
- bpf_program__attach_perf_event for attaching to any opened perf event FD,
  allowing users full control;
- bpf_program__attach_kprobe for attaching to kernel probes (both entry and
  return probes);
- bpf_program__attach_uprobe for attaching to user probes (both entry/return);
- bpf_program__attach_tracepoint for attaching to kernel tracepoints;
- bpf_program__attach_raw_tracepoint for attaching to raw kernel tracepoint
  (wrapper around bpf_raw_tracepoint_open);

This set of APIs makes libbpf more useful for tracing applications.

All attach APIs return abstract struct bpf_link that encapsulates logic of
detaching BPF program. See patch #2 for details. bpf_assoc was considered as
an alternative name for this opaque "handle", but bpf_link seems to be
appropriate semantically and is nice and short.

Pre-patch #1 makes internal libbpf_strerror_r helper function work w/ negative
error codes, lifting the burder off callers to keep track of error sign.
Patch #2 adds bpf_link abstraction.
Patch #3 adds attach_perf_event, which is the base for all other APIs.
Patch #4 adds kprobe/uprobe APIs.
Patch #5 adds tracepoint API.
Patch #6 adds raw_tracepoint API.
Patch #7 converts one existing test to use attach_perf_event.
Patch #8 adds new kprobe/uprobe tests.
Patch #9 converts some selftests currently using tracepoint to new APIs.

v4->v5:
- typo and small nits (Yonghong);
- validate pfd in attach_perf_event (Yonghong);
- parse_uint_from_file fixes (Yonghong);
- check for malloc failure in attach_raw_tracepoint (Yonghong);
- attach_probes selftests clean up fixes (Yonghong);
v3->v4:
- proper errno handling (Stanislav);
- bpf_fd -> prog_fd (Stanislav);
- switch to fprintf (Song);
v2->v3:
- added bpf_link concept (Daniel);
- didn't add generic bpf_link__attach_program for reasons described in [0];
- dropped Stanislav's Reviewed-by from patches #2-#6, in case he doesn't like
  the change;
v1->v2:
- preserve errno before close() call (Stanislav);
- use libbpf_perf_event_disable_and_close in selftest (Stanislav);
- remove unnecessary memset (Stanislav);

[0] https://lore.kernel.org/bpf/CAEf4BzZ7EM5eP2eaZn7T2Yb5QgVRiwAs+epeLR1g01TTx-6m6Q@mail.gmail.com/
====================

Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>