selftests/bpf: extend multi-uprobe tests with USDTs

Validate libbpf's USDT-over-multi-uprobe logic by adding USDTs to
existing multi-uprobe tests. This checks correct libbpf fallback to
singular uprobes (when run on older kernels with buggy PID filtering).
We reuse already established child process and child thread testing
infrastructure, so additions are minimal. These test fail on either
older kernels or older version of libbpf that doesn't detect PID
filtering problems.

Acked-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240521163401.3005045-6-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

selftests/bpf: extend multi-uprobe tests with child thread case

Extend existing multi-uprobe tests to test that PID filtering works
correctly. We already have child *process* tests, but we need also child
*thread* tests. This patch adds spawn_thread() helper to start child
thread, wait for it to be ready, and then instruct it to trigger desired
uprobes.

Additionally, we extend BPF-side code to track thread ID, not just
process ID. Also we detect whether extraneous triggerings with
unexpected process IDs happened, and validate that none of that happened
in practice.

These changes prove that fixed PID filtering logic for multi-uprobe
works as expected. These tests fail on old kernels.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20240521163401.3005045-5-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

selftests/bpf: Add extra link to uprobe_multi tests

Attaching extra program to same functions system wide for api
and link tests.

This way we can test the pid filter works properly when there's
extra system wide consumer on the same uprobe that will trigger
the original uprobe handler.

We expect to have the same counts as before.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20230809083440.3209381-29-jolsa@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

selftests/bpf: Add uprobe_multi pid filter tests

Running api and link tests also with pid filter and checking
the probe gets executed only for specific pid.

Spawning extra process to trigger attached uprobes and checking
we get correct counts from executed programs.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20230809083440.3209381-28-jolsa@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

selftests/bpf: Add uprobe_multi skel test

Adding uprobe_multi test for skeleton load/attach functions,
to test skeleton auto attach for uprobe_multi link.

Test that bpf_get_func_ip works properly for uprobe_multi
attachment.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20230809083440.3209381-20-jolsa@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>