selftests/bpf: Make perf_buffer selftests work on 4.9 kernel again

Recent change to use tp/syscalls/sys_enter_nanosleep for perf_buffer
selftests causes this selftest to fail on 4.9 kernel in libbpf CI ([0]):

  libbpf: prog 'handle_sys_enter': failed to attach to perf_event FD 6: Invalid argument
  libbpf: prog 'handle_sys_enter': failed to attach to tracepoint 'syscalls/sys_enter_nanosleep': Invalid argument

It's not exactly clear why, because perf_event itself is created for
this tracepoint, but I can't even compile 4.9 kernel locally, so it's
hard to figure this out. If anyone has better luck and would like to
help investigating this, I'd really appreciate this.

For now, unblock CI by switching back to raw_syscalls/sys_enter, but reduce
amount of unnecessary samples emitted by filter by process ID. Use
explicit ARRAY map for that to make it work on 4.9 as well, because
global data isn't yet supported there.

Fixes: aa274f98 ("selftests/bpf: Fix possible/online index mismatch in perf_buffer test")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211022201342.3490692-1-andrii@kernel.org

tools/testing/selftests/bpf/prog_tests/perf_buffer.c

@@ -46,6 +46,7 @@ int trigger_on_cpu(int cpu)
 void serial_test_perf_buffer(void)
 {
 	int err, on_len, nr_on_cpus = 0, nr_cpus, i, j;
+	int zero = 0, my_pid = getpid();
 	struct perf_buffer_opts pb_opts = {};
 	struct test_perf_buffer *skel;
 	cpu_set_t cpu_seen;
@@ -71,6 +72,10 @@ void serial_test_perf_buffer(void)
 	if (CHECK(!skel, "skel_load", "skeleton open/load failed\n"))
 		goto out_close;
 
+	err = bpf_map_update_elem(bpf_map__fd(skel->maps.my_pid_map), &zero, &my_pid, 0);
+	if (!ASSERT_OK(err, "my_pid_update"))
+		goto out_close;
+
 	/* attach probe */
 	err = test_perf_buffer__attach(skel);
 	if (CHECK(err, "attach_kprobe", "err %d\n", err))

tools/testing/selftests/bpf/progs/test_perf_buffer.c

@@ -6,20 +6,36 @@
 #include <bpf/bpf_helpers.h>
 #include <bpf/bpf_tracing.h>
 
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__type(key, int);
+	__type(value, int);
+	__uint(max_entries, 1);
+} my_pid_map SEC(".maps");
+
 struct {
 	__uint(type, BPF_MAP_TYPE_PERF_EVENT_ARRAY);
 	__type(key, int);
 	__type(value, int);
 } perf_buf_map SEC(".maps");
 
-SEC("tp/syscalls/sys_enter_nanosleep")
+SEC("tp/raw_syscalls/sys_enter")
 int handle_sys_enter(void *ctx)
 {
+	int zero = 0, *my_pid, cur_pid;
 	int cpu = bpf_get_smp_processor_id();
 
+	my_pid = bpf_map_lookup_elem(&my_pid_map, &zero);
+	if (!my_pid)
+		return 1;
+
+	cur_pid = bpf_get_current_pid_tgid() >> 32;
+	if (cur_pid != *my_pid)
+		return 1;
+
 	bpf_perf_event_output(ctx, &perf_buf_map, BPF_F_CURRENT_CPU,
 			      &cpu, sizeof(cpu));
-	return 0;
+	return 1;
 }
 
 char _license[] SEC("license") = "GPL";