selftests/bpf: Add selftest for attaching tracing to __btf_id functions

The reuslt:

  $ tools/testing/selftests/bpf/test_progs --name=tracing_btf_ids
  torvalds#467/1   tracing_btf_ids/btf_ids:OK
  torvalds#467     tracing_btf_ids:OK
  Summary: 1/1 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: KaFai Wan <mannkafai@gmail.com>

Author: mannkafai

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

@@ -0,0 +1,16 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <test_progs.h>
+#include "tracing_btf_ids.skel.h"
+
+void test_tracing_btf_ids(void)
+{
+	int btf_id;
+
+	// `migrate_disable` depends on `CONFIG_SMP`, may not exists
+	btf_id = libbpf_find_vmlinux_btf_id("migrate_disable", BPF_TRACE_FENTRY);
+	if (btf_id <= 0)
+		return;
+
+	RUN_TESTS(tracing_btf_ids);
+}

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

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include "bpf_misc.h"
+
+char _license[] SEC("license") = "GPL";
+
+SEC("fentry/migrate_disable")
+__failure __msg("Attaching tracing to __btf_id function 'migrate_disable' is rejected.")
+int BPF_PROG(btf_ids)
+{
+	return 0;
+}