selftests/bpf: add missing __weak kfunc log fixup test

Add test validating that libbpf correctly poisons and reports __weak
unresolved kfuncs in post-processed verifier log.

Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

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

···
 	test_log_fixup__destroy(skel);
 }
 
+static void missing_kfunc(void)
+{
+	char log_buf[8 * 1024];
+	struct test_log_fixup* skel;
+	int err;
+
+	skel = test_log_fixup__open();
+	if (!ASSERT_OK_PTR(skel, "skel_open"))
+		return;
+
+	bpf_program__set_autoload(skel->progs.use_missing_kfunc, true);
+	bpf_program__set_log_buf(skel->progs.use_missing_kfunc, log_buf, sizeof(log_buf));
+
+	err = test_log_fixup__load(skel);
+	if (!ASSERT_ERR(err, "load_fail"))
+		goto cleanup;
+
+	ASSERT_HAS_SUBSTR(log_buf,
+			  "0: <invalid kfunc call>\n"
+			  "kfunc 'bpf_nonexistent_kfunc' is referenced but wasn't resolved\n",
+			  "log_buf");
+
+	if (env.verbosity > VERBOSE_NONE)
+		printf("LOG:   \n=================\n%s=================\n", log_buf);
+
+cleanup:
+	test_log_fixup__destroy(skel);
+}
+
 void test_log_fixup(void)
 {
 	if (test__start_subtest("bad_core_relo_trunc_none"))
···
 		bad_core_relo_subprog();
 	if (test__start_subtest("missing_map"))
 		missing_map();
+	if (test__start_subtest("missing_kfunc"))
+		missing_kfunc();
 }

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

···
 	return value != NULL;
 }
 
+extern int bpf_nonexistent_kfunc(void) __ksym __weak;
+
+SEC("?raw_tp/sys_enter")
+int use_missing_kfunc(const void *ctx)
+{
+	bpf_nonexistent_kfunc();
+
+	return 0;
+}
+
 char _license[] SEC("license") = "GPL";