selftests/bpf: livepatch_trampoline: skip when prerequisites are missing

livepatch_trampoline relies on livepatch sysfs and livepatch-sample.ko.
When CONFIG_LIVEPATCH is disabled or the samples module isn't built, the
test fails with ENOENT and causes false failures in minimal CI configs.

Skip the test when livepatch sysfs or the sample module is unavailable.
Also avoid writing to livepatch sysfs when it's not present.

Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>

Author: ahjf-07

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

@@ -5,6 +5,8 @@
 #include "testing_helpers.h"
 #include "livepatch_trampoline.skel.h"
 
+#define LIVEPATCH_ENABLED_PATH "/sys/kernel/livepatch/livepatch_sample/enabled"
+
 static int load_livepatch(void)
 {
 	char path[4096];
@@ -19,7 +21,8 @@ static int load_livepatch(void)
 static void unload_livepatch(void)
 {
 	/* Disable the livepatch before unloading the module */
-	system("echo 0 > /sys/kernel/livepatch/livepatch_sample/enabled");
+	if (!access(LIVEPATCH_ENABLED_PATH, F_OK))
+		system("echo 0 > " LIVEPATCH_ENABLED_PATH);
 
 	unload_module("livepatch_sample", env_verbosity > VERBOSE_NONE);
 }
@@ -81,9 +84,22 @@ static void __test_livepatch_trampoline(bool fexit_first)
 void test_livepatch_trampoline(void)
 {
 	int retry_cnt = 0;
+	int err;
+
+	/* Skip if kernel was built without CONFIG_LIVEPATCH */
+	if (access("/sys/kernel/livepatch", F_OK)) {
+		test__skip();
+		return;
+	}
 
 retry:
-	if (load_livepatch()) {
+	err = load_livepatch();
+	if (err) {
+		if (err == -ENOENT) {
+			test__skip();
+			return;
+		}
+
 		if (retry_cnt) {
 			ASSERT_OK(1, "load_livepatch");
 			goto out;