selftests/timers/nanosleep: Add tests for return of remaining time

If interrupted by a signal clock_nanosleep() returns the remaining time
into the structure pointed to by the rmtp parameter. So far this
functionality was not tested by the timer selftests.

Extend the nanosleep selftest to cover this feature.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://patch.msgid.link/20251106-nanosleep-rtmp-selftest-v1-1-f9212fb295fe@linutronix.de

Author: t-8ch

tools/testing/selftests/timers/nanosleep.c

@@ -116,6 +116,56 @@ int nanosleep_test(int clockid, long long ns)
 	return 0;
 }
 
+static void dummy_event_handler(int val)
+{
+	/* No action needed */
+}
+
+static int nanosleep_test_remaining(int clockid)
+{
+	struct timespec rqtp = {}, rmtp = {};
+	struct itimerspec itimer = {};
+	struct sigaction sa = {};
+	timer_t timer;
+	int ret;
+
+	sa.sa_handler = dummy_event_handler;
+	ret = sigaction(SIGALRM, &sa, NULL);
+	if (ret)
+		return -1;
+
+	ret = timer_create(clockid, NULL, &timer);
+	if (ret)
+		return -1;
+
+	itimer.it_value.tv_nsec = NSEC_PER_SEC / 4;
+	ret = timer_settime(timer, 0, &itimer, NULL);
+	if (ret)
+		return -1;
+
+	rqtp.tv_nsec = NSEC_PER_SEC / 2;
+	ret = clock_nanosleep(clockid, 0, &rqtp, &rmtp);
+	if (ret != EINTR)
+		return -1;
+
+	ret = timer_delete(timer);
+	if (ret)
+		return -1;
+
+	sa.sa_handler = SIG_DFL;
+	ret = sigaction(SIGALRM, &sa, NULL);
+	if (ret)
+		return -1;
+
+	if (!in_order((struct timespec) {}, rmtp))
+		return -1;
+
+	if (!in_order(rmtp, rqtp))
+		return -1;
+
+	return 0;
+}
+
 int main(int argc, char **argv)
 {
 	long long length;
@@ -150,6 +200,11 @@ int main(int argc, char **argv)
 			}
 			length *= 100;
 		}
+		ret = nanosleep_test_remaining(clockid);
+		if (ret < 0) {
+			ksft_test_result_fail("%-31s\n", clockstring(clockid));
+			ksft_exit_fail();
+		}
 		ksft_test_result_pass("%-31s\n", clockstring(clockid));
 next:
 		ret = 0;