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* [LTP] [PATCH v2] clock_settime: Detect external clock adjustments via CLOCK_MONOTONIC
@ 2026-03-31 12:37 Andrea Cervesato
  2026-03-31 14:14 ` Cyril Hrubis
  0 siblings, 1 reply; 2+ messages in thread
From: Andrea Cervesato @ 2026-03-31 12:37 UTC (permalink / raw)
  To: Linux Test Project; +Cc: Claude

From: Andrea Cervesato <andrea.cervesato@suse.com>

These tests manipulate CLOCK_REALTIME to verify timer and
clock_nanosleep behavior, but NTP or VM time sync can also adjust
CLOCK_REALTIME during the test, causing timers to fire at wrong times
and producing spurious failures.

Use CLOCK_MONOTONIC as a sideband check to detect external interference.
If the monotonic elapsed time is anomalous, report UNTESTED instead of
a false FAIL. Guard the checks with _POSIX_MONOTONIC_CLOCK since
CLOCK_MONOTONIC is optional in POSIX.1-2001.

Co-authored-by: Claude <noreply@anthropic.com>
Signed-off-by: Andrea Cervesato <andrea.cervesato@suse.com>
---
I used Claude to generate the right patches to fix sporadic issues in the
clock_settime testing suite. We tried many attempt for fixing it, but the
CLOCK_REALTIME usage seems to be the real issue in here. The idea is that
we use internal Openposix mechanisms to skip tests if CLOCK_MONOTONIC
usage spots a possible interference from external tools.
---
Changes in v2:
- print a message if CLOCK_MONOTONIC is not available
- always verify that CLOCK_MONOTONIC is available
- Link to v1: https://lore.kernel.org/r/20260331-clock_settime_fix-v1-1-dfae06df2436@suse.com
---
 .../conformance/interfaces/clock_settime/4-1.c     | 36 ++++++++++++++++++
 .../conformance/interfaces/clock_settime/5-1.c     | 43 +++++++++++++++++++++-
 .../conformance/interfaces/clock_settime/5-2.c     | 43 +++++++++++++++++++++-
 .../conformance/interfaces/clock_settime/7-1.c     | 38 +++++++++++++++++++
 .../conformance/interfaces/clock_settime/7-2.c     | 41 ++++++++++++++++++++-
 5 files changed, 198 insertions(+), 3 deletions(-)

diff --git a/testcases/open_posix_testsuite/conformance/interfaces/clock_settime/4-1.c b/testcases/open_posix_testsuite/conformance/interfaces/clock_settime/4-1.c
index f8a3e9c542ca8c9767cdd0057005e519f58b2b55..5f3f816a5fc71a9e62bb3577e1a18a19f987a802 100644
--- a/testcases/open_posix_testsuite/conformance/interfaces/clock_settime/4-1.c
+++ b/testcases/open_posix_testsuite/conformance/interfaces/clock_settime/4-1.c
@@ -42,6 +42,9 @@ int main(void)
 {
 	struct sigevent ev;
 	struct timespec tpT0, tpT2, tpreset;
+#ifdef _POSIX_MONOTONIC_CLOCK
+	struct timespec mono_before, mono_after;
+#endif
 	struct itimerspec its;
 	timer_t tid;
 	int delta;
@@ -55,6 +58,10 @@ int main(void)
 		return PTS_UNTESTED;
 	}
 
+#ifndef _POSIX_MONOTONIC_CLOCK
+	printf("CLOCK_MONOTONIC unavailable, test may fail due to external clock adjustments\n");
+#endif
+
 	/*
 	 * set up sigevent for timer
 	 * set up signal set for sigwait
@@ -82,6 +89,13 @@ int main(void)
 		return PTS_UNRESOLVED;
 	}
 
+#ifdef _POSIX_MONOTONIC_CLOCK
+	if (clock_gettime(CLOCK_MONOTONIC, &mono_before) != 0) {
+		perror("clock_gettime() was not successful\n");
+		return PTS_UNRESOLVED;
+	}
+#endif
+
 	if (timer_create(CLOCK_REALTIME, &ev, &tid) != 0) {
 		perror("timer_create() did not return success\n");
 		return PTS_UNRESOLVED;
@@ -120,6 +134,28 @@ int main(void)
 	tpreset.tv_sec += tpT2.tv_sec - tpT0.tv_sec;
 	setBackTime(tpreset);
 
+#ifdef _POSIX_MONOTONIC_CLOCK
+	if (clock_gettime(CLOCK_MONOTONIC, &mono_after) != 0) {
+		perror("clock_gettime() was not successful\n");
+		return PTS_UNRESOLVED;
+	}
+
+	/*
+	 * The expected monotonic elapsed time is SLEEPTIME + TIMEROFFSET
+	 * since after resetting the clock, the timer must wait another
+	 * full TIMEROFFSET. If monotonic time is much shorter, an external
+	 * clock adjustment (NTP, VM sync) interfered with the test.
+	 */
+	if (mono_after.tv_sec - mono_before.tv_sec <
+	    SLEEPTIME + TIMEROFFSET - ACCEPTABLEDELTA) {
+		printf("UNTESTED: external clock adjustment detected "
+		       "(monotonic elapsed %ds, expected ~%ds)\n",
+		       (int)(mono_after.tv_sec - mono_before.tv_sec),
+		       SLEEPTIME + TIMEROFFSET);
+		return PTS_UNTESTED;
+	}
+#endif
+
 	printf("delta: %d\n", delta);
 	if ((delta <= ACCEPTABLEDELTA) && (delta >= 0)) {
 		printf("Test PASSED\n");
diff --git a/testcases/open_posix_testsuite/conformance/interfaces/clock_settime/5-1.c b/testcases/open_posix_testsuite/conformance/interfaces/clock_settime/5-1.c
index 75fa591e014601b2ef9308a118992a704365392b..da8cbae29fd77fcbca939ac215f5c4d6d450fe31 100644
--- a/testcases/open_posix_testsuite/conformance/interfaces/clock_settime/5-1.c
+++ b/testcases/open_posix_testsuite/conformance/interfaces/clock_settime/5-1.c
@@ -46,9 +46,13 @@ int main(void)
 	struct sigevent ev;
 	struct sigaction act;
 	struct timespec tsclock, ts, tsleft, tsreset;
+#ifdef _POSIX_MONOTONIC_CLOCK
+	struct timespec mono_before, mono_after;
+#endif
 	struct itimerspec its;
 	timer_t tid;
 	sigset_t set;
+	int ns_ret;
 
 	/* Check that we're root...can't call clock_settime with CLOCK_REALTIME otherwise */
 	if (getuid() != 0) {
@@ -56,6 +60,10 @@ int main(void)
 		return PTS_UNTESTED;
 	}
 
+#ifndef _POSIX_MONOTONIC_CLOCK
+	printf("CLOCK_MONOTONIC unavailable, test may fail due to external clock adjustments\n");
+#endif
+
 	/*
 	 * set up sigevent for timer
 	 * set up signal set for sigwait
@@ -97,6 +105,13 @@ int main(void)
 		return PTS_UNRESOLVED;
 	}
 
+#ifdef _POSIX_MONOTONIC_CLOCK
+	if (clock_gettime(CLOCK_MONOTONIC, &mono_before) != 0) {
+		perror("clock_gettime() was not successful\n");
+		return PTS_UNRESOLVED;
+	}
+#endif
+
 	if (clock_gettime(CLOCK_REALTIME, &tsclock) != 0) {
 		printf("clock_gettime() did not return success\n");
 		return PTS_UNRESOLVED;
@@ -112,7 +127,33 @@ int main(void)
 	ts.tv_sec = TIMERSEC + SLEEPDELTA;
 	ts.tv_nsec = 0;
 
-	if (nanosleep(&ts, &tsleft) != -1) {
+	ns_ret = nanosleep(&ts, &tsleft);
+
+#ifdef _POSIX_MONOTONIC_CLOCK
+	if (clock_gettime(CLOCK_MONOTONIC, &mono_after) != 0) {
+		perror("clock_gettime() was not successful\n");
+		return PTS_UNRESOLVED;
+	}
+
+	/*
+	 * The relative timer should fire after TIMERSEC regardless of
+	 * clock_settime changes. If monotonic elapsed time is too far
+	 * from TIMERSEC, an external clock adjustment (NTP, VM sync)
+	 * interfered with the test.
+	 */
+	if (labs(mono_after.tv_sec - mono_before.tv_sec - TIMERSEC) >
+	    ACCEPTABLEDELTA + 1) {
+		printf("UNTESTED: external clock adjustment detected "
+		       "(monotonic elapsed %ds, expected ~%ds)\n",
+		       (int)(mono_after.tv_sec - mono_before.tv_sec),
+		       TIMERSEC);
+		tsreset.tv_sec += mono_after.tv_sec - mono_before.tv_sec;
+		setBackTime(tsreset);
+		return PTS_UNTESTED;
+	}
+#endif
+
+	if (ns_ret != -1) {
 		printf("nanosleep() not interrupted\n");
 		return PTS_FAIL;
 	}
diff --git a/testcases/open_posix_testsuite/conformance/interfaces/clock_settime/5-2.c b/testcases/open_posix_testsuite/conformance/interfaces/clock_settime/5-2.c
index b8a60028c6d3d1a3c0d75922f39697b57329c440..d12d18861ca78b174b47bae69d6e4a1146393159 100644
--- a/testcases/open_posix_testsuite/conformance/interfaces/clock_settime/5-2.c
+++ b/testcases/open_posix_testsuite/conformance/interfaces/clock_settime/5-2.c
@@ -46,9 +46,13 @@ int main(void)
 	struct sigevent ev;
 	struct sigaction act;
 	struct timespec tsclock, ts, tsleft, tsreset;
+#ifdef _POSIX_MONOTONIC_CLOCK
+	struct timespec mono_before, mono_after;
+#endif
 	struct itimerspec its;
 	timer_t tid;
 	sigset_t set;
+	int ns_ret;
 
 	/* Check that we're root...can't call clock_settime with CLOCK_REALTIME otherwise */
 	if (getuid() != 0) {
@@ -56,6 +60,10 @@ int main(void)
 		return PTS_UNTESTED;
 	}
 
+#ifndef _POSIX_MONOTONIC_CLOCK
+	printf("CLOCK_MONOTONIC unavailable, test may fail due to external clock adjustments\n");
+#endif
+
 	/*
 	 * set up sigevent for timer
 	 * set up signal set for sigwait
@@ -97,6 +105,13 @@ int main(void)
 		return PTS_UNRESOLVED;
 	}
 
+#ifdef _POSIX_MONOTONIC_CLOCK
+	if (clock_gettime(CLOCK_MONOTONIC, &mono_before) != 0) {
+		perror("clock_gettime() was not successful\n");
+		return PTS_UNRESOLVED;
+	}
+#endif
+
 	if (clock_gettime(CLOCK_REALTIME, &tsclock) != 0) {
 		printf("clock_gettime() did not return success\n");
 		return PTS_UNRESOLVED;
@@ -112,7 +127,33 @@ int main(void)
 	ts.tv_sec = TIMERSEC + SLEEPDELTA;
 	ts.tv_nsec = 0;
 
-	if (nanosleep(&ts, &tsleft) != -1) {
+	ns_ret = nanosleep(&ts, &tsleft);
+
+#ifdef _POSIX_MONOTONIC_CLOCK
+	if (clock_gettime(CLOCK_MONOTONIC, &mono_after) != 0) {
+		perror("clock_gettime() was not successful\n");
+		return PTS_UNRESOLVED;
+	}
+
+	/*
+	 * The relative timer should fire after TIMERSEC regardless of
+	 * clock_settime changes. If monotonic elapsed time is too far
+	 * from TIMERSEC, an external clock adjustment (NTP, VM sync)
+	 * interfered with the test.
+	 */
+	if (labs(mono_after.tv_sec - mono_before.tv_sec - TIMERSEC) >
+	    ACCEPTABLEDELTA + 1) {
+		printf("UNTESTED: external clock adjustment detected "
+		       "(monotonic elapsed %ds, expected ~%ds)\n",
+		       (int)(mono_after.tv_sec - mono_before.tv_sec),
+		       TIMERSEC);
+		tsreset.tv_sec += mono_after.tv_sec - mono_before.tv_sec;
+		setBackTime(tsreset);
+		return PTS_UNTESTED;
+	}
+#endif
+
+	if (ns_ret != -1) {
 		printf("nanosleep() not interrupted\n");
 		return PTS_FAIL;
 	}
diff --git a/testcases/open_posix_testsuite/conformance/interfaces/clock_settime/7-1.c b/testcases/open_posix_testsuite/conformance/interfaces/clock_settime/7-1.c
index 569eae904ef2eff41441029d3427313107febe53..d372d2d48af0a29133eb833979ae37d682768170 100644
--- a/testcases/open_posix_testsuite/conformance/interfaces/clock_settime/7-1.c
+++ b/testcases/open_posix_testsuite/conformance/interfaces/clock_settime/7-1.c
@@ -36,6 +36,9 @@
 int main(void)
 {
 	struct timespec tsT0, tsT1;
+#ifdef _POSIX_MONOTONIC_CLOCK
+	struct timespec mono_before, mono_after;
+#endif
 	int pid;
 
 	/* Check that we're root...can't call clock_settime with CLOCK_REALTIME otherwise */
@@ -44,11 +47,22 @@ int main(void)
 		return PTS_UNTESTED;
 	}
 
+#ifndef _POSIX_MONOTONIC_CLOCK
+	printf("CLOCK_MONOTONIC unavailable, test may fail due to external clock adjustments\n");
+#endif
+
 	if (clock_gettime(CLOCK_REALTIME, &tsT0) != 0) {
 		perror("clock_gettime() did not return success\n");
 		return PTS_UNRESOLVED;
 	}
 
+#ifdef _POSIX_MONOTONIC_CLOCK
+	if (clock_gettime(CLOCK_MONOTONIC, &mono_before) != 0) {
+		perror("clock_gettime() was not successful\n");
+		return PTS_UNRESOLVED;
+	}
+#endif
+
 	if ((pid = fork()) == 0) {
 		/* child here */
 		int flags = 0;
@@ -103,6 +117,30 @@ int main(void)
 		getBeforeTime(&tsreset);	// get current time
 		tsreset.tv_sec += SMALLTIME;
 		setBackTime(tsreset);
+
+#ifdef _POSIX_MONOTONIC_CLOCK
+		if (clock_gettime(CLOCK_MONOTONIC, &mono_after) != 0) {
+			perror("clock_gettime() was not successful\n");
+			return PTS_UNRESOLVED;
+		}
+
+		/*
+		 * Expected monotonic elapsed time is SMALLTIME + SLEEPOFFSET
+		 * since after resetting the clock, the child must wait
+		 * another full SLEEPOFFSET. If monotonic time is much
+		 * shorter, an external clock adjustment (NTP, VM sync)
+		 * interfered with the test.
+		 */
+		if (mono_after.tv_sec - mono_before.tv_sec <
+		    SMALLTIME + SLEEPOFFSET - ACCEPTABLEDELTA - 1) {
+			printf("UNTESTED: external clock adjustment detected "
+			       "(monotonic elapsed %ds, expected ~%ds)\n",
+			       (int)(mono_after.tv_sec - mono_before.tv_sec),
+			       SMALLTIME + SLEEPOFFSET);
+			return PTS_UNTESTED;
+		}
+#endif
+
 		if (WIFEXITED(i) && WEXITSTATUS(i)) {
 			printf("Test PASSED\n");
 			return PTS_PASS;
diff --git a/testcases/open_posix_testsuite/conformance/interfaces/clock_settime/7-2.c b/testcases/open_posix_testsuite/conformance/interfaces/clock_settime/7-2.c
index 7c340b6c97ce03a1b04709f06996e5a66e1a653a..b9f10bfd5dfc517f0372662b1b1247298d2bee32 100644
--- a/testcases/open_posix_testsuite/conformance/interfaces/clock_settime/7-2.c
+++ b/testcases/open_posix_testsuite/conformance/interfaces/clock_settime/7-2.c
@@ -31,6 +31,7 @@
 
 #define CHILDPASS 1
 #define CHILDFAIL 0
+#define CHILDUNTESTED 2
 
 int main(void)
 {
@@ -43,6 +44,10 @@ int main(void)
 		return PTS_UNTESTED;
 	}
 
+#ifndef _POSIX_MONOTONIC_CLOCK
+	printf("CLOCK_MONOTONIC unavailable, test may fail due to external clock adjustments\n");
+#endif
+
 	if (clock_gettime(CLOCK_REALTIME, &tsT0) != 0) {
 		perror("clock_gettime() did not return success\n");
 		return PTS_UNRESOLVED;
@@ -58,13 +63,44 @@ int main(void)
 		/* child here */
 		int flags = 0;
 		struct timespec tsT3;
+#ifdef _POSIX_MONOTONIC_CLOCK
+		struct timespec mono_start, mono_end;
+#endif
 
 		flags |= TIMER_ABSTIME;
+
+#ifdef _POSIX_MONOTONIC_CLOCK
+		if (clock_gettime(CLOCK_MONOTONIC, &mono_start) != 0) {
+			perror("clock_gettime() was not successful\n");
+			return CHILDFAIL;
+		}
+#endif
+
 		if (clock_nanosleep(CLOCK_REALTIME, flags, &tsT1, NULL) != 0) {
 			printf("clock_nanosleep() did not return success\n");
 			return CHILDFAIL;
 		}
 
+#ifdef _POSIX_MONOTONIC_CLOCK
+		if (clock_gettime(CLOCK_MONOTONIC, &mono_end) != 0) {
+			perror("clock_gettime() was not successful\n");
+			return CHILDFAIL;
+		}
+
+		/*
+		 * The parent sleeps 1s before jumping the clock forward.
+		 * If clock_nanosleep returned in less than 1s monotonic,
+		 * an external clock adjustment (NTP, VM sync) woke us
+		 * instead of the parent's clock_settime.
+		 */
+		if (mono_end.tv_sec - mono_start.tv_sec < 1) {
+			printf("UNTESTED: external clock adjustment detected "
+			       "(monotonic elapsed %ds)\n",
+			       (int)(mono_end.tv_sec - mono_start.tv_sec));
+			return CHILDUNTESTED;
+		}
+#endif
+
 		if (clock_gettime(CLOCK_REALTIME, &tsT3) != 0) {
 			perror("clock_gettime() did not return success\n");
 			return CHILDFAIL;
@@ -105,7 +141,10 @@ int main(void)
 
 		setBackTime(tsreset);	//should be ~= before time
 
-		if (WIFEXITED(i) && WEXITSTATUS(i)) {
+		if (WIFEXITED(i) && WEXITSTATUS(i) == CHILDUNTESTED) {
+			printf("Test UNTESTED\n");
+			return PTS_UNTESTED;
+		} else if (WIFEXITED(i) && WEXITSTATUS(i)) {
 			printf("Test PASSED\n");
 			return PTS_PASS;
 		} else {

---
base-commit: 4688c20c01eece869b59e05ca3dd68c43e0d6af7
change-id: 20260331-clock_settime_fix-8e4117dcb609

Best regards,
-- 
Andrea Cervesato <andrea.cervesato@suse.com>


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2026-03-31 12:37 [LTP] [PATCH v2] clock_settime: Detect external clock adjustments via CLOCK_MONOTONIC Andrea Cervesato
2026-03-31 14:14 ` Cyril Hrubis

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