[PATCH] um: time-travel: fix time going backwards

[Vincent Whitchurch]

In basic time travel mode, I sometimes see "time goes backwards" panics
like the one below:

 Kernel panic: time-travel: time goes backwards 161689340000492 -> 161689339869814
 Call Trace:
  panic+0x1a1/0x3d7
  time_travel_update_time.cold+0xe9/0x133
  timer_read+0xc1/0x100
  ktime_get+0x10c/0x200
  copy_process+0x1899/0x2230
  kernel_clone+0x57/0x7a0
  kernel_thread+0x4a/0x50
  kthreadd+0x116/0x190

The problem is a race between time_travel_handle_real_alarm() and
timer_read().  time_travel_handle_real_alarm() changes the time after
time_read() reads the current time but before time_travel_update_time()
has had a chance to add the end event.

Fix this by doing the time read and event add atomically with respect to
time_travel_handle_real_alarm().

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@xxxxxxxx>
---
 arch/um/kernel/time.c | 48 ++++++++++++++++++++++++++++++++++--------------
 1 file changed, 34 insertions(+), 14 deletions(-)

diff --git a/arch/um/kernel/time.c b/arch/um/kernel/time.c
index fddd1dec27e6..38a94dd41b8f 100644
--- a/arch/um/kernel/time.c
+++ b/arch/um/kernel/time.c
@@ -392,17 +392,11 @@ bool time_travel_del_event(struct time_travel_event *e)
 	return true;
 }
 
-static void time_travel_update_time(unsigned long long next, bool idle)
+static void time_travel_update_to_event(struct time_travel_event *ne, bool idle)
 {
-	struct time_travel_event ne = {
-		.onstack = true,
-	};
 	struct time_travel_event *e;
 	bool finished = idle;
 
-	/* add it without a handler - we deal with that specifically below */
-	__time_travel_add_event(&ne, next);
-
 	do {
 		e = time_travel_first_event();
 
@@ -414,7 +408,7 @@ static void time_travel_update_time(unsigned long long next, bool idle)
 			BUG_ON(!time_travel_del_event(e));
 			BUG_ON(time_travel_time != e->time);
 
-			if (e == &ne) {
+			if (e == ne) {
 				finished = true;
 			} else {
 				if (e->onstack)
@@ -427,14 +421,38 @@ static void time_travel_update_time(unsigned long long next, bool idle)
 		e = time_travel_first_event();
 		if (e)
 			time_travel_ext_update_request(e->time);
-	} while (ne.pending && !finished);
+	} while (ne->pending && !finished);
+
+	time_travel_del_event(ne);
+}
 
-	time_travel_del_event(&ne);
+static void time_travel_update_time(unsigned long long next, bool idle)
+{
+	struct time_travel_event ne = {
+		.onstack = true,
+	};
+
+	__time_travel_add_event(&ne, next);
+	time_travel_update_to_event(&ne, idle);
+}
+
+static void time_travel_forward_time(unsigned long nsec, bool idle)
+{
+	struct time_travel_event ne = {
+		.onstack = true,
+	};
+	unsigned long flags;
+
+	local_irq_save(flags);
+	__time_travel_add_event(&ne, time_travel_time + nsec);
+	local_irq_restore(flags);
+
+	time_travel_update_to_event(&ne, idle);
 }
 
 void time_travel_ndelay(unsigned long nsec)
 {
-	time_travel_update_time(time_travel_time + nsec, false);
+	time_travel_forward_time(nsec, false);
 }
 EXPORT_SYMBOL(time_travel_ndelay);
 
@@ -572,6 +590,10 @@ static inline void time_travel_update_time(unsigned long long ns, bool retearly)
 {
 }
 
+static void time_travel_forward_time(unsigned long nsec, bool idle)
+{
+}
+
 static inline void time_travel_handle_real_alarm(void)
 {
 }
@@ -720,9 +742,7 @@ static u64 timer_read(struct clocksource *cs)
 		 */
 		if (!irqs_disabled() && !in_interrupt() && !in_softirq() &&
 		    !time_travel_ext_waiting)
-			time_travel_update_time(time_travel_time +
-						TIMER_MULTIPLIER,
-						false);
+			time_travel_forward_time(TIMER_MULTIPLIER, false);
 		return time_travel_time / TIMER_MULTIPLIER;
 	}
 

---
base-commit: 58720809f52779dc0f08e53e54b014209d13eebb
change-id: 20231020-uml-time-backwards-552c81aedd23

Best regards,
-- 
Vincent Whitchurch <vincent.whitchurch@xxxxxxxx>