Re: [PATCHv6 10/36] kernel: Add do_timens_ktime_to_host() helper

+ktime_t do_timens_ktime_to_host(clockid_t clockid, ktime_t tim,
+				struct timens_offsets *ns_offsets)
+{
+	ktime_t offset;
+
+	switch (clockid) {
+	case CLOCK_MONOTONIC:
+		offset = timespec64_to_ktime(ns_offsets->monotonic);
+		break;
+	case CLOCK_BOOTTIME:
+	case CLOCK_BOOTTIME_ALARM:
+		offset = timespec64_to_ktime(ns_offsets->boottime);
+		break;
+	default:
+		return tim;
+	}
+
+	/*
+	 * Check that @tim value is in [offset, KTIME_MAX + offset]
+	 * and subtract offset.
+	 */
+	if (tim < offset) {
+		/*
+		 * User can specify @tim *absolute* value - if it's lesser than
+		 * the time namespace's offset - it's already expired.
+		 */
+		tim = 0;
+	} else if (KTIME_MAX - tim < -offset) {
+		/*
+		 * User-supplied @tim may be close or even equal KTIME_MAX
+		 * and time namespace offset can be negative.
+		 * Let's check (tim - offset) for an overflow.
+		 * Return KTIME_MAX in such case, as the time value is
+		 * thousands *years* in future anyway.
+		 */
+		tim = KTIME_MAX;
+	} else {
+		tim = ktime_sub(tim, offset);
+	}