Fix overflow in KTime_GetEpochNS

2015-01-16 17:08:29 -08:00 · 2015-01-16 17:08:29 -08:00 · 114b35bbbc
commit 114b35bbbc
parent c408b47966
1 changed files with 6 additions and 1 deletions
--- a/sys/kern/ktime.c
+++ b/sys/kern/ktime.c
@ -194,7 +194,12 @@ KTime_GetEpochNS()

    Spinlock_Lock(&ktimeLock);
    tscDiff = Time_GetTSC() - ktimeLastTSC;
-    epoch = (ktimeLastEpoch * 1000000000) + (tscDiff * 1000000000 / ticksPerSecond);
+    /*
+     * This is ugly but it avoids overflowing tscDiff to time computation.  
+     * Note that the bottom bits of ticksPerSecond are not significant so it is 
+     * okay to discard them.
+     */
+    epoch = (ktimeLastEpoch * 1000000000ULL) + (tscDiff * 1000000ULL / ticksPerSecond * 1000ULL);
    Spinlock_Unlock(&ktimeLock);

    return epoch;