Allow filter sets that reduce the filter, not just increase

Previous behavior would only pass along filters that added new bits
(filters) to the current filter set, without considering whether some
bits were set only by the old HigherPacketFilter. Possibly related
to nmap#106, since this code was one of few changes in v0.9990.

packetWin7/npf/npf/Openclos.c

@@ -2609,12 +2609,11 @@ NOTE: Called at <= DISPATCH_LEVEL  (unlike a miniport's MiniportOidRequest)
 
 	TRACE_ENTER();
 
-	// Special case: if they're trying to set a packet filter that is a
-	// subset of our existing one, then we don't pass it down but just
-	// return success.
+	// Special case: if their new packet filter doesn't change the lower one
+	// then we don't pass it down but just return success.
 	if (Request->RequestType == NdisRequestSetInformation
 			&& Request->DATA.SET_INFORMATION.Oid == OID_GEN_CURRENT_PACKET_FILTER
-			&& (*(PULONG) Request->DATA.SET_INFORMATION.InformationBuffer & ~(pFiltMod->HigherPacketFilter | pFiltMod->MyPacketFilter)) == 0)
+			&& (*(PULONG) Request->DATA.SET_INFORMATION.InformationBuffer | pFiltMod->MyPacketFilter) == (pFiltMod->HigherPacketFilter | pFiltMod->MyPacketFilter))
 	{
 		pFiltMod->HigherPacketFilter = *(PULONG) Request->DATA.SET_INFORMATION.InformationBuffer;
 		Request->DATA.SET_INFORMATION.BytesRead = sizeof(ULONG);
@@ -3230,7 +3229,7 @@ NPF_SetPacketFilter(
 #endif
 	// If the new packet filter is the same as the old one...
 	if (NewPacketFilter == pFiltMod->MyPacketFilter
-		// ...or it wouldn't change the upper one
+		// ...or it wouldn't add to the upper one
 		|| (NewPacketFilter & (~pFiltMod->HigherPacketFilter)) == 0)
        	{
 		pFiltMod->MyPacketFilter = NewPacketFilter;