Fix problem where NTP times with the high-bit set to 0 (which RFC 2030

chapter 3 has redefined to mean years *after* 2036) were being represented as
times prior to 1968.

This has been broken since r35840 (apparently not many people see NTP
timestamps beyond 2036 :-)): apparently I over-optimized packet-ntp's code
while copying it into proto.c: that temporary variable is necessary for the
unsigned math to happen correctly before assigning the result to the (signed)
time_t.

Leave a comment in the code indicating why the temporary variable is needed.
Copy that comment to packet-ntp.c.

Fix the same problem in ntp_to_nstime(): it also did not use the temporary variable.

svn path=/trunk/; revision=45790

epan/dissectors/packet-ntp.c

@@ -644,6 +644,10 @@ tvb_ntp_fmt_ts(tvbuff_t *tvb, gint offset)
 		return "NULL";
 	}
 
+	/* We need a temporary variable here so the unsigned math
+	 * works correctly (for years > 2036 according to RFC 2030
+	 * chapter 3).
+	 */
 	temptime = tempstmp - (guint32) NTP_BASETIME;
 	bd = gmtime(&temptime);
 	if(!bd){
@@ -666,9 +670,18 @@ tvb_ntp_fmt_ts(tvbuff_t *tvb, gint offset)
 void
 ntp_to_nstime(tvbuff_t *tvb, gint offset, nstime_t *nstime)
 {
-	nstime->secs  = tvb_get_ntohl(tvb, offset);
-	if (nstime->secs)
-		nstime->secs -= NTP_BASETIME;
+	guint32		 tempstmp;
+
+	/* We need a temporary variable here so the unsigned math
+	 * works correctly (for years > 2036 according to RFC 2030
+	 * chapter 3).
+	 */
+	tempstmp  = tvb_get_ntohl(tvb, offset);
+	if (tempstmp)
+		nstime->secs = tempstmp - (guint32)NTP_BASETIME;
+	else
+		nstime->secs = tempstmp; /* 0 */
+
 	nstime->nsecs = (int)(tvb_get_ntohl(tvb, offset+4)/(NTP_FLOAT_DENOM/1000000000.0));
 }
 

epan/proto.c

@@ -618,7 +618,7 @@ void
 proto_tree_reset(proto_tree *tree)
 {
 	tree_data_t *tree_data = PTREE_DATA(tree);
-  
+
 	proto_tree_children_foreach(tree, proto_tree_free_node, NULL);
 
 	/* reset tree */
@@ -1230,6 +1230,7 @@ proto_tree_new_item(field_info *new_fi, proto_tree *tree,
 	double	    doubleval;
 	const char *string;
 	nstime_t    time_stamp;
+	guint32     tmpsecs;
 	GPtrArray  *ptrs;
 	gboolean    length_error;
 
@@ -1626,9 +1627,16 @@ proto_tree_new_item(field_info *new_fi, proto_tree *tree,
 
 /* XXX - where should this go? */
 #define NTP_BASETIME 2208988800ul
-				time_stamp.secs  = tvb_get_ntohl(tvb, start);
-				if (time_stamp.secs)
-					time_stamp.secs -= NTP_BASETIME;
+
+				/* We need a temporary variable here so the unsigned math
+				 * works correctly (for years > 2036 according to RFC 2030
+				 * chapter 3).
+				 */
+				tmpsecs  = tvb_get_ntohl(tvb, start);
+				if (tmpsecs)
+					time_stamp.secs = tmpsecs - (guint32)NTP_BASETIME;
+				else
+					time_stamp.secs = tmpsecs; /* 0 */
 
 				if (length == 8) {
 					/*
@@ -1648,9 +1656,11 @@ proto_tree_new_item(field_info *new_fi, proto_tree *tree,
 				/*
 				 * NTP time stamp, big-endian.
 				 */
-				time_stamp.secs  = tvb_get_letohl(tvb, start);
-				if (time_stamp.secs)
-					time_stamp.secs -= NTP_BASETIME;
+				tmpsecs  = tvb_get_letohl(tvb, start);
+				if (tmpsecs)
+					time_stamp.secs = tmpsecs - (guint32)NTP_BASETIME;
+				else
+					time_stamp.secs = tmpsecs; /* 0 */
 
 				if (length == 8) {
 					/*