net-test: packetdrill: fix error msg in loose segment check path

Fix the following misleading error msg introduced in commit:
  net-test: packetdrill: loose segmentation checks (non-strict by default now)
which implements loose segment check:
1. Subtract isn from tcp_seq/ack_seq for all segments so error msg
displays those sequence number correctly
2. when segment length is not expected, print expected and actual total
length seen apart from the expected and actual length of this segment.

Change-Id: I4138c357d5b69ac6025156ee7e91fa2bb128707a

gtests/net/packetdrill/run_packet.c

@@ -587,6 +587,26 @@ static int map_inbound_packet(
 	return STATUS_OK;
 }
 
+static int tcp_convert_seq_number(struct socket *socket, struct packet *packet,
+				  char **error)
+{
+	/* Rewrite TCP sequence number from live to script space. */
+	const bool is_syn = packet->tcp->syn;
+	const u32 seq_offset = local_seq_live_to_script_offset(socket, is_syn);
+	packet->tcp->seq =
+	    htonl(ntohl(packet->tcp->seq) + seq_offset);
+
+	/* Rewrite ACKs and SACKs from live to script space. */
+	const u32 ack_offset = remote_seq_live_to_script_offset(socket, is_syn);
+	if (packet->tcp->ack)
+		packet->tcp->ack_seq =
+		    htonl(ntohl(packet->tcp->ack_seq) + ack_offset);
+	if (offset_sack_blocks(packet, ack_offset, error))
+		return STATUS_ERR;
+
+	return STATUS_OK;
+}
+
 /* Transforms values in the 'actual_packet' by mapping outbound packet
  * values in the sniffed 'live_packet' (address 4-tuple, sequence
  * number in seq, timestamp value) from live values to script values
@@ -622,20 +642,6 @@ static int map_outbound_live_packet(
 	if (live_packet->tcp == NULL)
 		return STATUS_OK;
 
-	/* Rewrite TCP sequence number from live to script space. */
-	const bool is_syn = live_packet->tcp->syn;
-	const u32 seq_offset = local_seq_live_to_script_offset(socket, is_syn);
-	actual_packet->tcp->seq =
-	    htonl(ntohl(live_packet->tcp->seq) + seq_offset);
-
-	/* Rewrite ACKs and SACKs from live to script space. */
-	const u32 ack_offset = remote_seq_live_to_script_offset(socket, is_syn);
-	if (actual_packet->tcp->ack)
-		actual_packet->tcp->ack_seq =
-		    htonl(ntohl(live_packet->tcp->ack_seq) + ack_offset);
-	if (offset_sack_blocks(actual_packet, ack_offset, error))
-		return STATUS_ERR;
-
 	/* Extract location of script and actual TCP timestamp values. */
 	if (find_tcp_timestamp(script_packet, error))
 		return STATUS_ERR;
@@ -1404,7 +1410,8 @@ static int verify_outbound_live_packet(
 static int verify_packet_fragments(
 	const struct packet *current_fragment,
 	const struct packet *previous_fragment,
-	int current_payload, int expected_payload, char **error)
+	int current_payload, int sniffed_payload,
+	int expected_total_payload, char **error)
 {
 	/* Ensure that current fragment headers match the previous fragment
 	 * headers.
@@ -1416,12 +1423,16 @@ static int verify_packet_fragments(
 	/* If this is not the last fragment, also check that its payload matches
 	 * the payload of the previous fragment.
 	 */
-	if (current_payload < expected_payload && current_payload !=
-			packet_payload_len(previous_fragment)) {
+	if (current_payload < expected_total_payload - sniffed_payload &&
+	    current_payload != packet_payload_len(previous_fragment)) {
 		asprintf(error, "fragment payload: expected %d bytes vs "
+				"actual %d bytes; "
+				"total payload: expected %d bytes vs "
 				"actual %d bytes",
 			 packet_payload_len(previous_fragment),
-			 current_payload);
+			 current_payload,
+			 expected_total_payload,
+			 current_payload + sniffed_payload);
 		return STATUS_ERR;
 	}
 	return STATUS_OK;
@@ -1596,8 +1607,12 @@ static int do_outbound_script_packet(
 		/* Save the TCP header so we can reset the connection at the
 		 * end.
 		 */
-		if (live_packet->tcp)
+		if (live_packet->tcp) {
 			socket->last_outbound_tcp_header = *(live_packet->tcp);
+			/* Rewrite TCP sequence number */
+			if (tcp_convert_seq_number(socket, live_packet, error))
+				goto out;
+		}
 
 		sniffed = packet_list_new();
 		sniffed->packet = live_packet;
@@ -1641,8 +1656,8 @@ static int do_outbound_script_packet(
 						sniffed->packet,
 						sniffed_packets_end->packet,
 						live_payload,
-						expected_payload_len -
-							sniffed_payload_len,
+						sniffed_payload_len,
+						expected_payload_len,
 						error)) {
 					packet_list_free(sniffed);
 					goto out;