QBONE packet processor: pull the src address from `ProcessIpv6HeaderAndFilter`.

The original code had some mistaken labeling, claiming to extract the dest address.  We actually extract the src address,  which becomes the destination for ICMP replies.

This probably doesn't matter in practice but it bothered me

PiperOrigin-RevId: 934406303

quiche/quic/qbone/qbone_packet_processor.cc

diff --git a/quiche/quic/qbone/qbone_packet_processor.cc b/quiche/quic/qbone/qbone_packet_processor.cc
index 8ac86ee..5783bc4 100644
--- a/quiche/quic/qbone/qbone_packet_processor.cc
+++ b/quiche/quic/qbone/qbone_packet_processor.cc
@@ -28,7 +28,6 @@
 constexpr size_t kIPv6MinPacketSize = 1280;
 constexpr size_t kIcmpTtl = 64;
 constexpr size_t kICMPv6DestinationUnreachableDueToSourcePolicy = 5;
-constexpr size_t kIPv6DestinationOffset = 8;
 
 }  // namespace
 
@@ -82,13 +81,11 @@
   uint8_t transport_protocol;
   char* transport_data;
   icmp6_hdr icmp_header;
+  in6_addr src;
   memset(&icmp_header, 0, sizeof(icmp_header));
-  ProcessingResult result = ProcessIPv6HeaderAndFilter(
-      packet, direction, &transport_protocol, &transport_data, &icmp_header);
-
-  in6_addr dst;
-  // TODO(b/70339814): ensure this is actually a unicast address.
-  memcpy(&dst, &packet->data()[kIPv6DestinationOffset], kIPv6AddressSize);
+  ProcessingResult result =
+      ProcessIPv6HeaderAndFilter(packet, direction, &src, &transport_protocol,
+                                 &transport_data, &icmp_header);
 
   switch (result) {
     case ProcessingResult::OK:
@@ -112,14 +109,14 @@
         // need to take off both the IPv6 header and the ICMP6 header.
         auto icmp_body = absl::string_view(*packet).substr(sizeof(ip6_hdr) +
                                                            sizeof(icmp6_hdr));
-        SendIcmpResponse(dst, &icmp_header, icmp_body, direction);
+        SendIcmpResponse(/*dst=*/src, &icmp_header, icmp_body, direction);
       } else {
-        SendIcmpResponse(dst, &icmp_header, *packet, direction);
+        SendIcmpResponse(/*dst=*/src, &icmp_header, *packet, direction);
       }
       stats_->OnPacketDroppedWithIcmp(direction, traffic_class);
       break;
     case ProcessingResult::ICMP_AND_TCP_RESET:
-      SendIcmpResponse(dst, &icmp_header, *packet, direction);
+      SendIcmpResponse(/*dst=*/src, &icmp_header, *packet, direction);
       stats_->OnPacketDroppedWithIcmp(direction, traffic_class);
       SendTcpReset(*packet, direction);
       stats_->OnPacketDroppedWithTcpReset(direction, traffic_class);
@@ -134,11 +131,12 @@
 QbonePacketProcessor::ProcessingResult
 QbonePacketProcessor::ProcessIPv6HeaderAndFilter(std::string* packet,
                                                  Direction direction,
+                                                 in6_addr* src,
                                                  uint8_t* transport_protocol,
                                                  char** transport_data,
                                                  icmp6_hdr* icmp_header) {
   ProcessingResult result = ProcessIPv6Header(
-      packet, direction, transport_protocol, transport_data, icmp_header);
+      packet, direction, src, transport_protocol, transport_data, icmp_header);
 
   if (result == ProcessingResult::OK) {
     char* packet_data = &*packet->begin();
@@ -180,8 +178,9 @@
 }
 
 QbonePacketProcessor::ProcessingResult QbonePacketProcessor::ProcessIPv6Header(
-    std::string* packet, Direction direction, uint8_t* transport_protocol,
-    char** transport_data, icmp6_hdr* icmp_header) {
+    std::string* packet, Direction direction, in6_addr* src,
+    uint8_t* transport_protocol, char** transport_data,
+    icmp6_hdr* icmp_header) {
   // Check if the packet is big enough to have IPv6 header.
   if (packet->size() < kIPv6HeaderSize) {
     QUIC_DVLOG(1) << "Dropped malformed packet: IPv6 header too short";
@@ -209,6 +208,7 @@
   QuicIpAddress address_to_check;
   uint8_t address_reject_code;
   bool ip_parse_result;
+  *src = header->ip6_src;
   switch (direction) {
     case Direction::FROM_OFF_NETWORK:
       // Expect the source IP to match the client.

quiche/quic/qbone/qbone_packet_processor.h

diff --git a/quiche/quic/qbone/qbone_packet_processor.h b/quiche/quic/qbone/qbone_packet_processor.h
index 8cfc51f..c1465ae 100644
--- a/quiche/quic/qbone/qbone_packet_processor.h
+++ b/quiche/quic/qbone/qbone_packet_processor.h
@@ -160,6 +160,7 @@
   // packet may be decreased in the process.
   ProcessingResult ProcessIPv6HeaderAndFilter(std::string* packet,
                                               Direction direction,
+                                              in6_addr* src,
                                               uint8_t* transport_protocol,
                                               char** transport_data,
                                               icmp6_hdr* icmp_header);
@@ -187,7 +188,7 @@
   // Performs basic sanity and permission checks on the packet, and decreases
   // the TTL.
   ProcessingResult ProcessIPv6Header(std::string* packet, Direction direction,
-                                     uint8_t* transport_protocol,
+                                     in6_addr* src, uint8_t* transport_protocol,
                                      char** transport_data,
                                      icmp6_hdr* icmp_header);