Fix IPv4 ICMP echo handler to copy options

The IPv4 RFCs are specific (though obtuse) that an echo response
packet needs to contain all the options from the echo request,
much as if it been routed back to the sender, though apparently
with a new TTL. They suggest copying the incoming packet header
to achieve this so that is what this patch does.

PiperOrigin-RevId: 335559176

1 files changed, 17 insertions, 5 deletions

diff --git a/pkg/tcpip/transport/udp/udp_test.go b/pkg/tcpip/transport/udp/udp_test.go
index 4e14a5fc5..cddedb686 100644
--- a/pkg/tcpip/transport/udp/udp_test.go
+++ b/pkg/tcpip/transport/udp/udp_test.go
@@ -1780,16 +1780,26 @@ func TestV4UnknownDestination(t *testing.T) {
 				checker.ICMPv4Type(header.ICMPv4DstUnreachable),
 				checker.ICMPv4Code(header.ICMPv4PortUnreachable)))
 
+			// We need to compare the included data part of the UDP packet that is in
+			// the ICMP packet with the matching original data.
 			icmpPkt := header.ICMPv4(hdr.Payload())
 			payloadIPHeader := header.IPv4(icmpPkt.Payload())
+			incomingHeaderLength := header.IPv4MinimumSize + header.UDPMinimumSize
 			wantLen := len(payload)
 			if tc.largePayload {
-				wantLen = header.IPv4MinimumProcessableDatagramSize - header.IPv4MinimumSize*2 - header.ICMPv4MinimumSize - header.UDPMinimumSize
+				// To work out the data size we need to simulate what the sender would
+				// have done. The wanted size is the total available minus the sum of
+				// the headers in the UDP AND ICMP packets, given that we know the test
+				// had only a minimal IP header but the ICMP sender will have allowed
+				// for a maximally sized packet header.
+				wantLen = header.IPv4MinimumProcessableDatagramSize - header.IPv4MaximumHeaderSize - header.ICMPv4MinimumSize - incomingHeaderLength
+
 			}
 
-			// In case of large payloads the IP packet may be truncated. Update
+			// In the case of large payloads the IP packet may be truncated. Update
 			// the length field before retrieving the udp datagram payload.
-			payloadIPHeader.SetTotalLength(uint16(wantLen + header.UDPMinimumSize + header.IPv4MinimumSize))
+			// Add back the two headers within the payload.
+			payloadIPHeader.SetTotalLength(uint16(wantLen + incomingHeaderLength))
 
 			origDgram := header.UDP(payloadIPHeader.Payload())
 			if got, want := len(origDgram.Payload()), wantLen; got != want {
@@ -2015,7 +2025,8 @@ func TestPayloadModifiedV4(t *testing.T) {
 	payload := newPayload()
 	h := unicastV4.header4Tuple(incoming)
 	buf := c.buildV4Packet(payload, &h)
-	// Modify the payload so that the checksum value in the UDP header will be incorrect.
+	// Modify the payload so that the checksum value in the UDP header will be
+	// incorrect.
 	buf[len(buf)-1]++
 	c.linkEP.InjectInbound(ipv4.ProtocolNumber, stack.NewPacketBuffer(stack.PacketBufferOptions{
 		Data: buf.ToVectorisedView(),
@@ -2045,7 +2056,8 @@ func TestPayloadModifiedV6(t *testing.T) {
 	payload := newPayload()
 	h := unicastV6.header4Tuple(incoming)
 	buf := c.buildV6Packet(payload, &h)
-	// Modify the payload so that the checksum value in the UDP header will be incorrect.
+	// Modify the payload so that the checksum value in the UDP header will be
+	// incorrect.
 	buf[len(buf)-1]++
 	c.linkEP.InjectInbound(ipv6.ProtocolNumber, stack.NewPacketBuffer(stack.PacketBufferOptions{
 		Data: buf.ToVectorisedView(),