Honour the link's MaxHeaderLength when forwarding

This change also updates where the IP packet buffer is held in an
outbound tcpip.PacketBuffer from Header to Data. This change removes
unncessary copying of the IP packet buffer when forwarding.

Test: stack_test.TestNICForwarding
PiperOrigin-RevId: 300217972

5 files changed, 85 insertions, 53 deletions

diff --git a/pkg/tcpip/packet_buffer.go b/pkg/tcpip/packet_buffer.go
index ab24372e7..04852132c 100644
--- a/pkg/tcpip/packet_buffer.go
+++ b/pkg/tcpip/packet_buffer.go
@@ -39,8 +39,12 @@ type PacketBuffer struct {
 	// payload.
 	DataSize int
 
-	// Header holds the headers of outbound packets. As a packet is passed
-	// down the stack, each layer adds to Header.
+	// Header holds the headers of outbound packets generated by the netstack. As
+	// a packet is passed down the stack, each layer adds to Header.
+	//
+	// Note, if a packet is being forwarded at the IP layer, the headers for the
+	// IP layer and above (transport) will be held in Data as the packet was not
+	// passed down the stack it arrived at before being forwarded.
 	Header buffer.Prependable
 
 	// These fields are used by both inbound and outbound packets. They
diff --git a/pkg/tcpip/stack/forwarder_test.go b/pkg/tcpip/stack/forwarder_test.go
index 321b7524d..5a04590d5 100644
--- a/pkg/tcpip/stack/forwarder_test.go
+++ b/pkg/tcpip/stack/forwarder_test.go
@@ -473,7 +473,7 @@ func TestForwardingWithFakeResolverPartialTimeout(t *testing.T) {
 		t.Fatal("packet not forwarded")
 	}
 
-	b := p.Pkt.Header.View()
+	b := p.Pkt.Data.First()
 	if b[0] != 3 {
 		t.Fatalf("got b[0] = %d, want = 3", b[0])
 	}
@@ -517,7 +517,7 @@ func TestForwardingWithFakeResolverTwoPackets(t *testing.T) {
 			t.Fatal("packet not forwarded")
 		}
 
-		b := p.Pkt.Header.View()
+		b := p.Pkt.Data.First()
 		if b[0] != 3 {
 			t.Fatalf("got b[0] = %d, want = 3", b[0])
 		}
@@ -564,7 +564,7 @@ func TestForwardingWithFakeResolverManyPackets(t *testing.T) {
 			t.Fatal("packet not forwarded")
 		}
 
-		b := p.Pkt.Header.View()
+		b := p.Pkt.Data.First()
 		if b[0] != 3 {
 			t.Fatalf("got b[0] = %d, want = 3", b[0])
 		}
@@ -619,7 +619,7 @@ func TestForwardingWithFakeResolverManyResolutions(t *testing.T) {
 
 		// The first 5 packets (address 3 to 7) should not be forwarded
 		// because their address resolutions are interrupted.
-		b := p.Pkt.Header.View()
+		b := p.Pkt.Data.First()
 		if b[0] < 8 {
 			t.Fatalf("got b[0] = %d, want b[0] >= 8", b[0])
 		}
diff --git a/pkg/tcpip/stack/nic.go b/pkg/tcpip/stack/nic.go
index cd9202aed..e46bd86c6 100644
--- a/pkg/tcpip/stack/nic.go
+++ b/pkg/tcpip/stack/nic.go
@@ -1246,10 +1246,10 @@ func (n *NIC) DeliverNetworkPacket(linkEP LinkEndpoint, remote, local tcpip.Link
 }
 
 func (n *NIC) forwardPacket(r *Route, protocol tcpip.NetworkProtocolNumber, pkt tcpip.PacketBuffer) {
-	// TODO(b/143425874) Decrease the TTL field in forwarded packets.
-	pkt.Header = buffer.NewPrependableFromView(pkt.Data.First())
-	pkt.Data.RemoveFirst()
+	// TODO(b/143425874): Decrease the TTL field in forwarded packets.
 
+	// pkt.Header should have enough capacity to hold the link's headers.
+	pkt.Header = buffer.NewPrependable(int(n.linkEP.MaxHeaderLength()))
 	if err := n.linkEP.WritePacket(r, nil /* gso */, protocol, pkt); err != nil {
 		r.Stats().IP.OutgoingPacketErrors.Increment()
 		return
diff --git a/pkg/tcpip/stack/stack_test.go b/pkg/tcpip/stack/stack_test.go
index e15db40fb..9515426d6 100644
--- a/pkg/tcpip/stack/stack_test.go
+++ b/pkg/tcpip/stack/stack_test.go
@@ -2240,56 +2240,84 @@ func TestNICStats(t *testing.T) {
 }
 
 func TestNICForwarding(t *testing.T) {
-	// Create a stack with the fake network protocol, two NICs, each with
-	// an address.
-	s := stack.New(stack.Options{
-		NetworkProtocols: []stack.NetworkProtocol{fakeNetFactory()},
-	})
-	s.SetForwarding(true)
+	const nicID1 = 1
+	const nicID2 = 2
+	const dstAddr = tcpip.Address("\x03")
 
-	ep1 := channel.New(10, defaultMTU, "")
-	if err := s.CreateNIC(1, ep1); err != nil {
-		t.Fatal("CreateNIC #1 failed:", err)
-	}
-	if err := s.AddAddress(1, fakeNetNumber, "\x01"); err != nil {
-		t.Fatal("AddAddress #1 failed:", err)
+	tests := []struct {
+		name      string
+		headerLen uint16
+	}{
+		{
+			name: "Zero header length",
+		},
+		{
+			name:      "Non-zero header length",
+			headerLen: 16,
+		},
 	}
 
-	ep2 := channel.New(10, defaultMTU, "")
-	if err := s.CreateNIC(2, ep2); err != nil {
-		t.Fatal("CreateNIC #2 failed:", err)
-	}
-	if err := s.AddAddress(2, fakeNetNumber, "\x02"); err != nil {
-		t.Fatal("AddAddress #2 failed:", err)
-	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			s := stack.New(stack.Options{
+				NetworkProtocols: []stack.NetworkProtocol{fakeNetFactory()},
+			})
+			s.SetForwarding(true)
 
-	// Route all packets to address 3 to NIC 2.
-	{
-		subnet, err := tcpip.NewSubnet("\x03", "\xff")
-		if err != nil {
-			t.Fatal(err)
-		}
-		s.SetRouteTable([]tcpip.Route{{Destination: subnet, Gateway: "\x00", NIC: 2}})
-	}
+			ep1 := channel.New(10, defaultMTU, "")
+			if err := s.CreateNIC(nicID1, ep1); err != nil {
+				t.Fatalf("CreateNIC(%d, _): %s", nicID1, err)
+			}
+			if err := s.AddAddress(nicID1, fakeNetNumber, "\x01"); err != nil {
+				t.Fatalf("AddAddress(%d, %d, 0x01): %s", nicID1, fakeNetNumber, err)
+			}
 
-	// Send a packet to address 3.
-	buf := buffer.NewView(30)
-	buf[0] = 3
-	ep1.InjectInbound(fakeNetNumber, tcpip.PacketBuffer{
-		Data: buf.ToVectorisedView(),
-	})
+			ep2 := channelLinkWithHeaderLength{
+				Endpoint:     channel.New(10, defaultMTU, ""),
+				headerLength: test.headerLen,
+			}
+			if err := s.CreateNIC(nicID2, &ep2); err != nil {
+				t.Fatalf("CreateNIC(%d, _): %s", nicID2, err)
+			}
+			if err := s.AddAddress(nicID2, fakeNetNumber, "\x02"); err != nil {
+				t.Fatalf("AddAddress(%d, %d, 0x02): %s", nicID2, fakeNetNumber, err)
+			}
 
-	if _, ok := ep2.Read(); !ok {
-		t.Fatal("Packet not forwarded")
-	}
+			// Route all packets to dstAddr to NIC 2.
+			{
+				subnet, err := tcpip.NewSubnet(dstAddr, "\xff")
+				if err != nil {
+					t.Fatal(err)
+				}
+				s.SetRouteTable([]tcpip.Route{{Destination: subnet, Gateway: "\x00", NIC: nicID2}})
+			}
 
-	// Test that forwarding increments Tx stats correctly.
-	if got, want := s.NICInfo()[2].Stats.Tx.Packets.Value(), uint64(1); got != want {
-		t.Errorf("got Tx.Packets.Value() = %d, want = %d", got, want)
-	}
+			// Send a packet to dstAddr.
+			buf := buffer.NewView(30)
+			buf[0] = dstAddr[0]
+			ep1.InjectInbound(fakeNetNumber, tcpip.PacketBuffer{
+				Data: buf.ToVectorisedView(),
+			})
 
-	if got, want := s.NICInfo()[2].Stats.Tx.Bytes.Value(), uint64(len(buf)); got != want {
-		t.Errorf("got Tx.Bytes.Value() = %d, want = %d", got, want)
+			pkt, ok := ep2.Read()
+			if !ok {
+				t.Fatal("packet not forwarded")
+			}
+
+			// Test that the link's MaxHeaderLength is honoured.
+			if capacity, want := pkt.Pkt.Header.AvailableLength(), int(test.headerLen); capacity != want {
+				t.Errorf("got Header.AvailableLength() = %d, want = %d", capacity, want)
+			}
+
+			// Test that forwarding increments Tx stats correctly.
+			if got, want := s.NICInfo()[nicID2].Stats.Tx.Packets.Value(), uint64(1); got != want {
+				t.Errorf("got Tx.Packets.Value() = %d, want = %d", got, want)
+			}
+
+			if got, want := s.NICInfo()[nicID2].Stats.Tx.Bytes.Value(), uint64(len(buf)); got != want {
+				t.Errorf("got Tx.Bytes.Value() = %d, want = %d", got, want)
+			}
+		})
 	}
 }
 
diff --git a/pkg/tcpip/stack/transport_test.go b/pkg/tcpip/stack/transport_test.go
index 5d1da2f8b..3609a25b6 100644
--- a/pkg/tcpip/stack/transport_test.go
+++ b/pkg/tcpip/stack/transport_test.go
@@ -641,10 +641,10 @@ func TestTransportForwarding(t *testing.T) {
 		t.Fatal("Response packet not forwarded")
 	}
 
-	if dst := p.Pkt.Header.View()[0]; dst != 3 {
+	if dst := p.Pkt.Data.First()[0]; dst != 3 {
 		t.Errorf("Response packet has incorrect destination addresss: got = %d, want = 3", dst)
 	}
-	if src := p.Pkt.Header.View()[1]; src != 1 {
+	if src := p.Pkt.Data.First()[1]; src != 1 {
 		t.Errorf("Response packet has incorrect source addresss: got = %d, want = 3", src)
 	}
 }