Pass local link address to DeliverNetworkPacket

This allows a NetworkDispatcher to implement transparent bridging,
assuming all implementations of LinkEndpoint.WritePacket call eth.Encode
with header.EthernetFields.SrcAddr set to the passed
Route.LocalLinkAddress, if it is provided.

PiperOrigin-RevId: 213686651
Change-Id: I446a4ac070970202f0724ef796ff1056ae4dd72a

2 files changed, 60 insertions, 27 deletions

diff --git a/pkg/tcpip/link/fdbased/endpoint.go b/pkg/tcpip/link/fdbased/endpoint.go
index 40a10eb9b..ee99ada07 100644
--- a/pkg/tcpip/link/fdbased/endpoint.go
+++ b/pkg/tcpip/link/fdbased/endpoint.go
@@ -164,17 +164,24 @@ func (e *endpoint) WritePacket(r *stack.Route, hdr buffer.Prependable, payload b
 		views[0] = hdr.View()
 		views = append(views, payload.Views()...)
 		vv := buffer.NewVectorisedView(len(views[0])+payload.Size(), views)
-		e.dispatcher.DeliverNetworkPacket(e, r.RemoteLinkAddress, protocol, vv)
+		e.dispatcher.DeliverNetworkPacket(e, r.RemoteLinkAddress, r.LocalLinkAddress, protocol, vv)
 		return nil
 	}
 	if e.hdrSize > 0 {
 		// Add ethernet header if needed.
 		eth := header.Ethernet(hdr.Prepend(header.EthernetMinimumSize))
-		eth.Encode(&header.EthernetFields{
+		ethHdr := &header.EthernetFields{
 			DstAddr: r.RemoteLinkAddress,
-			SrcAddr: e.addr,
 			Type:    protocol,
-		})
+		}
+
+		// Preserve the src address if it's set in the route.
+		if r.LocalLinkAddress != "" {
+			ethHdr.SrcAddr = r.LocalLinkAddress
+		} else {
+			ethHdr.SrcAddr = e.addr
+		}
+		eth.Encode(ethHdr)
 	}
 
 	if payload.Size() == 0 {
@@ -223,12 +230,15 @@ func (e *endpoint) dispatch(largeV buffer.View) (bool, *tcpip.Error) {
 		return false, nil
 	}
 
-	var p tcpip.NetworkProtocolNumber
-	var addr tcpip.LinkAddress
+	var (
+		p                             tcpip.NetworkProtocolNumber
+		remoteLinkAddr, localLinkAddr tcpip.LinkAddress
+	)
 	if e.hdrSize > 0 {
 		eth := header.Ethernet(e.views[0])
 		p = eth.Type()
-		addr = eth.SourceAddress()
+		remoteLinkAddr = eth.SourceAddress()
+		localLinkAddr = eth.DestinationAddress()
 	} else {
 		// We don't get any indication of what the packet is, so try to guess
 		// if it's an IPv4 or IPv6 packet.
@@ -246,7 +256,7 @@ func (e *endpoint) dispatch(largeV buffer.View) (bool, *tcpip.Error) {
 	vv := buffer.NewVectorisedView(n, e.views[:used])
 	vv.TrimFront(e.hdrSize)
 
-	e.dispatcher.DeliverNetworkPacket(e, addr, p, vv)
+	e.dispatcher.DeliverNetworkPacket(e, remoteLinkAddr, localLinkAddr, p, vv)
 
 	// Prepare e.views for another packet: release used views.
 	for i := 0; i < used; i++ {
@@ -287,7 +297,7 @@ func (e *InjectableEndpoint) Attach(dispatcher stack.NetworkDispatcher) {
 
 // Inject injects an inbound packet.
 func (e *InjectableEndpoint) Inject(protocol tcpip.NetworkProtocolNumber, vv buffer.VectorisedView) {
-	e.dispatcher.DeliverNetworkPacket(e, "", protocol, vv)
+	e.dispatcher.DeliverNetworkPacket(e, "" /* remoteLinkAddr */, "" /* localLinkAddr */, protocol, vv)
 }
 
 // NewInjectable creates a new fd-based InjectableEndpoint.
diff --git a/pkg/tcpip/link/fdbased/endpoint_test.go b/pkg/tcpip/link/fdbased/endpoint_test.go
index 411ad7832..52e532ebb 100644
--- a/pkg/tcpip/link/fdbased/endpoint_test.go
+++ b/pkg/tcpip/link/fdbased/endpoint_test.go
@@ -31,6 +31,13 @@ import (
 	"gvisor.googlesource.com/gvisor/pkg/tcpip/stack"
 )
 
+const (
+	mtu   = 1500
+	laddr = tcpip.LinkAddress("\x11\x22\x33\x44\x55\x66")
+	raddr = tcpip.LinkAddress("\x77\x88\x99\xaa\xbb\xcc")
+	proto = 10
+)
+
 type packetInfo struct {
 	raddr    tcpip.LinkAddress
 	proto    tcpip.NetworkProtocolNumber
@@ -78,12 +85,11 @@ func (c *context) cleanup() {
 	syscall.Close(c.fds[1])
 }
 
-func (c *context) DeliverNetworkPacket(linkEP stack.LinkEndpoint, remoteLinkAddr tcpip.LinkAddress, protocol tcpip.NetworkProtocolNumber, vv buffer.VectorisedView) {
+func (c *context) DeliverNetworkPacket(linkEP stack.LinkEndpoint, remoteLinkAddr tcpip.LinkAddress, localLinkAddr tcpip.LinkAddress, protocol tcpip.NetworkProtocolNumber, vv buffer.VectorisedView) {
 	c.ch <- packetInfo{remoteLinkAddr, protocol, vv.ToView()}
 }
 
 func TestNoEthernetProperties(t *testing.T) {
-	const mtu = 1500
 	c := newContext(t, &Options{MTU: mtu})
 	defer c.cleanup()
 
@@ -97,7 +103,6 @@ func TestNoEthernetProperties(t *testing.T) {
 }
 
 func TestEthernetProperties(t *testing.T) {
-	const mtu = 1500
 	c := newContext(t, &Options{EthernetHeader: true, MTU: mtu})
 	defer c.cleanup()
 
@@ -111,7 +116,6 @@ func TestEthernetProperties(t *testing.T) {
 }
 
 func TestAddress(t *testing.T) {
-	const mtu = 1500
 	addrs := []tcpip.LinkAddress{"", "abc", "def"}
 	for _, a := range addrs {
 		t.Run(fmt.Sprintf("Address: %q", a), func(t *testing.T) {
@@ -126,13 +130,6 @@ func TestAddress(t *testing.T) {
 }
 
 func TestWritePacket(t *testing.T) {
-	const (
-		mtu   = 1500
-		laddr = tcpip.LinkAddress("\x11\x22\x33\x44\x55\x66")
-		raddr = tcpip.LinkAddress("\x77\x88\x99\xaa\xbb\xcc")
-		proto = 10
-	)
-
 	lengths := []int{0, 100, 1000}
 	eths := []bool{true, false}
 
@@ -197,14 +194,40 @@ func TestWritePacket(t *testing.T) {
 	}
 }
 
-func TestDeliverPacket(t *testing.T) {
-	const (
-		mtu   = 1500
-		laddr = tcpip.LinkAddress("\x11\x22\x33\x44\x55\x66")
-		raddr = tcpip.LinkAddress("\x77\x88\x99\xaa\xbb\xcc")
-		proto = 10
-	)
+func TestPreserveSrcAddress(t *testing.T) {
+	baddr := tcpip.LinkAddress("\xcc\xbb\xaa\x77\x88\x99")
 
+	c := newContext(t, &Options{Address: laddr, MTU: mtu, EthernetHeader: true})
+	defer c.cleanup()
+
+	// Set LocalLinkAddress in route to the value of the bridged address.
+	r := &stack.Route{
+		RemoteLinkAddress: raddr,
+		LocalLinkAddress:  baddr,
+	}
+
+	// WritePacket panics given a prependable with anything less than
+	// the minimum size of the ethernet header.
+	hdr := buffer.NewPrependable(header.EthernetMinimumSize)
+	if err := c.ep.WritePacket(r, hdr, buffer.VectorisedView{}, proto); err != nil {
+		t.Fatalf("WritePacket failed: %v", err)
+	}
+
+	// Read from the FD, then compare with what we wrote.
+	b := make([]byte, mtu)
+	n, err := syscall.Read(c.fds[0], b)
+	if err != nil {
+		t.Fatalf("Read failed: %v", err)
+	}
+	b = b[:n]
+	h := header.Ethernet(b)
+
+	if a := h.SourceAddress(); a != baddr {
+		t.Fatalf("SourceAddress() = %v, want %v", a, baddr)
+	}
+}
+
+func TestDeliverPacket(t *testing.T) {
 	lengths := []int{100, 1000}
 	eths := []bool{true, false}