Consider ethernet header size when returning MTU

...so that upper-layers do not send larger than expected packets.

PiperOrigin-RevId: 416645503

pkg/tcpip/link/ethernet/ethernet.go

@@ -50,6 +50,14 @@ func (e *Endpoint) LinkAddress() tcpip.LinkAddress {
 	return header.UnspecifiedEthernetAddress
 }
 
+// MTU implements stack.LinkEndpoint.
+func (e *Endpoint) MTU() uint32 {
+	if mtu := e.Endpoint.MTU(); mtu > header.EthernetMinimumSize {
+		return mtu - header.EthernetMinimumSize
+	}
+	return 0
+}
+
 // DeliverNetworkPacket implements stack.NetworkDispatcher.
 func (e *Endpoint) DeliverNetworkPacket(_, _ tcpip.LinkAddress, _ tcpip.NetworkProtocolNumber, pkt *stack.PacketBuffer) {
 	hdr, ok := pkt.LinkHeader().Consume(header.EthernetMinimumSize)

pkg/tcpip/link/ethernet/ethernet_test.go

@@ -15,6 +15,7 @@
 package ethernet_test
 
 import (
+	"fmt"
 	"testing"
 
 	"gvisor.dev/gvisor/pkg/tcpip"
@@ -69,3 +70,52 @@ func TestDeliverNetworkPacket(t *testing.T) {
 		t.Fatalf("got networkDispatcher.networkPackets = %d, want = 1", networkDispatcher.networkPackets)
 	}
 }
+
+type testLinkEndpoint struct {
+	stack.LinkEndpoint
+
+	mtu uint32
+}
+
+func (t *testLinkEndpoint) MTU() uint32 {
+	return t.mtu
+}
+
+func TestMTU(t *testing.T) {
+	const maxFrameSize = 1500
+
+	tests := []struct {
+		maxFrameSize uint32
+		expectedMTU  uint32
+	}{
+		{
+			maxFrameSize: 0,
+			expectedMTU:  0,
+		},
+		{
+			maxFrameSize: header.EthernetMinimumSize - 1,
+			expectedMTU:  0,
+		},
+		{
+			maxFrameSize: header.EthernetMinimumSize,
+			expectedMTU:  0,
+		},
+		{
+			maxFrameSize: header.EthernetMinimumSize + 1,
+			expectedMTU:  1,
+		},
+		{
+			maxFrameSize: maxFrameSize,
+			expectedMTU:  maxFrameSize - header.EthernetMinimumSize,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(fmt.Sprintf("MaxFrameSize=%d", test.maxFrameSize), func(t *testing.T) {
+			e := ethernet.New(&testLinkEndpoint{mtu: test.maxFrameSize})
+			if got := e.MTU(); got != test.expectedMTU {
+				t.Errorf("got e.MTU() = %d, want = %d", got, test.expectedMTU)
+			}
+		})
+	}
+}