Don't include link header when forwarding packets

Before this change, if a link header was included in an incoming packet
that is forwarded, the packet that gets sent out will take the original
packet and add a link header to it while keeping the old link header.
This would make the sent packet look like:

   OUTGOING LINK HDR | INCOMING LINK HDR | NETWORK HDR | ...

Obviously this is incorrect as we should drop the incoming link header
and only include the outgoing link header. This change fixes this bug.

Test: integration_test.TestForwarding
PiperOrigin-RevId: 337571447

2 files changed, 114 insertions, 0 deletions

diff --git a/pkg/tcpip/link/ethernet/BUILD b/pkg/tcpip/link/ethernet/BUILD
new file mode 100644
index 000000000..ec92ed623
--- /dev/null
+++ b/pkg/tcpip/link/ethernet/BUILD
@@ -0,0 +1,15 @@
+load("//tools:defs.bzl", "go_library")
+
+package(licenses = ["notice"])
+
+go_library(
+    name = "ethernet",
+    srcs = ["ethernet.go"],
+    visibility = ["//visibility:public"],
+    deps = [
+        "//pkg/tcpip",
+        "//pkg/tcpip/header",
+        "//pkg/tcpip/link/nested",
+        "//pkg/tcpip/stack",
+    ],
+)
diff --git a/pkg/tcpip/link/ethernet/ethernet.go b/pkg/tcpip/link/ethernet/ethernet.go
new file mode 100644
index 000000000..3eef7cd56
--- /dev/null
+++ b/pkg/tcpip/link/ethernet/ethernet.go
@@ -0,0 +1,99 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package ethernet provides an implementation of an ethernet link endpoint that
+// wraps an inner link endpoint.
+package ethernet
+
+import (
+	"gvisor.dev/gvisor/pkg/tcpip"
+	"gvisor.dev/gvisor/pkg/tcpip/header"
+	"gvisor.dev/gvisor/pkg/tcpip/link/nested"
+	"gvisor.dev/gvisor/pkg/tcpip/stack"
+)
+
+var _ stack.NetworkDispatcher = (*Endpoint)(nil)
+var _ stack.LinkEndpoint = (*Endpoint)(nil)
+
+// New returns an ethernet link endpoint that wraps an inner link endpoint.
+func New(ep stack.LinkEndpoint) *Endpoint {
+	var e Endpoint
+	e.Endpoint.Init(ep, &e)
+	return &e
+}
+
+// Endpoint is an ethernet endpoint.
+//
+// It adds an ethernet header to packets before sending them out through its
+// inner link endpoint and consumes an ethernet header before sending the
+// packet to the stack.
+type Endpoint struct {
+	nested.Endpoint
+}
+
+// DeliverNetworkPacket implements stack.NetworkDispatcher.
+func (e *Endpoint) DeliverNetworkPacket(_, _ tcpip.LinkAddress, _ tcpip.NetworkProtocolNumber, pkt *stack.PacketBuffer) {
+	hdr, ok := pkt.LinkHeader().Consume(header.EthernetMinimumSize)
+	if !ok {
+		return
+	}
+
+	eth := header.Ethernet(hdr)
+	if dst := eth.DestinationAddress(); dst == e.Endpoint.LinkAddress() || dst == header.EthernetBroadcastAddress || header.IsMulticastEthernetAddress(dst) {
+		e.Endpoint.DeliverNetworkPacket(eth.SourceAddress() /* remote */, dst /* local */, eth.Type() /* protocol */, pkt)
+	}
+}
+
+// Capabilities implements stack.LinkEndpoint.
+func (e *Endpoint) Capabilities() stack.LinkEndpointCapabilities {
+	return stack.CapabilityResolutionRequired | e.Endpoint.Capabilities()
+}
+
+// WritePacket implements stack.LinkEndpoint.
+func (e *Endpoint) WritePacket(r *stack.Route, gso *stack.GSO, proto tcpip.NetworkProtocolNumber, pkt *stack.PacketBuffer) *tcpip.Error {
+	e.AddHeader(e.Endpoint.LinkAddress(), r.RemoteLinkAddress, proto, pkt)
+	return e.Endpoint.WritePacket(r, gso, proto, pkt)
+}
+
+// WritePackets implements stack.LinkEndpoint.
+func (e *Endpoint) WritePackets(r *stack.Route, gso *stack.GSO, pkts stack.PacketBufferList, proto tcpip.NetworkProtocolNumber) (int, *tcpip.Error) {
+	linkAddr := e.Endpoint.LinkAddress()
+
+	for pkt := pkts.Front(); pkt != nil; pkt = pkt.Next() {
+		e.AddHeader(linkAddr, r.RemoteLinkAddress, proto, pkt)
+	}
+
+	return e.Endpoint.WritePackets(r, gso, pkts, proto)
+}
+
+// MaxHeaderLength implements stack.LinkEndpoint.
+func (e *Endpoint) MaxHeaderLength() uint16 {
+	return header.EthernetMinimumSize + e.Endpoint.MaxHeaderLength()
+}
+
+// ARPHardwareType implements stack.LinkEndpoint.
+func (*Endpoint) ARPHardwareType() header.ARPHardwareType {
+	return header.ARPHardwareEther
+}
+
+// AddHeader implements stack.LinkEndpoint.
+func (*Endpoint) AddHeader(local, remote tcpip.LinkAddress, proto tcpip.NetworkProtocolNumber, pkt *stack.PacketBuffer) {
+	eth := header.Ethernet(pkt.LinkHeader().Push(header.EthernetMinimumSize))
+	fields := header.EthernetFields{
+		SrcAddr: local,
+		DstAddr: remote,
+		Type:    proto,
+	}
+	eth.Encode(&fields)
+}