Remove linux-compat loopback hacks from packet endpoint

Previously, gVisor did not represent loopback devices as an ethernet
device as Linux does. To maintain Linux API compatibility for packet
sockets, a workaround was used to add an ethernet header if a link
header was not already present in the packet buffer delivered to a
packet endpoint.

However, this workaround is a bug for non-ethernet based interfaces; not
all links use an ethernet header (e.g. pure L3/TUN interfaces).

As of 3b4bb947517d0d9010120aaa1c3989fd6abf278e, gVisor represents
loopback devices as an ethernet-based device so this workaround can
now be removed.

BUG: https://fxbug.dev/81592
Updates #6530, #6531.
PiperOrigin-RevId: 395819151

2 files changed, 44 insertions, 65 deletions

diff --git a/pkg/tcpip/link/channel/channel.go b/pkg/tcpip/link/channel/channel.go
index d02eea93c..658557d62 100644
--- a/pkg/tcpip/link/channel/channel.go
+++ b/pkg/tcpip/link/channel/channel.go
@@ -28,7 +28,9 @@ import (
 
 // PacketInfo holds all the information about an outbound packet.
 type PacketInfo struct {
-	Pkt   *stack.PacketBuffer
+	Pkt *stack.PacketBuffer
+
+	// TODO(https://gvisor.dev/issue/6537): Remove these fields.
 	Proto tcpip.NetworkProtocolNumber
 	Route stack.RouteInfo
 }
@@ -244,7 +246,10 @@ func (e *Endpoint) WritePacket(r stack.RouteInfo, protocol tcpip.NetworkProtocol
 		Route: r,
 	}
 
-	e.q.Write(p)
+	// Write returns false if the queue is full. A full queue is not an error
+	// from the perspective of a LinkEndpoint so we ignore Write's return
+	// value and always return nil from this method.
+	_ = e.q.Write(p)
 
 	return nil
 }
@@ -292,4 +297,16 @@ func (*Endpoint) AddHeader(tcpip.LinkAddress, tcpip.LinkAddress, tcpip.NetworkPr
 }
 
 // WriteRawPacket implements stack.LinkEndpoint.
-func (*Endpoint) WriteRawPacket(*stack.PacketBuffer) tcpip.Error { return &tcpip.ErrNotSupported{} }
+func (e *Endpoint) WriteRawPacket(pkt *stack.PacketBuffer) tcpip.Error {
+	p := PacketInfo{
+		Pkt:   pkt,
+		Proto: pkt.NetworkProtocolNumber,
+	}
+
+	// Write returns false if the queue is full. A full queue is not an error
+	// from the perspective of a LinkEndpoint so we ignore Write's return
+	// value and always return nil from this method.
+	_ = e.q.Write(p)
+
+	return nil
+}
diff --git a/pkg/tcpip/transport/packet/endpoint.go b/pkg/tcpip/transport/packet/endpoint.go
index 89b4720aa..0554d2f4a 100644
--- a/pkg/tcpip/transport/packet/endpoint.go
+++ b/pkg/tcpip/transport/packet/endpoint.go
@@ -25,7 +25,6 @@
 package packet
 
 import (
-	"fmt"
 	"io"
 	"time"
 
@@ -424,76 +423,39 @@ func (ep *endpoint) HandlePacket(nicID tcpip.NICID, localAddr tcpip.LinkAddress,
 
 	wasEmpty := ep.rcvBufSize == 0
 
-	// Push new packet into receive list and increment the buffer size.
-	var packet packet
+	rcvdPkt := packet{
+		packetInfo: tcpip.LinkPacketInfo{
+			Protocol: netProto,
+			PktType:  pkt.PktType,
+		},
+		senderAddr: tcpip.FullAddress{
+			NIC: nicID,
+		},
+		receivedAt: ep.stack.Clock().Now(),
+	}
+
 	if !pkt.LinkHeader().View().IsEmpty() {
-		// Get info directly from the ethernet header.
 		hdr := header.Ethernet(pkt.LinkHeader().View())
-		packet.senderAddr = tcpip.FullAddress{
-			NIC:  nicID,
-			Addr: tcpip.Address(hdr.SourceAddress()),
-		}
-		packet.packetInfo.Protocol = netProto
-		packet.packetInfo.PktType = pkt.PktType
-	} else {
-		// Guess the would-be ethernet header.
-		packet.senderAddr = tcpip.FullAddress{
-			NIC:  nicID,
-			Addr: tcpip.Address(localAddr),
-		}
-		packet.packetInfo.Protocol = netProto
-		packet.packetInfo.PktType = pkt.PktType
+		rcvdPkt.senderAddr.Addr = tcpip.Address(hdr.SourceAddress())
 	}
 
 	if ep.cooked {
-		// Cooked packets can simply be queued.
-		switch pkt.PktType {
-		case tcpip.PacketHost:
-			packet.data = pkt.Data().ExtractVV()
-		case tcpip.PacketOutgoing:
-			// Strip Link Header.
-			var combinedVV buffer.VectorisedView
-			if v := pkt.NetworkHeader().View(); !v.IsEmpty() {
-				combinedVV.AppendView(v)
-			}
-			if v := pkt.TransportHeader().View(); !v.IsEmpty() {
-				combinedVV.AppendView(v)
-			}
-			combinedVV.Append(pkt.Data().ExtractVV())
-			packet.data = combinedVV
-		default:
-			panic(fmt.Sprintf("unexpected PktType in pkt: %+v", pkt))
+		// Cooked packet endpoints don't include the link-headers in received
+		// packets.
+		if v := pkt.NetworkHeader().View(); !v.IsEmpty() {
+			rcvdPkt.data.AppendView(v)
 		}
-	} else {
-		// Raw packets need their ethernet headers prepended before
-		// queueing.
-		var linkHeader buffer.View
-		if pkt.PktType != tcpip.PacketOutgoing {
-			if pkt.LinkHeader().View().IsEmpty() {
-				// We weren't provided with an actual ethernet header,
-				// so fake one.
-				ethFields := header.EthernetFields{
-					SrcAddr: tcpip.LinkAddress([]byte{0x00, 0x00, 0x00, 0x00, 0x00, 0x00}),
-					DstAddr: localAddr,
-					Type:    netProto,
-				}
-				fakeHeader := make(header.Ethernet, header.EthernetMinimumSize)
-				fakeHeader.Encode(&ethFields)
-				linkHeader = buffer.View(fakeHeader)
-			} else {
-				linkHeader = append(buffer.View(nil), pkt.LinkHeader().View()...)
-			}
-			combinedVV := linkHeader.ToVectorisedView()
-			combinedVV.Append(pkt.Data().ExtractVV())
-			packet.data = combinedVV
-		} else {
-			packet.data = buffer.NewVectorisedView(pkt.Size(), pkt.Views())
+		if v := pkt.TransportHeader().View(); !v.IsEmpty() {
+			rcvdPkt.data.AppendView(v)
 		}
+		rcvdPkt.data.Append(pkt.Data().ExtractVV())
+	} else {
+		// Raw packet endpoints include link-headers in received packets.
+		rcvdPkt.data = buffer.NewVectorisedView(pkt.Size(), pkt.Views())
 	}
-	packet.receivedAt = ep.stack.Clock().Now()
 
-	ep.rcvList.PushBack(&packet)
-	ep.rcvBufSize += packet.data.Size()
+	ep.rcvList.PushBack(&rcvdPkt)
+	ep.rcvBufSize += rcvdPkt.data.Size()
 
 	ep.rcvMu.Unlock()
 	ep.stats.PacketsReceived.Increment()