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

1 files changed, 20 insertions, 3 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
+}