TUN inbound: Enhance Darwin interface support (#5598)

* Proxy: TUN: Enhance Darwin interface support.
- reduce number of actions done to create/configure the interface in the system
- assign synthetic static link-local ipv4/ipv6 addresses to the interface, that are required by the OS for the routing to work
- make tun_darwin_endpoint be implemented significantly more similar to tun_windows_enpoint, preparing them for potential unification

* Proxy: TUN: Unify Darwin/Windows endpoint, which are now extremely similar, into one GVisorEndpoint.
Making darwin/windows tun implement GVisorDevice with simple readpacket/writepacket methods that GVisorEndpoint untilise

proxy/tun/tun_windows.go

@@ -3,19 +3,28 @@
 package tun
 
 import (
+	"errors"
+	_ "unsafe"
+
 	"golang.org/x/sys/windows"
 	"golang.zx2c4.com/wintun"
+	"gvisor.dev/gvisor/pkg/buffer"
+	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/stack"
 )
 
+//go:linkname procyield runtime.procyield
+func procyield(cycles uint32)
+
 // WindowsTun is an object that handles tun network interface on Windows
 // current version is heavily stripped to do nothing more,
 // then create a network interface, to be provided as endpoint to gVisor ip stack
 type WindowsTun struct {
-	options TunOptions
-	adapter *wintun.Adapter
-	session wintun.Session
-	MTU     uint32
+	options  TunOptions
+	adapter  *wintun.Adapter
+	session  wintun.Session
+	readWait windows.Handle
+	MTU      uint32
 }
 
 // WindowsTun implements Tun
@@ -24,6 +33,9 @@ var _ Tun = (*WindowsTun)(nil)
 // WindowsTun implements GVisorTun
 var _ GVisorTun = (*WindowsTun)(nil)
 
+// WindowsTun implements GVisorDevice
+var _ GVisorDevice = (*WindowsTun)(nil)
+
 // NewTun creates a Wintun interface with the given name. Should a Wintun
 // interface with the same name exist, it tried to be reused.
 func NewTun(options TunOptions) (Tun, error) {
@@ -41,9 +53,10 @@ func NewTun(options TunOptions) (Tun, error) {
 	}
 
 	tun := &WindowsTun{
-		options: options,
-		adapter: adapter,
-		session: session,
+		options:  options,
+		adapter:  adapter,
+		session:  session,
+		readWait: session.ReadWaitEvent(),
 		// there is currently no iphndl.dll support, which is the netlink library for windows
 		// so there is nowhere to change MTU for the Wintun interface, and we take its default value
 		MTU: wintun.PacketSizeMax,
@@ -78,7 +91,54 @@ func (t *WindowsTun) Close() error {
 	return nil
 }
 
-// newEndpoint builds new gVisor stack.LinkEndpoint (WintunEndpoint) on top of WindowsTun
-func (t *WindowsTun) newEndpoint() (stack.LinkEndpoint, error) {
-	return &WintunEndpoint{tun: t}, nil
+// WritePacket implements GVisorDevice method to write one packet to the tun device
+func (t *WindowsTun) WritePacket(packetBuffer *stack.PacketBuffer) tcpip.Error {
+	// request buffer from Wintun
+	packet, err := t.session.AllocateSendPacket(packetBuffer.Size())
+	if err != nil {
+		return &tcpip.ErrAborted{}
+	}
+
+	// copy the bytes of slices that compose the packet into the allocated buffer
+	var index int
+	for _, packetElement := range packetBuffer.AsSlices() {
+		index += copy(packet[index:], packetElement)
+	}
+
+	// signal Wintun to send that buffer as the packet
+	t.session.SendPacket(packet)
+
+	return nil
+}
+
+// ReadPacket implements GVisorDevice method to read one packet from the tun device
+// It is expected that the method will not block, rather return ErrQueueEmpty when there is nothing on the line,
+// which will make the stack call Wait which should implement desired push-back
+func (t *WindowsTun) ReadPacket() (byte, *stack.PacketBuffer, error) {
+	packet, err := t.session.ReceivePacket()
+	if errors.Is(err, windows.ERROR_NO_MORE_ITEMS) {
+		return 0, nil, ErrQueueEmpty
+	}
+	if err != nil {
+		return 0, nil, err
+	}
+
+	version := packet[0] >> 4
+	packetBuffer := buffer.MakeWithView(buffer.NewViewWithData(packet))
+	return version, stack.NewPacketBuffer(stack.PacketBufferOptions{
+		Payload:           packetBuffer,
+		IsForwardedPacket: true,
+		OnRelease: func() {
+			t.session.ReleaseReceivePacket(packet)
+		},
+	}), nil
+}
+
+func (t *WindowsTun) Wait() {
+	procyield(1)
+	_, _ = windows.WaitForSingleObject(t.readWait, windows.INFINITE)
+}
+
+func (t *WindowsTun) newEndpoint() (stack.LinkEndpoint, error) {
+	return &LinkEndpoint{deviceMTU: t.options.MTU, device: t}, nil
 }