syscalls_tun_windows.go

package water

import (
	"crypto/rand"
	_ "embed"
	"encoding/binary"
	"errors"
	"fmt"
	"os"
	"sync"
	"sync/atomic"
	"time"
	_ "unsafe"

	"golang.org/x/sys/windows"
	"golang.zx2c4.com/wintun"
)

const (
	rateMeasurementGranularity = uint64((time.Second / 2) / time.Nanosecond)
	spinloopRateThreshold      = 800000000 / 8                                   // 800mbps
	spinloopDuration           = uint64(time.Millisecond / 80 / time.Nanosecond) // ~1gbit/s
)

type Event int

const (
	EventUp = 1 << iota
	EventDown
	EventMTUUpdate
)

type rateJuggler struct {
	current       uint64
	nextByteCount uint64
	nextStartTime int64
	changing      int32
}

type NativeTun struct {
	wt        *wintun.Adapter
	name      string
	handle    windows.Handle
	rate      rateJuggler
	session   wintun.Session
	readWait  windows.Handle
	events    chan Event
	running   sync.WaitGroup
	closeOnce sync.Once
	close     int32
	forcedMTU int
}

type WTun struct {
	dev *NativeTun
}

func (w *WTun) Close() error {
	return w.dev.Close()
}

func (w *WTun) Write(b []byte) (int, error) {
	return w.dev.Write(b, 0)
}

func (w *WTun) Read(b []byte) (int, error) {
	return w.dev.Read(b, 0)
}

var (
	WintunTunnelType          = "Wintun"
	WintunStaticRequestedGUID *windows.GUID
)

//go:linkname procyield runtime.procyield
func procyield(cycles uint32)

//go:linkname nanotime runtime.nanotime
func nanotime() int64

func generateRandomGUID() (*windows.GUID, error) {
	var guid windows.GUID

	// Generate random values for the Data1, Data2, and Data3 fields
	if err := binary.Read(rand.Reader, binary.LittleEndian, &guid.Data1); err != nil {
		return nil, err
	}
	if err := binary.Read(rand.Reader, binary.LittleEndian, &guid.Data2); err != nil {
		return nil, err
	}
	if err := binary.Read(rand.Reader, binary.LittleEndian, &guid.Data3); err != nil {
		return nil, err
	}

	// Generate random bytes for the Data4 field
	if _, err := rand.Read(guid.Data4[:]); err != nil {
		return nil, err
	}

	return &guid, nil
}

func openTunDev(config Config) (ifce *Interface, err error) {
	/*
		gUID := &windows.GUID{
			0x0000000,
			0xFFFF,
			0xFFFF,
			[8]byte{0xFF, 0xe9, 0x76, 0xe5, 0x8c, 0x74, 0x06, 0x3e},
		}
	*/

	// We'll geneerate a random GUID for the Wintun interface
	// This is to work around some issue in the Wintun driver that causes
	// it to fail to create an interface as claims it already exists
	gUID, _ := generateRandomGUID()

	if config.PlatformSpecificParams.Name == "" {
		config.PlatformSpecificParams.Name = "WaterIface"
	}
	nativeTunDevice, err := CreateTUNWithRequestedGUID(config.PlatformSpecificParams.Name, gUID, 0)
	if err != nil {
		// Windows 10 has an issue with unclean shutdowns not fully cleaning up the wintun device.
		// Trying a second time resolves the issue.
		time.Sleep(1 * time.Second)
		nativeTunDevice, err = CreateTUNWithRequestedGUID(config.PlatformSpecificParams.Name, gUID, 0)
		if err != nil {
			return nil, err
		}
	}
	ifce = &Interface{
		isTAP:           config.DeviceType == TAP,
		ReadWriteCloser: &WTun{dev: nativeTunDevice},
		name:            config.PlatformSpecificParams.Name,
	}
	return ifce, nil
}

// CreateTUN creates a Wintun interface with the given name. Should a Wintun
// interface with the same name exist, it is reused.
func CreateTUN(ifname string, mtu int) (*NativeTun, error) {
	return CreateTUNWithRequestedGUID(ifname, WintunStaticRequestedGUID, mtu)
}

// CreateTUNWithRequestedGUID creates a Wintun interface with the given name and
// a requested GUID. Should a Wintun interface with the same name exist, it is reused.
func CreateTUNWithRequestedGUID(ifname string, requestedGUID *windows.GUID, mtu int) (*NativeTun, error) {
	wt, err := wintun.CreateAdapter(ifname, WintunTunnelType, requestedGUID)
	if err != nil {
		return nil, fmt.Errorf("Error creating interface: %w", err)
	}

	forcedMTU := 1420
	if mtu > 0 {
		forcedMTU = mtu
	}

	tun := &NativeTun{
		wt:        wt,
		name:      ifname,
		handle:    windows.InvalidHandle,
		events:    make(chan Event, 10),
		forcedMTU: forcedMTU,
	}

	tun.session, err = wt.StartSession(0x800000) // Ring capacity, 8 MiB
	if err != nil {
		tun.wt.Close()
		close(tun.events)
		return nil, fmt.Errorf("Error starting session: %w", err)
	}
	tun.readWait = tun.session.ReadWaitEvent()
	return tun, nil
}

func (tun *NativeTun) Name() (string, error) {
	return tun.name, nil
}

func (tun *NativeTun) File() *os.File {
	return nil
}

func (tun *NativeTun) Events() chan Event {
	return tun.events
}

func (tun *NativeTun) Close() error {
	var err error
	tun.closeOnce.Do(func() {
		atomic.StoreInt32(&tun.close, 1)
		windows.SetEvent(tun.readWait)
		tun.running.Wait()
		tun.session.End()
		if tun.wt != nil {
			tun.wt.Close()
		}
		close(tun.events)
	})
	return err
}

func (tun *NativeTun) MTU() (int, error) {
	return tun.forcedMTU, nil
}

// TODO: This is a temporary hack. We really need to be monitoring the interface in real time and adapting to MTU changes.
func (tun *NativeTun) ForceMTU(mtu int) {
	update := tun.forcedMTU != mtu
	tun.forcedMTU = mtu
	if update {
		tun.events <- EventMTUUpdate
	}
}

// Note: Read() and Write() assume the caller comes only from a single thread; there's no locking.

func (tun *NativeTun) Read(buff []byte, offset int) (int, error) {
	tun.running.Add(1)
	defer tun.running.Done()
retry:
	if atomic.LoadInt32(&tun.close) == 1 {
		return 0, os.ErrClosed
	}
	start := nanotime()
	shouldSpin := atomic.LoadUint64(&tun.rate.current) >= spinloopRateThreshold && uint64(start-atomic.LoadInt64(&tun.rate.nextStartTime)) <= rateMeasurementGranularity*2
	for {
		if atomic.LoadInt32(&tun.close) == 1 {
			return 0, os.ErrClosed
		}
		packet, err := tun.session.ReceivePacket()
		switch err {
		case nil:
			packetSize := len(packet)
			copy(buff[offset:], packet)
			tun.session.ReleaseReceivePacket(packet)
			tun.rate.update(uint64(packetSize))
			return packetSize, nil
		case windows.ERROR_NO_MORE_ITEMS:
			if !shouldSpin || uint64(nanotime()-start) >= spinloopDuration {
				windows.WaitForSingleObject(tun.readWait, windows.INFINITE)
				goto retry
			}
			procyield(1)
			continue
		case windows.ERROR_HANDLE_EOF:
			return 0, os.ErrClosed
		case windows.ERROR_INVALID_DATA:
			return 0, errors.New("Send ring corrupt")
		}
		return 0, fmt.Errorf("Read failed: %w", err)
	}
}

func (tun *NativeTun) Flush() error {
	return nil
}

func (tun *NativeTun) Write(buff []byte, offset int) (int, error) {
	tun.running.Add(1)
	defer tun.running.Done()
	if atomic.LoadInt32(&tun.close) == 1 {
		return 0, os.ErrClosed
	}

	packetSize := len(buff) - offset
	tun.rate.update(uint64(packetSize))

	packet, err := tun.session.AllocateSendPacket(packetSize)
	if err == nil {
		copy(packet, buff[offset:])
		tun.session.SendPacket(packet)
		return packetSize, nil
	}
	switch err {
	case windows.ERROR_HANDLE_EOF:
		return 0, os.ErrClosed
	case windows.ERROR_BUFFER_OVERFLOW:
		return 0, nil // Dropping when ring is full.
	}
	return 0, fmt.Errorf("Write failed: %w", err)
}

// LUID returns Windows interface instance ID.
func (tun *NativeTun) LUID() uint64 {
	tun.running.Add(1)
	defer tun.running.Done()
	if atomic.LoadInt32(&tun.close) == 1 {
		return 0
	}
	return tun.wt.LUID()
}

// RunningVersion returns the running version of the Wintun driver.
func (tun *NativeTun) RunningVersion() (version uint32, err error) {
	return wintun.RunningVersion()
}

func (rate *rateJuggler) update(packetLen uint64) {
	now := nanotime()
	total := atomic.AddUint64(&rate.nextByteCount, packetLen)
	period := uint64(now - atomic.LoadInt64(&rate.nextStartTime))
	if period >= rateMeasurementGranularity {
		if !atomic.CompareAndSwapInt32(&rate.changing, 0, 1) {
			return
		}
		atomic.StoreInt64(&rate.nextStartTime, now)
		atomic.StoreUint64(&rate.current, total*uint64(time.Second/time.Nanosecond)/period)
		atomic.StoreUint64(&rate.nextByteCount, 0)
		atomic.StoreInt32(&rate.changing, 0)
	}
}