checkpt

overlay/tun_linux.go

@@ -16,11 +16,11 @@ import (
 	"unsafe"
 
 	"github.com/gaissmai/bart"
-	"github.com/hetznercloud/virtio-go/tuntap"
-	"github.com/hetznercloud/virtio-go/vhostnet"
 	"github.com/hetznercloud/virtio-go/virtio"
 	"github.com/sirupsen/logrus"
 	"github.com/slackhq/nebula/config"
+	"github.com/slackhq/nebula/overlay/tuntap"
+	"github.com/slackhq/nebula/overlay/vhostnet"
 	"github.com/slackhq/nebula/routing"
 	"github.com/slackhq/nebula/util"
 	"github.com/vishvananda/netlink"
@@ -121,9 +121,9 @@ func newTun(c *config.C, l *logrus.Logger, vpnNetworks []netip.Prefix, multiqueu
 	name := strings.Trim(c.GetString("tun.dev", ""), "\x00")
 	tundev, err := tuntap.NewDevice(
 		tuntap.WithName(name),
-		tuntap.WithDeviceType(tuntap.DeviceTypeTUN),
-		tuntap.WithVirtioNetHdr(true),
-		tuntap.WithOffloads(0x0), //todo
+		tuntap.WithDeviceType(tuntap.DeviceTypeTUN),                          //todo wtf
+		tuntap.WithVirtioNetHdr(true),                                        //todo hmm
+		tuntap.WithOffloads(unix.TUN_F_CSUM|unix.TUN_F_USO4|unix.TUN_F_USO6), //todo
 	)
 	if err != nil {
 		return nil, err
@@ -137,7 +137,7 @@ func newTun(c *config.C, l *logrus.Logger, vpnNetworks []netip.Prefix, multiqueu
 	t.Device = name
 
 	vdev, err := vhostnet.NewDevice(
-		vhostnet.WithBackendDevice(tundev),
+		vhostnet.WithBackendFD(int(tundev.File().Fd())),
 		vhostnet.WithQueueSize(8), //todo config
 	)
 	if err != nil {
@@ -264,9 +264,12 @@ func (t *tun) RoutesFor(ip netip.Addr) routing.Gateways {
 }
 
 func (t *tun) Read(p []byte) (int, error) {
-	_, out, err := t.vdev.ReceivePacket()
+	hdr, out, err := t.vdev.ReceivePacket()
 	if err != nil {
 		return 0, err
+	}
+	if hdr.NumBuffers == 0 {
+
 	}
 	p = p[:len(out)]
 	copy(p, out)
@@ -278,14 +281,18 @@ func (t *tun) Write(b []byte) (int, error) {
 
 	hdr := virtio.NetHdr{ //todo
 		Flags:      0,
-		GSOType:    0,
+		GSOType:    unix.VIRTIO_NET_HDR_GSO_NONE,
 		HdrLen:     0,
 		GSOSize:    0,
 		CsumStart:  0,
 		CsumOffset: 0,
 		NumBuffers: 0,
 	}
+	//todo wow fuck this
+	//bb := make([]byte, maximum+14)
+	//copy(bb[14:], b)
 	err := t.vdev.TransmitPacket(hdr, b)
+	//err := t.vdev.TransmitPacket2(b)
 	if err != nil {
 		return 0, err
 	}

overlay/tuntap/device.go

@@ -0,0 +1,232 @@
+package tuntap
+
+import (
+	"fmt"
+	"net"
+	"os"
+	"time"
+	"unsafe"
+
+	"github.com/hetznercloud/virtio-go/virtio"
+	"golang.org/x/sys/unix"
+)
+
+// Documentation:
+//   https://docs.kernel.org/networking/tuntap.html
+// Also worth a read:
+//   https://blog.cloudflare.com/virtual-networking-101-understanding-tap/
+
+// Device represents a TUN/TAP device.
+type Device struct {
+	name    string
+	ifindex uint32
+	mac     net.HardwareAddr
+	file    *os.File
+}
+
+// NewDevice creates a new TUN/TAP device, brings it up, and returns a [Device]
+// instance providing access to it.
+//
+// There are multiple options that can be passed to this constructor to
+// influence device creation:
+//   - [WithName]
+//   - [WithDeviceType]
+//   - [WithVirtioNetHdr]
+//   - [WithInterfaceFlags]
+//
+// Remember to call [Device.Close] after use to free up resources.
+func NewDevice(options ...Option) (_ *Device, err error) {
+	opts := optionDefaults
+	opts.apply(options)
+	if err = opts.validate(); err != nil {
+		return nil, fmt.Errorf("invalid options: %w", err)
+	}
+
+	// Get a file descriptor. The device will exist as long as we keep this
+	// file descriptor open.
+	fd, err := unix.Open("/dev/net/tun", os.O_RDWR, 0666)
+	if err != nil {
+		return nil, fmt.Errorf("access tuntap driver: %w", err)
+	}
+
+	// Create an interface request. When the name is empty, the kernel will
+	// auto-select one.
+	ifreq, err := unix.NewIfreq(opts.name)
+	if err != nil {
+		_ = unix.Close(fd)
+		return nil, fmt.Errorf("new ifreq: %w", err)
+	}
+
+	// Create the new device.
+	ifreq.SetUint16(opts.ifreqFlags())
+	if err = unix.IoctlIfreq(fd, unix.TUNSETIFF, ifreq); err != nil {
+		_ = unix.Close(fd)
+		return nil, fmt.Errorf("create device: %w", err)
+	}
+
+	dev := Device{
+		// The TUNSETIFF ioctl writes the actual name that was chosen for the
+		// device back to the request, so use that.
+		name: ifreq.Name(),
+	}
+
+	// Make the file descriptor of the device non-blocking. This enables us to
+	// cancel reads after a timeout when no packets are arriving.
+	// This, and the call to NewFile has to happen after creating the device:
+	// https://github.com/golang/go/issues/30426#issuecomment-470330742
+	// NewFile will recognize that the file descriptor is non-blocking and will
+	// configure polling for it.
+	if err = unix.SetNonblock(fd, true); err != nil {
+		_ = unix.Close(fd)
+		return nil, fmt.Errorf("make file descriptor non-blocking: %w", err)
+	}
+
+	// By wrapping the file descriptor as an os.File, we not only have a
+	// convenient way to read and write, but also register a finalizer that
+	// closes the file descriptor when it's being garbage collected.
+	dev.file = os.NewFile(uintptr(fd), dev.name)
+
+	// Make sure the device is removed when one of the following initialization
+	// steps fails.
+	defer func() {
+		if err != nil {
+			_ = dev.Close()
+		}
+	}()
+
+	if opts.virtioNetHdr {
+		// Tell the device which size we use for our virtio_net_hdr.
+		err = unix.IoctlSetPointerInt(fd, unix.TUNSETVNETHDRSZ, virtio.NetHdrSize)
+		if err != nil {
+			return nil, fmt.Errorf("set vnethdr size: %w", err)
+		}
+	}
+
+	// Tell the device which offloads are supported.
+	err = unix.IoctlSetInt(fd, unix.TUNSETOFFLOAD, opts.offloads)
+	if err != nil {
+		return nil, fmt.Errorf("set offloads: %w", err)
+	}
+
+	// For the following ioctls we need just any AF_INET socket, so create one.
+	inet, err := unix.Socket(unix.AF_INET, unix.SOCK_DGRAM, 0)
+	if err != nil {
+		return nil, fmt.Errorf("open inet socket: %w", err)
+	}
+	defer func() { _ = unix.Close(inet) }()
+
+	// Set the interface flags to bring it up.
+	ifreq.SetUint16(unix.IFF_UP | opts.interfaceFlags)
+	if err = unix.IoctlIfreq(inet, unix.SIOCSIFFLAGS, ifreq); err != nil {
+		return nil, fmt.Errorf("set interface flags: %w", err)
+	}
+
+	// Get the interface index.
+	if err = unix.IoctlIfreq(inet, unix.SIOCGIFINDEX, ifreq); err != nil {
+		return nil, fmt.Errorf("get interface index: %w", err)
+	}
+	dev.ifindex = ifreq.Uint32()
+
+	// Get the MAC address.
+	// This ioctl writes a sockaddr into the data ifru section of the interface
+	// request struct. The MAC address is in the beginning of the
+	// sockaddr.sa_data section.
+	if err = unix.IoctlIfreq(inet, unix.SIOCGIFHWADDR, ifreq); err != nil {
+		return nil, fmt.Errorf("get mac address: %w", err)
+	}
+	dev.mac = unsafe.Slice((*byte)(unsafe.Pointer(ifreq)), 32)[16+2 : 16+8]
+
+	return &dev, nil
+}
+
+// Close closes the file descriptor behind this device. This will cause the
+// TUN/TAP device to be removed.
+func (dev *Device) Close() error {
+	if err := dev.file.Close(); err != nil {
+		return fmt.Errorf("close file descriptor: %w", err)
+	}
+	dev.file = nil
+	return nil
+}
+
+// Name returns the name of this device.
+func (dev *Device) Name() string {
+	dev.ensureInitialized()
+	return dev.name
+}
+
+// Ifindex returns the interface index of this device.
+func (dev *Device) Ifindex() uint32 {
+	dev.ensureInitialized()
+	return dev.ifindex
+}
+
+// MAC returns the hardware address of this device.
+func (dev *Device) MAC() net.HardwareAddr {
+	dev.ensureInitialized()
+	return dev.mac
+}
+
+// File returns the [os.File] that is used to communicate with this device.
+// If you access it directly, please be careful to not interfere with this
+// implementation.
+func (dev *Device) File() *os.File {
+	dev.ensureInitialized()
+	return dev.file
+}
+
+// WritePacket writes the given packet to the TUN/TAP device.
+// When the [WithVirtioNetHdr] option was enabled, then the caller is
+// responsible to prepend the packet with a [virtio.NetHdr].
+func (dev *Device) WritePacket(packet []byte) error {
+	dev.ensureInitialized()
+
+	_, err := dev.file.Write(packet)
+	if err != nil {
+		return fmt.Errorf("write %d bytes: %w", len(packet), err)
+	}
+
+	return nil
+}
+
+// ReadPacket reads the next available packet from the TUN/TAP device into the
+// given buffer. Make sure that the buffer is large enough, otherwise only a
+// part of the packet may be read. The number of read bytes will be returned.
+//
+// When the [WithVirtioNetHdr] option was enabled, then the read packet will be
+// prepended with a [virtio.NetHdr]. The caller is responsible to handle it
+// accordingly.
+//
+// A timeout can be given to limit the time this operation blocks. If no packet
+// arrives within the given timeout, the read is canceled and an error that
+// wraps [os.ErrDeadlineExceeded] is returned. Pass a timeout of zero to make
+// this operation block infinitely.
+func (dev *Device) ReadPacket(buf []byte, timeout time.Duration) (int, error) {
+	dev.ensureInitialized()
+
+	// Make sure the read times out. This only works for files that support
+	// polling (see above).
+	// When no timeout is desired, passing the zero time removes the deadline.
+	var deadline time.Time
+	if timeout > 0 {
+		deadline = time.Now().Add(timeout)
+	}
+	if err := dev.file.SetReadDeadline(deadline); err != nil {
+		return 0, fmt.Errorf("set deadline: %w", err)
+	}
+
+	n, err := dev.file.Read(buf)
+	if err != nil {
+		return n, fmt.Errorf("read up to %d bytes: %w", len(buf), err)
+	}
+
+	return n, nil
+}
+
+// ensureInitialized is used as a guard to prevent methods to be called on an
+// uninitialized instance.
+func (dev *Device) ensureInitialized() {
+	if dev.file == nil {
+		panic("device is not initialized")
+	}
+}

overlay/tuntap/device_test.go

@@ -0,0 +1,132 @@
+package tuntap_test
+
+import (
+	"net"
+	"os"
+	"testing"
+	"time"
+
+	"github.com/gopacket/gopacket/afpacket"
+	"github.com/hetznercloud/virtio-go/internal/testsupport"
+	"github.com/hetznercloud/virtio-go/tuntap"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"golang.org/x/sys/unix"
+)
+
+func TestNewDevice(t *testing.T) {
+	testsupport.VirtrunOnly(t)
+
+	t.Run("with static name", func(t *testing.T) {
+		const name = "test42"
+		dev, err := tuntap.NewDevice(
+			tuntap.WithDeviceType(tuntap.DeviceTypeTAP),
+			tuntap.WithName(name),
+		)
+		require.NoError(t, err)
+		t.Cleanup(func() {
+			assert.NoError(t, dev.Close())
+		})
+
+		assert.Equal(t, name, dev.Name())
+
+		iface, err := net.InterfaceByIndex(int(dev.Ifindex()))
+		assert.NoError(t, err)
+		assert.Equal(t, name, iface.Name)
+		assert.Equal(t, dev.MAC(), iface.HardwareAddr)
+	})
+
+	t.Run("with auto selected name", func(t *testing.T) {
+		dev, err := tuntap.NewDevice(
+			tuntap.WithDeviceType(tuntap.DeviceTypeTAP),
+		)
+		require.NoError(t, err)
+		t.Cleanup(func() {
+			assert.NoError(t, dev.Close())
+		})
+
+		assert.Contains(t, dev.Name(), "tap")
+
+		iface, err := net.InterfaceByIndex(int(dev.Ifindex()))
+		assert.NoError(t, err)
+		assert.Equal(t, dev.Name(), iface.Name)
+		assert.Equal(t, dev.MAC(), iface.HardwareAddr)
+	})
+}
+
+func TestDevice_WritePacket(t *testing.T) {
+	testsupport.VirtrunOnly(t)
+
+	dev, tPacket := setupTestDevice(t)
+
+	// Write a test packet to the TAP device.
+	_, pkt := testsupport.TestPacket(t, dev.MAC(), 64)
+	assert.NoError(t, dev.WritePacket(pkt))
+
+	// Check if the packet arrived in the RAW socket.
+	data, _, err := tPacket.ReadPacketData()
+	assert.NoError(t, err)
+	assert.Equal(t, pkt, data)
+}
+
+func TestDevice_ReadPacket(t *testing.T) {
+	testsupport.VirtrunOnly(t)
+
+	dev, tPacket := setupTestDevice(t)
+
+	// Write a test packet to the RAW socket.
+	_, pkt := testsupport.TestPacket(t, dev.MAC(), 64)
+	assert.NoError(t, tPacket.WritePacketData(pkt))
+
+	// Check if the packet arrived at the TAP device.
+	receiveBuf := make([]byte, 1024)
+	n, err := dev.ReadPacket(receiveBuf, time.Second)
+	assert.NoError(t, err)
+	assert.Equal(t, len(pkt), n)
+	assert.Equal(t, pkt, receiveBuf[:n])
+}
+
+func TestDevice_ReadPacket_Timeout(t *testing.T) {
+	testsupport.VirtrunOnly(t)
+
+	dev, _ := setupTestDevice(t)
+
+	// Try to receive a packet on the TAP device when none was sent.
+	// This should time out.
+	receiveBuf := make([]byte, 1024)
+	_, err := dev.ReadPacket(receiveBuf, 500*time.Millisecond)
+	assert.ErrorIs(t, err, os.ErrDeadlineExceeded)
+}
+
+func setupTestDevice(t *testing.T) (*tuntap.Device, *afpacket.TPacket) {
+	t.Helper()
+	testsupport.VirtrunOnly(t)
+
+	// Make sure the Linux kernel does not send router solicitations that may
+	// interfere with these tests.
+	testsupport.SetSysctl(t, "net.ipv6.conf.all.disable_ipv6", "1")
+
+	// Create a TAP device.
+	dev, err := tuntap.NewDevice(
+		tuntap.WithDeviceType(tuntap.DeviceTypeTAP),
+		// Helps to stop the Linux kernel from sending packets on this
+		// interface.
+		tuntap.WithInterfaceFlags(unix.IFF_NOARP),
+	)
+	require.NoError(t, err)
+	t.Cleanup(func() {
+		assert.NoError(t, dev.Close())
+	})
+
+	// Open a RAW socket to capture packets arriving at the TAP device or
+	// write packets to it.
+	tPacket, err := afpacket.NewTPacket(
+		afpacket.SocketRaw,
+		afpacket.TPacketVersion3,
+		afpacket.OptInterface(dev.Name()),
+	)
+	require.NoError(t, err)
+	t.Cleanup(tPacket.Close)
+
+	return dev, tPacket
+}

overlay/tuntap/doc.go

@@ -0,0 +1,3 @@
+// Package tuntap provides methods to create TUN/TAP devices and send and
+// receive packets on them.
+package tuntap

overlay/tuntap/options.go

@@ -0,0 +1,116 @@
+package tuntap
+
+import (
+	"errors"
+
+	"golang.org/x/sys/unix"
+)
+
+// DeviceType is the TUN/TAP device type.
+type DeviceType int
+
+const (
+	// DeviceTypeTUN can be used to create TUN devices that operate on layer 3.
+	// Packets that are transported over TUN devices do not have an Ethernet
+	// header.
+	DeviceTypeTUN DeviceType = unix.IFF_TUN
+	// DeviceTypeTAP can be used to create TAP devices that operate on layer 2.
+	// Packets that are transported over TAP devices do have an Ethernet header.
+	DeviceTypeTAP DeviceType = unix.IFF_TAP
+)
+
+type optionValues struct {
+	name           string
+	deviceType     DeviceType
+	virtioNetHdr   bool
+	offloads       int
+	interfaceFlags uint16
+}
+
+func (o *optionValues) apply(options []Option) {
+	for _, option := range options {
+		option(o)
+	}
+}
+
+func (o *optionValues) validate() error {
+	if len(o.name) >= unix.IFNAMSIZ {
+		return errors.New("name must not be longer that 15 characters")
+	}
+	if o.deviceType != DeviceTypeTUN && o.deviceType != DeviceTypeTAP {
+		return errors.New("device type is required and must be either TUN or TAP")
+	}
+	return nil
+}
+
+func (o *optionValues) ifreqFlags() uint16 {
+	flags := uint16(o.deviceType)
+
+	// Disable the packet information prefix.
+	flags |= unix.IFF_NO_PI
+
+	// Ensure the ioctl fails when a device with the same name already exists.
+	flags |= unix.IFF_TUN_EXCL
+
+	if o.virtioNetHdr {
+		// Also requires the TUNSETVNETHDRSZ ioctl at a later time.
+		flags |= unix.IFF_VNET_HDR
+	}
+
+	return flags
+}
+
+var optionDefaults = optionValues{
+	// Let the kernel auto-select a name.
+	name: "",
+	// Required.
+	deviceType: -1,
+	// Don't enable it by default to avoid surprises.
+	virtioNetHdr: false,
+	// Optional. No offload support advertised by default.
+	offloads: 0,
+	// Optional. IFF_UP will always be set.
+	interfaceFlags: 0,
+}
+
+// Option can be passed to [NewDevice] to influence device creation.
+type Option func(*optionValues)
+
+// WithName returns an [Option] that sets the name of the to be created device.
+// This is optional. When no name is specified, the kernel will auto-select a
+// name using the scheme "tunX" or "tapX".
+func WithName(name string) Option {
+	return func(o *optionValues) { o.name = name }
+}
+
+// WithDeviceType returns an [Option] that sets the type of device that should
+// be created.
+// This is required.
+func WithDeviceType(deviceType DeviceType) Option {
+	return func(o *optionValues) { o.deviceType = deviceType }
+}
+
+// WithVirtioNetHdr returns an [Option] that sets whether packets that are
+// transported over the device are prepended with a [virtio.NetHdr].
+// This is optional and disabled by default.
+func WithVirtioNetHdr(enable bool) Option {
+	return func(o *optionValues) { o.virtioNetHdr = enable }
+}
+
+// WithOffloads returns an [Option] that sets the supported offloads that the
+// device should advertise. This tells the kernel which offloads the owner of
+// the device can deal with ([unix.TUN_F_CSUM] for example).
+// This is optional. By default, no offloads are supported.
+// When configured, then [WithVirtioNetHdr] should also be enabled.
+func WithOffloads(offloads int) Option {
+	return func(o *optionValues) { o.offloads = offloads }
+}
+
+// WithInterfaceFlags returns an [Option] that sets the flags that should be
+// used when taking the created interface up.
+// This is optional. The [unix.IFF_UP] flag will always be set.
+// The [unix.IFF_NOARP] flag may be useful in some scenarios to avoid packets
+// from the Linux networking stack interfering with your application.
+func WithInterfaceFlags(flags uint16) Option {
+	return func(o *optionValues) { o.interfaceFlags = flags }
+}

overlay/tuntap/options_internal_test.go

@@ -0,0 +1,79 @@
+package tuntap
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"golang.org/x/sys/unix"
+)
+
+func TestOptionValues_Apply(t *testing.T) {
+	opts := optionDefaults
+	opts.apply([]Option{
+		WithName("name"),
+		WithDeviceType(DeviceTypeTAP),
+		WithVirtioNetHdr(true),
+		WithOffloads(unix.TUN_F_CSUM),
+		WithInterfaceFlags(unix.IFF_NOARP),
+	})
+
+	assert.Equal(t, optionValues{
+		name:           "name",
+		deviceType:     DeviceTypeTAP,
+		virtioNetHdr:   true,
+		offloads:       unix.TUN_F_CSUM,
+		interfaceFlags: unix.IFF_NOARP,
+	}, opts)
+}
+
+func TestOptionValues_Validate(t *testing.T) {
+	tests := []struct {
+		name      string
+		values    optionValues
+		assertErr assert.ErrorAssertionFunc
+	}{
+		{
+			name: "name too long",
+			values: optionValues{
+				name:       "thisisaverylongname",
+				deviceType: DeviceTypeTAP,
+			},
+			assertErr: assert.Error,
+		},
+		{
+			name:      "device type missing",
+			values:    optionValues{},
+			assertErr: assert.Error,
+		},
+		{
+			name: "invalid device type",
+			values: optionValues{
+				deviceType: 999,
+			},
+			assertErr: assert.Error,
+		},
+		{
+			name: "valid minimal",
+			values: optionValues{
+				deviceType: DeviceTypeTAP,
+			},
+			assertErr: assert.NoError,
+		},
+		{
+			name: "valid full",
+			values: optionValues{
+				name:           "name",
+				deviceType:     DeviceTypeTAP,
+				virtioNetHdr:   true,
+				offloads:       unix.TUN_F_CSUM,
+				interfaceFlags: unix.IFF_NOARP,
+			},
+			assertErr: assert.NoError,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			tt.assertErr(t, tt.values.validate())
+		})
+	}
+}

overlay/vhostnet/device.go

@@ -0,0 +1,412 @@
+package vhostnet
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"runtime"
+
+	"github.com/hetznercloud/virtio-go/vhost"
+	"github.com/hetznercloud/virtio-go/virtio"
+	"github.com/hetznercloud/virtio-go/virtqueue"
+	"golang.org/x/sys/unix"
+)
+
+// ErrDeviceClosed is returned when the [Device] is closed while operations are
+// still running.
+var ErrDeviceClosed = errors.New("device was closed")
+
+// The indexes for the receive and transmit queues.
+const (
+	receiveQueueIndex  = 0
+	transmitQueueIndex = 1
+)
+
+// Device represents a vhost networking device within the kernel-level virtio
+// implementation and provides methods to interact with it.
+type Device struct {
+	initialized bool
+	controlFD   int
+
+	receiveQueue  *virtqueue.SplitQueue
+	transmitQueue *virtqueue.SplitQueue
+
+	// transmitted contains channels for each possible descriptor chain head
+	// index. This is used for packet transmit notifications.
+	// When a packet was transmitted and the descriptor chain was used by the
+	// device, the corresponding channel receives the [virtqueue.UsedElement]
+	// instance provided by the device.
+	transmitted []chan virtqueue.UsedElement
+}
+
+// NewDevice initializes a new vhost networking device within the
+// kernel-level virtio implementation, sets up the virtqueues and returns a
+// [Device] instance that can be used to communicate with that vhost device.
+//
+// There are multiple options that can be passed to this constructor to
+// influence device creation:
+//   - [WithQueueSize]
+//   - [WithBackendFD]
+//   - [WithBackendDevice]
+//
+// Remember to call [Device.Close] after use to free up resources.
+func NewDevice(options ...Option) (_ *Device, err error) {
+	opts := optionDefaults
+	opts.apply(options)
+	if err = opts.validate(); err != nil {
+		return nil, fmt.Errorf("invalid options: %w", err)
+	}
+
+	dev := Device{
+		controlFD: -1,
+	}
+
+	// Clean up a partially initialized device when something fails.
+	defer func() {
+		if err != nil {
+			_ = dev.Close()
+		}
+	}()
+
+	// Retrieve a new control file descriptor. This will be used to configure
+	// the vhost networking device in the kernel.
+	dev.controlFD, err = unix.Open("/dev/vhost-net", os.O_RDWR, 0666)
+	if err != nil {
+		return nil, fmt.Errorf("get control file descriptor: %w", err)
+	}
+	if err = vhost.OwnControlFD(dev.controlFD); err != nil {
+		return nil, fmt.Errorf("own control file descriptor: %w", err)
+	}
+
+	// Advertise the supported features. This isn't much for now.
+	// TODO: Add feature options and implement proper feature negotiation.
+	features := virtio.FeatureVersion1 // | virtio.FeatureNetMergeRXBuffers
+	if err = vhost.SetFeatures(dev.controlFD, features); err != nil {
+		return nil, fmt.Errorf("set features: %w", err)
+	}
+
+	// Initialize and register the queues needed for the networking device.
+	if dev.receiveQueue, err = createQueue(dev.controlFD, receiveQueueIndex, opts.queueSize); err != nil {
+		return nil, fmt.Errorf("create receive queue: %w", err)
+	}
+	if dev.transmitQueue, err = createQueue(dev.controlFD, transmitQueueIndex, opts.queueSize); err != nil {
+		return nil, fmt.Errorf("create transmit queue: %w", err)
+	}
+
+	// Set up memory mappings for all buffers used by the queues. This has to
+	// happen before a backend for the queues can be registered.
+	memoryLayout := vhost.NewMemoryLayoutForQueues(
+		[]*virtqueue.SplitQueue{dev.receiveQueue, dev.transmitQueue},
+	)
+	if err = vhost.SetMemoryLayout(dev.controlFD, memoryLayout); err != nil {
+		return nil, fmt.Errorf("setup memory layout: %w", err)
+	}
+
+	// Set the queue backends. This activates the queues within the kernel.
+	if err = SetQueueBackend(dev.controlFD, receiveQueueIndex, opts.backendFD); err != nil {
+		return nil, fmt.Errorf("set receive queue backend: %w", err)
+	}
+	if err = SetQueueBackend(dev.controlFD, transmitQueueIndex, opts.backendFD); err != nil {
+		return nil, fmt.Errorf("set transmit queue backend: %w", err)
+	}
+
+	// Fully populate the receive queue with available buffers which the device
+	// can write new packets into.
+	if err = dev.refillReceiveQueue(); err != nil {
+		return nil, fmt.Errorf("refill receive queue: %w", err)
+	}
+
+	// Initialize channels for transmit notifications.
+	dev.transmitted = make([]chan virtqueue.UsedElement, dev.transmitQueue.Size())
+	for i := range len(dev.transmitted) {
+		// It is important to use a single-element buffered channel here.
+		// When the channel was unbuffered and the monitorTransmitQueue
+		// goroutine would write into it, the writing would block which could
+		// lead to deadlocks in case transmit notifications do not arrive in
+		// order.
+		// When the goroutine would use fire-and-forget to write into that
+		// channel, there may be a chance that the TransmitPacket does not
+		// receive the transmit notification due to this being a race condition.
+		// Buffering a single transmit notification resolves this without race
+		// conditions or possible deadlocks.
+		dev.transmitted[i] = make(chan virtqueue.UsedElement, 1)
+	}
+
+	// Monitor transmit queue in background.
+	go dev.monitorTransmitQueue()
+
+	dev.initialized = true
+
+	// Make sure to clean up even when the device gets garbage collected without
+	// Close being called first.
+	devPtr := &dev
+	runtime.SetFinalizer(devPtr, (*Device).Close)
+
+	return devPtr, nil
+}
+
+// monitorTransmitQueue waits for the device to advertise used descriptor chains
+// in the transmit queue and produces a transmit notification via the
+// corresponding channel.
+func (dev *Device) monitorTransmitQueue() {
+	usedChan := dev.transmitQueue.UsedDescriptorChains()
+	for {
+		used, ok := <-usedChan
+		if !ok {
+			// The queue was closed.
+			return
+		}
+		if int(used.DescriptorIndex) > len(dev.transmitted) {
+			panic(fmt.Sprintf("device provided a used descriptor index (%d) that is out of range",
+				used.DescriptorIndex))
+		}
+
+		dev.transmitted[used.DescriptorIndex] <- used
+	}
+}
+
+// TransmitPacket writes the given packet into the transmit queue of this
+// device. The packet will be prepended with the [virtio.NetHdr].
+//
+// When the queue is full, this will block until the queue has enough room to
+// transmit the packet. This method will not return before the packet was
+// transmitted and the device notifies that it has used the packet buffer.
+func (dev *Device) TransmitPacket(vnethdr virtio.NetHdr, packet []byte) error {
+	// Prepend the packet with its virtio-net header.
+	vnethdrBuf := make([]byte, virtio.NetHdrSize) //todo WHY
+	if err := vnethdr.Encode(vnethdrBuf); err != nil {
+		return fmt.Errorf("encode vnethdr: %w", err)
+	}
+	outBuffers := [][]byte{vnethdrBuf, packet}
+
+	chainIndex, err := dev.transmitQueue.OfferDescriptorChain(outBuffers, 0, true)
+	if err != nil {
+		return fmt.Errorf("offer descriptor chain: %w", err)
+	}
+
+	// Wait for the packet to have been transmitted.
+	<-dev.transmitted[chainIndex]
+
+	if err = dev.transmitQueue.FreeDescriptorChain(chainIndex); err != nil {
+		return fmt.Errorf("free descriptor chain: %w", err)
+	}
+
+	return nil
+}
+
+// ReceivePacket reads the next available packet from the receive queue of this
+// device and returns its [virtio.NetHdr] and packet data separately.
+//
+// When no packet is available, this will block until there is one.
+//
+// When this method returns an error, the receive queue will likely be in a
+// broken state which this implementation cannot recover from. The caller should
+// close the device and not attempt any additional receives.
+func (dev *Device) ReceivePacket() (virtio.NetHdr, []byte, error) {
+	var (
+		chainHeads []uint16
+
+		vnethdr virtio.NetHdr
+		buffers [][]byte
+
+		// Each packet starts with a virtio-net header which we have to subtract
+		// from the total length.
+		packetLength = -virtio.NetHdrSize
+	)
+
+	// We presented FeatureNetMergeRXBuffers to the device, so one packet may be
+	// made of multiple descriptor chains which are to be merged.
+	for remainingChains := 1; remainingChains > 0; remainingChains-- {
+		// Get the next descriptor chain.
+		usedElement, ok := <-dev.receiveQueue.UsedDescriptorChains()
+		if !ok {
+			return virtio.NetHdr{}, nil, ErrDeviceClosed
+		}
+
+		// Track this chain to be freed later.
+		head := uint16(usedElement.DescriptorIndex)
+		chainHeads = append(chainHeads, head)
+
+		outBuffers, inBuffers, err := dev.receiveQueue.GetDescriptorChain(head)
+		if err != nil {
+			// When this fails we may miss to free some descriptor chains. We
+			// could try to mitigate this by deferring the freeing somehow, but
+			// it's not worth the hassle. When this method fails, the queue will
+			// be in a broken state anyway.
+			return virtio.NetHdr{}, nil, fmt.Errorf("get descriptor chain: %w", err)
+		}
+		if len(outBuffers) > 0 {
+			// How did this happen!?
+			panic("receive queue contains device-readable buffers")
+		}
+		if len(inBuffers) == 0 {
+			// Empty descriptor chains should not be possible.
+			panic("descriptor chain contains no buffers")
+		}
+
+		// The device tells us how many bytes of the descriptor chain it has
+		// actually written to. The specification forces the device to fully
+		// fill up all but the last descriptor chain when multiple descriptor
+		// chains are being merged, but being more compatible here doesn't hurt.
+		inBuffers = truncateBuffers(inBuffers, int(usedElement.Length))
+		packetLength += int(usedElement.Length)
+
+		// Is this the first descriptor chain we process?
+		if len(buffers) == 0 {
+			// The specification requires that the first descriptor chain starts
+			// with a virtio-net header. It is not clear, whether it is also
+			// required to be fully contained in the first buffer of that
+			// descriptor chain, but it is reasonable to assume that this is
+			// always the case.
+			// The decode method already does the buffer length check.
+			if err = vnethdr.Decode(inBuffers[0]); err != nil {
+				// The device misbehaved. There is no way we can gracefully
+				// recover from this, because we don't know how many of the
+				// following descriptor chains belong to this packet.
+				return virtio.NetHdr{}, nil, fmt.Errorf("decode vnethdr: %w", err)
+			}
+			inBuffers[0] = inBuffers[0][virtio.NetHdrSize:]
+
+			// The virtio-net header tells us how many descriptor chains this
+			// packet is long.
+			remainingChains = int(vnethdr.NumBuffers)
+		}
+
+		buffers = append(buffers, inBuffers...)
+	}
+
+	// Copy all the buffers together to produce the complete packet slice.
+	packet := make([]byte, packetLength)
+	copied := 0
+	for _, buffer := range buffers {
+		copied += copy(packet[copied:], buffer)
+	}
+	if copied != packetLength {
+		panic(fmt.Sprintf("expected to copy %d bytes but only copied %d bytes", packetLength, copied))
+	}
+
+	// Now that we have copied all buffers, we can free the used descriptor
+	// chains again.
+	// TODO: Recycling the descriptor chains would be more efficient than
+	//  freeing them just to offer them again right after.
+	for _, head := range chainHeads {
+		if err := dev.receiveQueue.FreeDescriptorChain(head); err != nil {
+			return virtio.NetHdr{}, nil, fmt.Errorf("free descriptor chain with head index %d: %w", head, err)
+		}
+	}
+
+	// It's advised to always keep the receive queue fully populated with
+	// available buffers which the device can write new packets into.
+	if err := dev.refillReceiveQueue(); err != nil {
+		return virtio.NetHdr{}, nil, fmt.Errorf("refill receive queue: %w", err)
+	}
+
+	return vnethdr, packet, nil
+}
+
+// TODO: Make above methods cancelable by taking a context.Context argument?
+// TODO: Implement zero-copy variants to transmit and receive packets?
+
+// refillReceiveQueue offers as many new device-writable buffers to the device
+// as the queue can fit. The device will then use these to write received
+// packets.
+func (dev *Device) refillReceiveQueue() error {
+	for {
+		_, err := dev.receiveQueue.OfferDescriptorChain(nil, 1, false)
+		if err != nil {
+			if errors.Is(err, virtqueue.ErrNotEnoughFreeDescriptors) {
+				// Queue is full, job is done.
+				return nil
+			}
+			return fmt.Errorf("offer descriptor chain: %w", err)
+		}
+	}
+}
+
+// Close cleans up the vhost networking device within the kernel and releases
+// all resources used for it.
+// The implementation will try to release as many resources as possible and
+// collect potential errors before returning them.
+func (dev *Device) Close() error {
+	dev.initialized = false
+
+	// Closing the control file descriptor will unregister all queues from the
+	// kernel.
+	if dev.controlFD >= 0 {
+		if err := unix.Close(dev.controlFD); err != nil {
+			// Return an error and do not continue, because the memory used for
+			// the queues should not be released before they were unregistered
+			// from the kernel.
+			return fmt.Errorf("close control file descriptor: %w", err)
+		}
+		dev.controlFD = -1
+	}
+
+	var errs []error
+
+	if dev.receiveQueue != nil {
+		if err := dev.receiveQueue.Close(); err == nil {
+			dev.receiveQueue = nil
+		} else {
+			errs = append(errs, fmt.Errorf("close receive queue: %w", err))
+		}
+	}
+
+	if dev.transmitQueue != nil {
+		if err := dev.transmitQueue.Close(); err == nil {
+			dev.transmitQueue = nil
+		} else {
+			errs = append(errs, fmt.Errorf("close transmit queue: %w", err))
+		}
+	}
+
+	if len(errs) == 0 {
+		// Everything was cleaned up. No need to run the finalizer anymore.
+		runtime.SetFinalizer(dev, nil)
+	}
+
+	return errors.Join(errs...)
+}
+
+// ensureInitialized is used as a guard to prevent methods to be called on an
+// uninitialized instance.
+func (dev *Device) ensureInitialized() {
+	if !dev.initialized {
+		panic("device is not initialized")
+	}
+}
+
+// createQueue creates a new virtqueue and registers it with the vhost device
+// using the given index.
+func createQueue(controlFD int, queueIndex int, queueSize int) (*virtqueue.SplitQueue, error) {
+	var (
+		queue *virtqueue.SplitQueue
+		err   error
+	)
+	if queue, err = virtqueue.NewSplitQueue(queueSize); err != nil {
+		return nil, fmt.Errorf("create virtqueue: %w", err)
+	}
+	if err = vhost.RegisterQueue(controlFD, uint32(queueIndex), queue); err != nil {
+		return nil, fmt.Errorf("register virtqueue with index %d: %w", queueIndex, err)
+	}
+	return queue, nil
+}
+
+// truncateBuffers returns a new list of buffers whose combined length matches
+// exactly the specified length. When the specified length exceeds the length of
+// the buffers, this is an error. When it is smaller, the buffer list will be
+// truncated accordingly.
+func truncateBuffers(buffers [][]byte, length int) (out [][]byte) {
+	for _, buffer := range buffers {
+		if length < len(buffer) {
+			out = append(out, buffer[:length])
+			return
+		}
+		out = append(out, buffer)
+		length -= len(buffer)
+	}
+	if length > 0 {
+		panic("length exceeds the combined length of all buffers")
+	}
+	return
+}

overlay/vhostnet/device_internal_test.go

@@ -0,0 +1,86 @@
+package vhostnet
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestTruncateBuffers(t *testing.T) {
+	tests := []struct {
+		name     string
+		buffers  [][]byte
+		length   int
+		expected [][]byte
+	}{
+		{
+			name:     "no buffers",
+			buffers:  nil,
+			length:   0,
+			expected: nil,
+		},
+		{
+			name: "single buffer correct length",
+			buffers: [][]byte{
+				make([]byte, 100),
+			},
+			length: 100,
+			expected: [][]byte{
+				make([]byte, 100),
+			},
+		},
+		{
+			name: "single buffer truncated",
+			buffers: [][]byte{
+				make([]byte, 100),
+			},
+			length: 90,
+			expected: [][]byte{
+				make([]byte, 90),
+			},
+		},
+		{
+			name: "multiple buffers correct length",
+			buffers: [][]byte{
+				make([]byte, 200),
+				make([]byte, 100),
+			},
+			length: 300,
+			expected: [][]byte{
+				make([]byte, 200),
+				make([]byte, 100),
+			},
+		},
+		{
+			name: "multiple buffers truncated",
+			buffers: [][]byte{
+				make([]byte, 200),
+				make([]byte, 100),
+			},
+			length: 250,
+			expected: [][]byte{
+				make([]byte, 200),
+				make([]byte, 50),
+			},
+		},
+		{
+			name: "multiple buffers truncated buffer list",
+			buffers: [][]byte{
+				make([]byte, 200),
+				make([]byte, 200),
+				make([]byte, 200),
+			},
+			length: 350,
+			expected: [][]byte{
+				make([]byte, 200),
+				make([]byte, 150),
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			actual := truncateBuffers(tt.buffers, tt.length)
+			assert.Equal(t, tt.expected, actual)
+		})
+	}
+}

overlay/vhostnet/device_test.go

@@ -0,0 +1,224 @@
+package vhostnet_test
+
+import (
+	"fmt"
+	"os"
+	"sync"
+	"testing"
+
+	"github.com/gopacket/gopacket/afpacket"
+	"github.com/hetznercloud/virtio-go/internal/testsupport"
+	"github.com/hetznercloud/virtio-go/tuntap"
+	"github.com/hetznercloud/virtio-go/vhostnet"
+	"github.com/hetznercloud/virtio-go/virtio"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"golang.org/x/sys/unix"
+)
+
+// Here is the general idea of how the following tests work to verify the
+// correct communication with the vhost-net device within the kernel:
+//
+//   +-----------------------------------+
+//   |   go test running in user space   |
+//   +-----------------------------------+
+//            ^                    ^
+//            |                    |
+//    capture / write      transmit / receive
+//    using AF_PACKET      using this package
+//            |                    |
+//            v                    v
+//   +----------------+      +-----------+
+//   | tun (TAP mode) |<---->| vhost-net |
+//   +----------------+      +-----------+
+//
+
+func TestDevice_TransmitPacket(t *testing.T) {
+	testsupport.VirtrunOnly(t)
+
+	fx := NewTestFixture(t)
+
+	for _, length := range []int{64, 1514, 9014, 64100} {
+		t.Run(fmt.Sprintf("%d byte packet", length), func(t *testing.T) {
+			vnethdr, pkt := testsupport.TestPacket(t, fx.TAPDevice.MAC(), length)
+
+			// Transmit the packet over the vhost-net device.
+			require.NoError(t, fx.NetDevice.TransmitPacket(vnethdr, pkt))
+
+			// Check if the packet arrived at the TAP device. The virtio-net
+			// header should have been stripped by the TAP device.
+			data, _, err := fx.TPacket.ReadPacketData()
+			assert.NoError(t, err)
+			assert.Equal(t, pkt, data)
+		})
+	}
+}
+
+func TestDevice_ReceivePacket(t *testing.T) {
+	testsupport.VirtrunOnly(t)
+
+	fx := NewTestFixture(t)
+
+	for _, length := range []int{64, 1514, 9014, 64100} {
+		t.Run(fmt.Sprintf("%d byte packet", length), func(t *testing.T) {
+			vnethdr, pkt := testsupport.TestPacket(t, fx.TAPDevice.MAC(), length)
+			prependedPkt := testsupport.PrependPacket(t, vnethdr, pkt)
+
+			// Write the prepended packet to the TAP device.
+			require.NoError(t, fx.TPacket.WritePacketData(prependedPkt))
+
+			// Try to receive the packet on the vhost-net device.
+			vnethdr, data, err := fx.NetDevice.ReceivePacket()
+			assert.NoError(t, err)
+			assert.Equal(t, pkt, data)
+
+			// Large packets should have been received as multiple buffers.
+			assert.Equal(t, (len(prependedPkt)/os.Getpagesize())+1, int(vnethdr.NumBuffers))
+		})
+	}
+}
+
+func TestDevice_TransmitManyPackets(t *testing.T) {
+	testsupport.VirtrunOnly(t)
+
+	fx := NewTestFixture(t)
+
+	// Test with a packet which does not fit into a single memory page.
+	vnethdr, pkt := testsupport.TestPacket(t, fx.TAPDevice.MAC(), 9014)
+
+	const count = 1024
+	var received int
+
+	var wg sync.WaitGroup
+	wg.Go(func() {
+		for range count {
+			err := fx.NetDevice.TransmitPacket(vnethdr, pkt)
+			if !assert.NoError(t, err) {
+				return
+			}
+		}
+	})
+	wg.Go(func() {
+		for range count {
+			data, _, err := fx.TPacket.ReadPacketData()
+			if !assert.NoError(t, err) {
+				return
+			}
+			assert.Equal(t, pkt, data)
+			received++
+		}
+	})
+	wg.Wait()
+
+	assert.Equal(t, count, received)
+}
+
+func TestDevice_ReceiveManyPackets(t *testing.T) {
+	testsupport.VirtrunOnly(t)
+
+	fx := NewTestFixture(t)
+
+	// Test with a packet which does not fit into a single memory page.
+	vnethdr, pkt := testsupport.TestPacket(t, fx.TAPDevice.MAC(), 9014)
+	prependedPkt := testsupport.PrependPacket(t, vnethdr, pkt)
+
+	const count = 1024
+	var received int
+
+	var wg sync.WaitGroup
+	wg.Go(func() {
+		for range count {
+			err := fx.TPacket.WritePacketData(prependedPkt)
+			if !assert.NoError(t, err) {
+				return
+			}
+		}
+	})
+	wg.Go(func() {
+		for range count {
+			_, data, err := fx.NetDevice.ReceivePacket()
+			if !assert.NoError(t, err) {
+				return
+			}
+			assert.Equal(t, pkt, data)
+			received++
+		}
+	})
+	wg.Wait()
+
+	assert.Equal(t, count, received)
+}
+
+type TestFixture struct {
+	TAPDevice *tuntap.Device
+	NetDevice *vhostnet.Device
+	TPacket   *afpacket.TPacket
+}
+
+func NewTestFixture(t *testing.T) *TestFixture {
+	testsupport.VirtrunOnly(t)
+
+	// In case something doesn't work, some more debug logging from the kernel
+	// modules may be very helpful.
+	testsupport.EnableDynamicDebug(t, "module tun")
+	testsupport.EnableDynamicDebug(t, "module vhost")
+	testsupport.EnableDynamicDebug(t, "module vhost_net")
+
+	// Make sure the Linux kernel does not send router solicitations that may
+	// interfere with these tests.
+	testsupport.SetSysctl(t, "net.ipv6.conf.all.disable_ipv6", "1")
+
+	var (
+		fx  TestFixture
+		err error
+	)
+
+	// Create a TAP device.
+	fx.TAPDevice, err = tuntap.NewDevice(
+		tuntap.WithDeviceType(tuntap.DeviceTypeTAP),
+		// Helps to stop the Linux kernel from sending packets on this
+		// interface.
+		tuntap.WithInterfaceFlags(unix.IFF_NOARP),
+		// Packets going over this device are prepended with a virtio-net
+		// header. When this is not set, then packets written to the TAP device
+		// will be passed to the Linux network stack without their virtio-net
+		// header stripped.
+		tuntap.WithVirtioNetHdr(true),
+		// When writing packets into the TAP device using the RAW socket, we
+		// don't want the offloads to be applied by the kernel. Advertising
+		// offload support makes the kernel pass the offload request along to
+		// our vhost-net device.
+		tuntap.WithOffloads(unix.TUN_F_CSUM|unix.TUN_F_USO4|unix.TUN_F_USO6),
+	)
+	require.NoError(t, err)
+	t.Cleanup(func() {
+		assert.NoError(t, fx.TAPDevice.Close())
+	})
+
+	// Create a vhost-net device that uses the TAP device as the backend.
+	fx.NetDevice, err = vhostnet.NewDevice(
+		vhostnet.WithQueueSize(32),
+		vhostnet.WithBackendDevice(fx.TAPDevice),
+	)
+	require.NoError(t, err)
+	t.Cleanup(func() {
+		assert.NoError(t, fx.NetDevice.Close())
+	})
+
+	// Open a RAW socket to capture packets arriving at the TAP device or
+	// write packets into it.
+	fx.TPacket, err = afpacket.NewTPacket(
+		afpacket.SocketRaw,
+		afpacket.TPacketVersion3,
+		afpacket.OptInterface(fx.TAPDevice.Name()),
+
+		// Tell the kernel that packets written to this socket are prepended
+		// with a virto-net header. This is used to communicate the use of GSO
+		// for large packets.
+		afpacket.OptVNetHdrSize(virtio.NetHdrSize),
+	)
+	require.NoError(t, err)
+	t.Cleanup(fx.TPacket.Close)
+
+	return &fx
+}

overlay/vhostnet/doc.go

@@ -0,0 +1,3 @@
+// Package vhostnet implements methods to initialize vhost networking devices
+// within the kernel-level virtio implementation and communicate with them.
+package vhostnet

overlay/vhostnet/ioctl.go

@@ -0,0 +1,31 @@
+package vhostnet
+
+import (
+	"fmt"
+	"unsafe"
+
+	"github.com/hetznercloud/virtio-go/vhost"
+)
+
+const (
+	// vhostNetIoctlSetBackend can be used to attach a virtqueue to a RAW socket
+	// or TAP device.
+	//
+	// Request payload: [vhost.QueueFile]
+	// Kernel name: VHOST_NET_SET_BACKEND
+	vhostNetIoctlSetBackend = 0x4008af30
+)
+
+// SetQueueBackend attaches a virtqueue of the vhost networking device
+// described by controlFD to the given backend file descriptor.
+// The backend file descriptor can either be a RAW socket or a TAP device. When
+// it is -1, the queue will be detached.
+func SetQueueBackend(controlFD int, queueIndex uint32, backendFD int) error {
+	if err := vhost.IoctlPtr(controlFD, vhostNetIoctlSetBackend, unsafe.Pointer(&vhost.QueueFile{
+		QueueIndex: queueIndex,
+		FD:         int32(backendFD),
+	})); err != nil {
+		return fmt.Errorf("set queue backend file descriptor: %w", err)
+	}
+	return nil
+}

overlay/vhostnet/options.go

@@ -0,0 +1,70 @@
+package vhostnet
+
+import (
+	"errors"
+
+	"github.com/hetznercloud/virtio-go/tuntap"
+	"github.com/hetznercloud/virtio-go/virtqueue"
+)
+
+type optionValues struct {
+	queueSize int
+	backendFD int
+}
+
+func (o *optionValues) apply(options []Option) {
+	for _, option := range options {
+		option(o)
+	}
+}
+
+func (o *optionValues) validate() error {
+	if o.queueSize == -1 {
+		return errors.New("queue size is required")
+	}
+	if err := virtqueue.CheckQueueSize(o.queueSize); err != nil {
+		return err
+	}
+	if o.backendFD == -1 {
+		return errors.New("backend file descriptor is required")
+	}
+	return nil
+}
+
+var optionDefaults = optionValues{
+	// Required.
+	queueSize: -1,
+	// Required.
+	backendFD: -1,
+}
+
+// Option can be passed to [NewDevice] to influence device creation.
+type Option func(*optionValues)
+
+// WithQueueSize returns an [Option] that sets the size of the TX and RX queues
+// that are to be created for the device. It specifies the number of
+// entries/buffers each queue can hold. This also affects the memory
+// consumption.
+// This is required and must be an integer from 1 to 32768 that is also a power
+// of 2.
+func WithQueueSize(queueSize int) Option {
+	return func(o *optionValues) { o.queueSize = queueSize }
+}
+
+// WithBackendFD returns an [Option] that sets the file descriptor of the
+// backend that will be used for the queues of the device. The device will write
+// and read packets to/from that backend. The file descriptor can either be of a
+// RAW socket or TUN/TAP device.
+// Either this or [WithBackendDevice] is required.
+func WithBackendFD(backendFD int) Option {
+	return func(o *optionValues) { o.backendFD = backendFD }
+}
+
+// WithBackendDevice returns an [Option] that sets the given TAP device as the
+// backend that will be used for the queues of the device. The device will
+// write and read packets to/from that backend. The TAP device should have been
+// created with the [tuntap.WithVirtioNetHdr] option enabled.
+// Either this or [WithBackendFD] is required.
+func WithBackendDevice(dev *tuntap.Device) Option {
+	return func(o *optionValues) { o.backendFD = int(dev.File().Fd()) }
+}

overlay/vhostnet/options_internal_test.go

@@ -0,0 +1,66 @@
+package vhostnet
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestOptionValues_Apply(t *testing.T) {
+	opts := optionDefaults
+	opts.apply([]Option{
+		WithQueueSize(256),
+		WithBackendFD(99),
+	})
+
+	assert.Equal(t, optionValues{
+		queueSize: 256,
+		backendFD: 99,
+	}, opts)
+}
+
+func TestOptionValues_Validate(t *testing.T) {
+	tests := []struct {
+		name      string
+		values    optionValues
+		assertErr assert.ErrorAssertionFunc
+	}{
+		{
+			name: "queue size missing",
+			values: optionValues{
+				queueSize: -1,
+				backendFD: 99,
+			},
+			assertErr: assert.Error,
+		},
+		{
+			name: "invalid queue size",
+			values: optionValues{
+				queueSize: 24,
+				backendFD: 99,
+			},
+			assertErr: assert.Error,
+		},
+		{
+			name: "backend fd missing",
+			values: optionValues{
+				queueSize: 256,
+				backendFD: -1,
+			},
+			assertErr: assert.Error,
+		},
+		{
+			name: "valid",
+			values: optionValues{
+				queueSize: 256,
+				backendFD: 99,
+			},
+			assertErr: assert.NoError,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			tt.assertErr(t, tt.values.validate())
+		})
+	}
+}