claude does TUN virtio header support

overlay/tun_linux.go

@@ -24,6 +24,11 @@ import (
 	"golang.org/x/sys/unix"
 )
 
+const (
+	// virtioNetHdrLen is the length of virtio_net_hdr (without mergeable buffers)
+	virtioNetHdrLen = 10
+)
+
 type tun struct {
 	io.ReadWriteCloser
 	fd          int
@@ -35,6 +40,12 @@ type tun struct {
 	deviceIndex int
 	ioctlFd     uintptr
 	nonBlocking bool // true if fd is in non-blocking mode
+	vnetHdr     bool // true if IFF_VNET_HDR is enabled on the TUN device
+
+	// readBuf is used when vnetHdr is enabled to read the full packet+header
+	// before stripping the header. This is needed because caller-provided
+	// buffers are sized for MTU but kernel writes MTU+10 with virtio header.
+	readBuf []byte
 
 	Routes                    atomic.Pointer[[]Route]
 	routeTree                 atomic.Pointer[bart.Table[routing.Gateways]]
@@ -54,6 +65,23 @@ func (t *tun) Networks() []netip.Prefix {
 	return t.vpnNetworks
 }
 
+// tunVnetHdrSupported checks if the kernel supports IFF_VNET_HDR on TUN devices
+func tunVnetHdrSupported() bool {
+	fd, err := unix.Open("/dev/net/tun", unix.O_RDONLY, 0)
+	if err != nil {
+		return false
+	}
+	defer unix.Close(fd)
+
+	var features uint32
+	err = ioctl(uintptr(fd), uintptr(unix.TUNGETFEATURES), uintptr(unsafe.Pointer(&features)))
+	if err != nil {
+		return false
+	}
+
+	return features&unix.IFF_VNET_HDR != 0
+}
+
 type ifReq struct {
 	Name  [16]byte
 	Flags uint16
@@ -108,11 +136,18 @@ func newTun(c *config.C, l *logrus.Logger, vpnNetworks []netip.Prefix, multiqueu
 		}
 	}
 
+	// Check if VNET_HDR is supported before trying to use it
+	useVnetHdr := tunVnetHdrSupported()
+
 	var req ifReq
 	req.Flags = uint16(unix.IFF_TUN | unix.IFF_NO_PI)
 	if multiqueue {
 		req.Flags |= unix.IFF_MULTI_QUEUE
 	}
+	if useVnetHdr {
+		req.Flags |= unix.IFF_VNET_HDR
+	}
+
 	nameStr := c.GetString("tun.dev", "")
 	copy(req.Name[:], nameStr)
 	if err = ioctl(uintptr(fd), uintptr(unix.TUNSETIFF), uintptr(unsafe.Pointer(&req))); err != nil {
@@ -123,6 +158,13 @@ func newTun(c *config.C, l *logrus.Logger, vpnNetworks []netip.Prefix, multiqueu
 	}
 	name := strings.Trim(string(req.Name[:]), "\x00")
 
+	// Track if VNET_HDR is in use
+	// Note: We don't call TUNSETOFFLOAD - just handle the headers manually
+	vnetHdrEnabled := useVnetHdr
+	if vnetHdrEnabled {
+		l.Info("TUN VNET_HDR enabled")
+	}
+
 	file := os.NewFile(uintptr(fd), "/dev/net/tun")
 	t, err := newTunGeneric(c, l, file, vpnNetworks)
 	if err != nil {
@@ -130,6 +172,13 @@ func newTun(c *config.C, l *logrus.Logger, vpnNetworks []netip.Prefix, multiqueu
 	}
 
 	t.Device = name
+	t.vnetHdr = vnetHdrEnabled
+
+	// Allocate read buffer for virtio header handling
+	// Buffer needs to be large enough for virtio header + max packet
+	if t.vnetHdr {
+		t.readBuf = make([]byte, t.MaxMTU+virtioNetHdrLen)
+	}
 
 	return t, nil
 }
@@ -247,25 +296,48 @@ func (t *tun) NewMultiQueueReader() (io.ReadWriteCloser, error) {
 
 	var req ifReq
 	req.Flags = uint16(unix.IFF_TUN | unix.IFF_NO_PI | unix.IFF_MULTI_QUEUE)
+	if t.vnetHdr {
+		req.Flags |= unix.IFF_VNET_HDR
+	}
 	copy(req.Name[:], t.Device)
 	if err = ioctl(uintptr(fd), uintptr(unix.TUNSETIFF), uintptr(unsafe.Pointer(&req))); err != nil {
 		return nil, err
 	}
 
-	return &tunBatchReader{fd: fd, device: t.Device}, nil
+	reader := &tunBatchReader{fd: fd, device: t.Device, vnetHdr: t.vnetHdr}
+	if t.vnetHdr {
+		reader.readBuf = make([]byte, t.MaxMTU+virtioNetHdrLen)
+	}
+	return reader, nil
 }
 
 // tunBatchReader implements BatchReader for efficient batch packet reading
 type tunBatchReader struct {
-	fd     int
+	fd      int
-	device string
+	device  string
+	vnetHdr bool
+	readBuf []byte // internal buffer for virtio header handling
 }
 
 func (r *tunBatchReader) Read(b []byte) (int, error) {
+	// Choose buffer: use internal buffer for vnetHdr, caller's buffer otherwise
+	readBuf := b
+	if r.vnetHdr {
+		readBuf = r.readBuf
+	}
+
 	// Use poll to wait for data, then read
 	for {
-		n, err := unix.Read(r.fd, b)
+		n, err := unix.Read(r.fd, readBuf)
 		if err == nil {
+			if r.vnetHdr && n > virtioNetHdrLen {
+				packetLen := n - virtioNetHdrLen
+				copy(b, readBuf[virtioNetHdrLen:n])
+				return packetLen, nil
+			}
+			if r.vnetHdr {
+				return 0, nil // No packet data
+			}
 			return n, nil
 		}
 		if err == unix.EAGAIN || err == unix.EWOULDBLOCK {
@@ -285,7 +357,24 @@ func (r *tunBatchReader) Read(b []byte) (int, error) {
 }
 
 func (r *tunBatchReader) Write(b []byte) (int, error) {
-	return unix.Write(r.fd, b)
+	if !r.vnetHdr {
+		return unix.Write(r.fd, b)
+	}
+
+	// Use writev to prepend virtio header without copying the packet data
+	// Header is all zeros = no GSO, no checksum offload
+	var hdr [virtioNetHdrLen]byte
+	bufs := [][]byte{hdr[:], b}
+
+	n, err := unix.Writev(r.fd, bufs)
+	if err != nil {
+		return 0, err
+	}
+	// Return only the packet bytes written (exclude header)
+	if n > virtioNetHdrLen {
+		return n - virtioNetHdrLen, nil
+	}
+	return 0, nil
 }
 
 func (r *tunBatchReader) Close() error {
@@ -302,10 +391,31 @@ func (r *tunBatchReader) ReadBatch(packets [][]byte, sizes []int) (int, error) {
 		maxPackets = len(sizes)
 	}
 
+	// Choose read buffer based on vnetHdr
+	readBuf := packets[0] // Will be updated in loop for non-vnetHdr
+	if r.vnetHdr {
+		readBuf = r.readBuf
+	}
+
 	for count < maxPackets {
-		n, err := unix.Read(r.fd, packets[count])
+		if !r.vnetHdr {
+			readBuf = packets[count]
+		}
+
+		n, err := unix.Read(r.fd, readBuf)
 		if err == nil && n > 0 {
-			sizes[count] = n
+			if r.vnetHdr {
+				if n > virtioNetHdrLen {
+					packetLen := n - virtioNetHdrLen
+					copy(packets[count], readBuf[virtioNetHdrLen:n])
+					sizes[count] = packetLen
+				} else {
+					// Malformed packet (no data after header), skip
+					continue
+				}
+			} else {
+				sizes[count] = n
+			}
 			count++
 			continue
 		}
@@ -353,6 +463,27 @@ func (t *tun) RoutesFor(ip netip.Addr) routing.Gateways {
 }
 
 func (t *tun) Write(b []byte) (int, error) {
+	if !t.vnetHdr {
+		return t.writeSimple(b)
+	}
+
+	// Use writev to prepend virtio header without copying the packet data
+	// Header is all zeros = no GSO, no checksum offload
+	var hdr [virtioNetHdrLen]byte
+	bufs := [][]byte{hdr[:], b}
+
+	n, err := unix.Writev(t.fd, bufs)
+	if err != nil {
+		return 0, err
+	}
+	// Return only the packet bytes written (exclude header)
+	if n > virtioNetHdrLen {
+		return n - virtioNetHdrLen, nil
+	}
+	return 0, nil
+}
+
+func (t *tun) writeSimple(b []byte) (int, error) {
 	var nn int
 	maximum := len(b)
 
@@ -389,21 +520,64 @@ func (t *tun) EnableBatchReading() error {
 	return nil
 }
 
-// Read overrides the default Read to handle non-blocking mode
+// Read overrides the default Read to handle non-blocking mode and virtio headers
 func (t *tun) Read(b []byte) (int, error) {
+	if !t.vnetHdr {
+		return t.readSimple(b)
+	}
+
+	// With VNET_HDR, read into internal buffer (which has space for header)
+	// then copy packet data to caller's buffer
 	if !t.nonBlocking {
-		// Use the embedded ReadWriteCloser for blocking reads
+		n, err := t.ReadWriteCloser.Read(t.readBuf)
-		return t.ReadWriteCloser.Read(b)
+		if err != nil {
+			return 0, err
+		}
+		if n <= virtioNetHdrLen {
+			return 0, nil // No packet data
+		}
+		packetLen := n - virtioNetHdrLen
+		copy(b, t.readBuf[virtioNetHdrLen:n])
+		return packetLen, nil
 	}
 
 	// Non-blocking read with poll
+	for {
+		n, err := unix.Read(t.fd, t.readBuf)
+		if err == nil {
+			if n <= virtioNetHdrLen {
+				return 0, nil // No packet data
+			}
+			packetLen := n - virtioNetHdrLen
+			copy(b, t.readBuf[virtioNetHdrLen:n])
+			return packetLen, nil
+		}
+		if err == unix.EAGAIN || err == unix.EWOULDBLOCK {
+			pfds := []unix.PollFd{{Fd: int32(t.fd), Events: unix.POLLIN}}
+			_, err = unix.Poll(pfds, -1)
+			if err != nil {
+				if err == unix.EINTR {
+					continue
+				}
+				return 0, err
+			}
+			continue
+		}
+		return n, err
+	}
+}
+
+func (t *tun) readSimple(b []byte) (int, error) {
+	if !t.nonBlocking {
+		return t.ReadWriteCloser.Read(b)
+	}
+
 	for {
 		n, err := unix.Read(t.fd, b)
 		if err == nil {
 			return n, nil
 		}
 		if err == unix.EAGAIN || err == unix.EWOULDBLOCK {
-			// Wait for data
 			pfds := []unix.PollFd{{Fd: int32(t.fd), Events: unix.POLLIN}}
 			_, err = unix.Poll(pfds, -1)
 			if err != nil {
@@ -423,7 +597,7 @@ func (t *tun) Read(b []byte) (int, error) {
 func (t *tun) ReadBatch(packets [][]byte, sizes []int) (int, error) {
 	if !t.nonBlocking {
 		// Fallback to single read if non-blocking not enabled
-		n, err := t.ReadWriteCloser.Read(packets[0])
+		n, err := t.Read(packets[0])
 		if err != nil {
 			return 0, err
 		}
@@ -437,10 +611,33 @@ func (t *tun) ReadBatch(packets [][]byte, sizes []int) (int, error) {
 		maxPackets = len(sizes)
 	}
 
+	// Choose read buffer based on vnetHdr
+	// With vnetHdr, we need to read into internal buffer (has space for header)
+	// then copy packet data to caller's buffer
+	readBuf := packets[0] // Will be updated in the loop
+	if t.vnetHdr {
+		readBuf = t.readBuf
+	}
+
 	for count < maxPackets {
-		n, err := unix.Read(t.fd, packets[count])
+		if !t.vnetHdr {
+			readBuf = packets[count]
+		}
+
+		n, err := unix.Read(t.fd, readBuf)
 		if err == nil && n > 0 {
-			sizes[count] = n
+			if t.vnetHdr {
+				if n > virtioNetHdrLen {
+					packetLen := n - virtioNetHdrLen
+					copy(packets[count], readBuf[virtioNetHdrLen:n])
+					sizes[count] = packetLen
+				} else {
+					// Malformed packet (no data after header), skip
+					continue
+				}
+			} else {
+				sizes[count] = n
+			}
 			count++
 			continue
 		}