Fix udp/icmp not work on gso with mixed stack

tun_linux.go

@@ -39,6 +39,7 @@ type NativeTun struct {
 	writeAccess         sync.Mutex
 	vnetHdr             bool
 	writeBuffer         []byte
+	vnetHdrWriteBuf     []byte
 	gsoToWrite          []int
 	tcpGROTable         *tcpGROTable
 	udpGroAccess        sync.Mutex

tun_linux_gvisor.go

@@ -3,6 +3,8 @@
 package tun
 
 import (
+	"fmt"
+
 	"github.com/sagernet/gvisor/pkg/rawfile"
 	"github.com/sagernet/gvisor/pkg/tcpip/link/fdbased"
 	"github.com/sagernet/gvisor/pkg/tcpip/stack"
@@ -18,6 +20,37 @@ var _ GVisorTun = (*NativeTun)(nil)
 
 func (t *NativeTun) WritePacket(pkt *stack.PacketBuffer) (int, error) {
 	iovecs := t.iovecsOutputDefault
+	if t.vnetHdr {
+		if t.vnetHdrWriteBuf == nil {
+			t.vnetHdrWriteBuf = make([]byte, virtioNetHdrLen)
+		}
+		vnetHdr := virtioNetHdr{}
+		if pkt.GSOOptions.Type != stack.GSONone {
+			vnetHdr.hdrLen = uint16(pkt.HeaderSize())
+			if pkt.GSOOptions.NeedsCsum {
+				vnetHdr.flags = unix.VIRTIO_NET_HDR_F_NEEDS_CSUM
+				vnetHdr.csumStart = pkt.GSOOptions.L3HdrLen
+				vnetHdr.csumOffset = pkt.GSOOptions.CsumOffset
+			}
+			if uint16(pkt.Data().Size()) > pkt.GSOOptions.MSS {
+				switch pkt.GSOOptions.Type {
+				case stack.GSOTCPv4:
+					vnetHdr.gsoType = unix.VIRTIO_NET_HDR_GSO_TCPV4
+				case stack.GSOTCPv6:
+					vnetHdr.gsoType = unix.VIRTIO_NET_HDR_GSO_TCPV6
+				default:
+					panic(fmt.Sprintf("Unknown gso type: %v", pkt.GSOOptions.Type))
+				}
+				vnetHdr.gsoSize = pkt.GSOOptions.MSS
+			}
+		}
+		if err := vnetHdr.encode(t.vnetHdrWriteBuf); err != nil {
+			return 0, err
+		}
+		iovec := unix.Iovec{Base: &t.vnetHdrWriteBuf[0]}
+		iovec.SetLen(virtioNetHdrLen)
+		iovecs = append(iovecs, iovec)
+	}
 	var dataLen int
 	for _, packetSlice := range pkt.AsSlices() {
 		dataLen += len(packetSlice)