what about with bad GRO on UDP

packet/outpacket.go

@@ -0,0 +1,23 @@
+package packet
+
+type OutPacket struct {
+	Segments [][]byte
+	//todo virtio header?
+	SegSize      int
+	SegCounter   int
+	Valid        bool
+	wasSegmented bool
+
+	Scratch []byte
+}
+
+func NewOut() *OutPacket {
+	out := new(OutPacket)
+	const numSegments = 64
+	out.Segments = make([][]byte, numSegments)
+	for i := 0; i < numSegments; i++ { //todo this is dumb
+		out.Segments[i] = make([]byte, Size)
+	}
+	out.Scratch = make([]byte, Size)
+	return out
+}

packet/packet.go

@@ -0,0 +1,117 @@
+package packet
+
+import (
+	"encoding/binary"
+	"iter"
+	"net/netip"
+
+	"golang.org/x/sys/unix"
+)
+
+const Size = 0xffff
+
+type Packet struct {
+	Payload []byte
+	Control []byte
+	Name    []byte
+	SegSize int
+
+	//todo should this hold out as well?
+	OutLen int
+
+	wasSegmented bool
+	isV4         bool
+	//Addr    netip.AddrPort
+}
+
+func New(isV4 bool) *Packet {
+	return &Packet{
+		Payload: make([]byte, Size),
+		Control: make([]byte, unix.CmsgSpace(2)),
+		Name:    make([]byte, unix.SizeofSockaddrInet6),
+		isV4:    isV4,
+	}
+}
+
+func (p *Packet) AddrPort() netip.AddrPort {
+	var ip netip.Addr
+	// Its ok to skip the ok check here, the slicing is the only error that can occur and it will panic
+	if p.isV4 {
+		ip, _ = netip.AddrFromSlice(p.Name[4:8])
+	} else {
+		ip, _ = netip.AddrFromSlice(p.Name[8:24])
+	}
+	return netip.AddrPortFrom(ip.Unmap(), binary.BigEndian.Uint16(p.Name[2:4]))
+}
+
+func (p *Packet) updateCtrl(ctrlLen int) {
+	p.SegSize = len(p.Payload)
+	p.wasSegmented = false
+	if ctrlLen == 0 {
+		return
+	}
+	if len(p.Control) == 0 {
+		return
+	}
+	cmsgs, err := unix.ParseSocketControlMessage(p.Control)
+	if err != nil {
+		return // oh well
+	}
+
+	for _, c := range cmsgs {
+		if c.Header.Level == unix.SOL_UDP && c.Header.Type == unix.UDP_GRO && len(c.Data) >= 2 {
+			p.wasSegmented = true
+			p.SegSize = int(binary.LittleEndian.Uint16(c.Data[:2]))
+			return
+		}
+	}
+}
+
+// Update sets a Packet into "just received, not processed" state
+func (p *Packet) Update(ctrlLen int) {
+	p.OutLen = -1
+	p.updateCtrl(ctrlLen)
+}
+
+func (p *Packet) Segments() iter.Seq[[]byte] {
+	return func(yield func([]byte) bool) {
+		//cursor := 0
+		for offset := 0; offset < len(p.Payload); offset += p.SegSize {
+			end := offset + p.SegSize
+			if end > len(p.Payload) {
+				end = len(p.Payload)
+			}
+			if !yield(p.Payload[offset:end]) {
+				return
+			}
+		}
+
+		//if p.SegSize > 0 && p.SegSize < len(p.Payload) {
+		//
+		//} else {
+		//	f.readOutsidePackets(p.Addr, nil, result2[:0], p.Payload, h, fwPacket2, lhh, nb2, i, conntrackCache.Get(f.l))
+		//}
+
+	}
+}
+
+//type Pool struct {
+//	pool sync.Pool
+//}
+//
+//var bigPool = &Pool{
+//	pool: sync.Pool{New: func() any { return New() }},
+//}
+//
+//func GetPool() *Pool {
+//	return bigPool
+//}
+//
+//func (p *Pool) Get() *Packet {
+//	return p.pool.Get().(*Packet)
+//}
+//
+//func (p *Pool) Put(x *Packet) {
+//	x.Payload = x.Payload[:Size]
+//	p.pool.Put(x)
+//}