checkpt

interface.go

@@ -16,6 +16,8 @@ import (
 	"github.com/slackhq/nebula/firewall"
 	"github.com/slackhq/nebula/header"
 	"github.com/slackhq/nebula/overlay"
+	"github.com/slackhq/nebula/overlay/coalesce"
+	"github.com/slackhq/nebula/overlay/tio"
 	"github.com/slackhq/nebula/udp"
 )
 
@@ -85,11 +87,11 @@ type Interface struct {
 	conntrackCacheTimeout time.Duration
 
 	writers []udp.Conn
-	readers []overlay.Queue
+	readers []tio.Queue
 	// tunCoalescers is one tcpCoalescer per tun queue, wrapping readers[i].
 	// decryptToTun sends plaintext into the coalescer; listenOut calls its
 	// Flush at the end of each UDP recvmmsg batch.
-	tunCoalescers []*tcpCoalescer
+	tunCoalescers []*coalesce.TCPCoalescer
 	wg            sync.WaitGroup
 
 	// fatalErr holds the first unexpected reader error that caused shutdown.
@@ -187,8 +189,8 @@ func NewInterface(ctx context.Context, c *InterfaceConfig) (*Interface, error) {
 		routines:              c.routines,
 		version:               c.version,
 		writers:               make([]udp.Conn, c.routines),
-		readers:               make([]overlay.Queue, c.routines),
-		tunCoalescers:         make([]*tcpCoalescer, c.routines),
+		readers:               make([]tio.Queue, c.routines),
+		tunCoalescers:         make([]*coalesce.TCPCoalescer, c.routines),
 		myVpnNetworks:         cs.myVpnNetworks,
 		myVpnNetworksTable:    cs.myVpnNetworksTable,
 		myVpnAddrs:            cs.myVpnAddrs,
@@ -243,16 +245,17 @@ func (f *Interface) activate() error {
 	metrics.GetOrRegisterGauge("routines", nil).Update(int64(f.routines))
 
 	// Prepare n tun queues
-	var reader overlay.Queue = f.inside
 	for i := 0; i < f.routines; i++ {
 		if i > 0 {
-			reader, err = f.inside.NewMultiQueueReader()
+			err = f.inside.NewMultiQueueReader()
 			if err != nil {
 				return err
 			}
 		}
-		f.readers[i] = reader
-		f.tunCoalescers[i] = newTCPCoalescer(reader)
+	}
+	f.readers = f.inside.Readers()
+	for i := range f.readers {
+		f.tunCoalescers[i] = coalesce.NewTCPCoalescer(f.readers[i]) //todo don't always do this
 	}
 
 	f.wg.Add(1) // for us to wait on Close() to return
@@ -342,7 +345,7 @@ func (f *Interface) listenOut(i int) {
 	f.l.Debugf("underlay reader %v is done", i)
 }
 
-func (f *Interface) listenIn(reader overlay.Queue, i int) {
+func (f *Interface) listenIn(reader tio.Queue, i int) {
 	rejectBuf := make([]byte, mtu)
 	batch := newSendBatch(sendBatchCap, udp.MTU+32)
 	fwPacket := &firewall.Packet{}

main.go

@@ -3,7 +3,10 @@ package nebula
 import (
 	"context"
 	"fmt"
+	"log"
 	"net"
+	"net/http"
+	_ "net/http/pprof"
 	"net/netip"
 	"runtime/debug"
 	"strings"
@@ -49,6 +52,11 @@ func Main(c *config.C, configTest bool, buildVersion string, logger *logrus.Logg
 		l.Println(string(b))
 	}
 
+	//todo!!!
+	go func() {
+		log.Println(http.ListenAndServe("localhost:6060", nil))
+	}()
+
 	err := configLogger(l, c)
 	if err != nil {
 		return nil, util.ContextualizeIfNeeded("Failed to configure the logger", err)

tcp_coalesce.go → overlay/coalesce/tcp_coalesce.go

@@ -1,11 +1,11 @@
-package nebula
+package coalesce
 
 import (
 	"bytes"
 	"encoding/binary"
 	"io"
 
-	"github.com/slackhq/nebula/overlay"
+	"github.com/slackhq/nebula/overlay/tio"
 )
 
 // ipProtoTCP is the IANA protocol number for TCP. Hardcoded instead of
@@ -66,14 +66,14 @@ type coalesceSlot struct {
 	payIovs [][]byte
 }
 
-// tcpCoalescer accumulates adjacent in-flow TCP data segments across
+// TCPCoalescer accumulates adjacent in-flow TCP data segments across
 // multiple concurrent flows and emits each flow's run as a single TSO
-// superpacket via overlay.GSOWriter. All output — coalesced or not — is
+// superpacket via tio.GSOWriter. All output — coalesced or not — is
 // deferred until Flush so arrival order is preserved on the wire. Owns
 // no locks; one coalescer per TUN write queue.
-type tcpCoalescer struct {
+type TCPCoalescer struct {
 	plainW io.Writer
-	gsoW   overlay.GSOWriter // nil when the queue doesn't support TSO
+	gsoW   tio.GSOWriter // nil when the queue doesn't support TSO
 
 	// slots is the ordered event queue. Flush walks it once and emits each
 	// entry as either a WriteGSO (coalesced) or a plainW.Write (passthrough).
@@ -86,14 +86,14 @@ type tcpCoalescer struct {
 	pool      []*coalesceSlot // free list for reuse
 }
 
-func newTCPCoalescer(w io.Writer) *tcpCoalescer {
-	c := &tcpCoalescer{
+func NewTCPCoalescer(w io.Writer) *TCPCoalescer {
+	c := &TCPCoalescer{
 		plainW:    w,
 		slots:     make([]*coalesceSlot, 0, initialSlots),
 		openSlots: make(map[flowKey]*coalesceSlot, initialSlots),
 		pool:      make([]*coalesceSlot, 0, initialSlots),
 	}
-	if gw, ok := w.(overlay.GSOWriter); ok && gw.GSOSupported() {
+	if gw, ok := w.(tio.GSOWriter); ok && gw.GSOSupported() {
 		c.gsoW = gw
 	}
 	return c
@@ -197,7 +197,7 @@ func (p parsedTCP) coalesceable() bool {
 // Add borrows pkt. The caller must keep pkt valid until the next Flush,
 // whether or not the packet was coalesced — passthrough (non-admissible)
 // packets are queued and written at Flush time, not synchronously.
-func (c *tcpCoalescer) Add(pkt []byte) error {
+func (c *TCPCoalescer) Add(pkt []byte) error {
 	if c.gsoW == nil {
 		c.addPassthrough(pkt)
 		return nil
@@ -237,7 +237,7 @@ func (c *tcpCoalescer) Add(pkt []byte) error {
 // via WriteGSO; passthrough slots go out via plainW.Write. Returns the
 // first error observed; keeps draining so one bad packet doesn't hold up
 // the rest. After Flush returns, borrowed payload slices may be recycled.
-func (c *tcpCoalescer) Flush() error {
+func (c *TCPCoalescer) Flush() error {
 	var first error
 	for _, s := range c.slots {
 		var err error
@@ -261,14 +261,14 @@ func (c *tcpCoalescer) Flush() error {
 	return first
 }
 
-func (c *tcpCoalescer) addPassthrough(pkt []byte) {
+func (c *TCPCoalescer) addPassthrough(pkt []byte) {
 	s := c.take()
 	s.passthrough = true
 	s.rawPkt = pkt
 	c.slots = append(c.slots, s)
 }
 
-func (c *tcpCoalescer) seed(pkt []byte, info parsedTCP) {
+func (c *TCPCoalescer) seed(pkt []byte, info parsedTCP) {
 	if info.hdrLen > tcpCoalesceHdrCap || info.hdrLen+info.payLen > tcpCoalesceBufSize {
 		// Pathological shape — can't fit our scratch, emit as-is.
 		c.addPassthrough(pkt)
@@ -297,7 +297,7 @@ func (c *tcpCoalescer) seed(pkt []byte, info parsedTCP) {
 // canAppend reports whether info's packet extends the slot's seed: same
 // header shape and stable contents, adjacent seq, not oversized, chain not
 // closed.
-func (c *tcpCoalescer) canAppend(s *coalesceSlot, pkt []byte, info parsedTCP) bool {
+func (c *TCPCoalescer) canAppend(s *coalesceSlot, pkt []byte, info parsedTCP) bool {
 	if s.psh {
 		return false
 	}
@@ -322,7 +322,7 @@ func (c *tcpCoalescer) canAppend(s *coalesceSlot, pkt []byte, info parsedTCP) bo
 	return true
 }
 
-func (c *tcpCoalescer) appendPayload(s *coalesceSlot, pkt []byte, info parsedTCP) {
+func (c *TCPCoalescer) appendPayload(s *coalesceSlot, pkt []byte, info parsedTCP) {
 	s.payIovs = append(s.payIovs, pkt[info.hdrLen:info.hdrLen+info.payLen])
 	s.numSeg++
 	s.totalPay += info.payLen
@@ -332,7 +332,7 @@ func (c *tcpCoalescer) appendPayload(s *coalesceSlot, pkt []byte, info parsedTCP
 	}
 }
 
-func (c *tcpCoalescer) take() *coalesceSlot {
+func (c *TCPCoalescer) take() *coalesceSlot {
 	if n := len(c.pool); n > 0 {
 		s := c.pool[n-1]
 		c.pool[n-1] = nil
@@ -342,7 +342,7 @@ func (c *tcpCoalescer) take() *coalesceSlot {
 	return &coalesceSlot{}
 }
 
-func (c *tcpCoalescer) release(s *coalesceSlot) {
+func (c *TCPCoalescer) release(s *coalesceSlot) {
 	s.passthrough = false
 	s.rawPkt = nil
 	for i := range s.payIovs {
@@ -357,7 +357,7 @@ func (c *tcpCoalescer) release(s *coalesceSlot) {
 
 // flushSlot patches the header and calls WriteGSO. Does not remove the
 // slot from c.slots.
-func (c *tcpCoalescer) flushSlot(s *coalesceSlot) error {
+func (c *TCPCoalescer) flushSlot(s *coalesceSlot) error {
 	total := s.hdrLen + s.totalPay
 	l4Len := total - s.ipHdrLen
 	hdr := s.hdrBuf[:s.hdrLen]

tcp_coalesce_test.go → overlay/coalesce/tcp_coalesce_test.go

@@ -1,4 +1,4 @@
-package nebula
+package coalesce
 
 import (
 	"encoding/binary"
@@ -114,7 +114,7 @@ const (
 
 func TestCoalescerPassthroughWhenGSOUnavailable(t *testing.T) {
 	w := &fakeTunWriter{gsoEnabled: false}
-	c := newTCPCoalescer(w)
+	c := NewTCPCoalescer(w)
 	pkt := buildTCPv4(1000, tcpAck, []byte("hello"))
 	if err := c.Add(pkt); err != nil {
 		t.Fatal(err)
@@ -133,7 +133,7 @@ func TestCoalescerPassthroughWhenGSOUnavailable(t *testing.T) {
 
 func TestCoalescerNonTCPPassthrough(t *testing.T) {
 	w := &fakeTunWriter{gsoEnabled: true}
-	c := newTCPCoalescer(w)
+	c := NewTCPCoalescer(w)
 	pkt := make([]byte, 28)
 	pkt[0] = 0x45
 	binary.BigEndian.PutUint16(pkt[2:4], 28)
@@ -153,7 +153,7 @@ func TestCoalescerNonTCPPassthrough(t *testing.T) {
 
 func TestCoalescerSeedThenFlushAlone(t *testing.T) {
 	w := &fakeTunWriter{gsoEnabled: true}
-	c := newTCPCoalescer(w)
+	c := NewTCPCoalescer(w)
 	pkt := buildTCPv4(1000, tcpAck, make([]byte, 1000))
 	if err := c.Add(pkt); err != nil {
 		t.Fatal(err)
@@ -180,7 +180,7 @@ func TestCoalescerSeedThenFlushAlone(t *testing.T) {
 
 func TestCoalescerCoalescesAdjacentACKs(t *testing.T) {
 	w := &fakeTunWriter{gsoEnabled: true}
-	c := newTCPCoalescer(w)
+	c := NewTCPCoalescer(w)
 	pay := make([]byte, 1200)
 	if err := c.Add(buildTCPv4(1000, tcpAck, pay)); err != nil {
 		t.Fatal(err)
@@ -220,7 +220,7 @@ func TestCoalescerCoalescesAdjacentACKs(t *testing.T) {
 
 func TestCoalescerRejectsSeqGap(t *testing.T) {
 	w := &fakeTunWriter{gsoEnabled: true}
-	c := newTCPCoalescer(w)
+	c := NewTCPCoalescer(w)
 	pay := make([]byte, 1200)
 	if err := c.Add(buildTCPv4(1000, tcpAck, pay)); err != nil {
 		t.Fatal(err)
@@ -239,7 +239,7 @@ func TestCoalescerRejectsSeqGap(t *testing.T) {
 
 func TestCoalescerRejectsFlagMismatch(t *testing.T) {
 	w := &fakeTunWriter{gsoEnabled: true}
-	c := newTCPCoalescer(w)
+	c := NewTCPCoalescer(w)
 	pay := make([]byte, 1200)
 	if err := c.Add(buildTCPv4(1000, tcpAck, pay)); err != nil {
 		t.Fatal(err)
@@ -260,7 +260,7 @@ func TestCoalescerRejectsFlagMismatch(t *testing.T) {
 
 func TestCoalescerRejectsFIN(t *testing.T) {
 	w := &fakeTunWriter{gsoEnabled: true}
-	c := newTCPCoalescer(w)
+	c := NewTCPCoalescer(w)
 	fin := buildTCPv4(1000, tcpAck|tcpFin, []byte("x"))
 	if err := c.Add(fin); err != nil {
 		t.Fatal(err)
@@ -276,7 +276,7 @@ func TestCoalescerRejectsFIN(t *testing.T) {
 
 func TestCoalescerShortLastSegmentClosesChain(t *testing.T) {
 	w := &fakeTunWriter{gsoEnabled: true}
-	c := newTCPCoalescer(w)
+	c := NewTCPCoalescer(w)
 	full := make([]byte, 1200)
 	half := make([]byte, 500)
 	if err := c.Add(buildTCPv4(1000, tcpAck, full)); err != nil {
@@ -311,7 +311,7 @@ func TestCoalescerShortLastSegmentClosesChain(t *testing.T) {
 
 func TestCoalescerPSHFinalizesChain(t *testing.T) {
 	w := &fakeTunWriter{gsoEnabled: true}
-	c := newTCPCoalescer(w)
+	c := NewTCPCoalescer(w)
 	pay := make([]byte, 1200)
 	if err := c.Add(buildTCPv4(1000, tcpAck, pay)); err != nil {
 		t.Fatal(err)
@@ -336,7 +336,7 @@ func TestCoalescerPSHFinalizesChain(t *testing.T) {
 
 func TestCoalescerRejectsDifferentFlow(t *testing.T) {
 	w := &fakeTunWriter{gsoEnabled: true}
-	c := newTCPCoalescer(w)
+	c := NewTCPCoalescer(w)
 	pay := make([]byte, 1200)
 	p1 := buildTCPv4(1000, tcpAck, pay)
 	p2 := buildTCPv4(2200, tcpAck, pay)
@@ -358,7 +358,7 @@ func TestCoalescerRejectsDifferentFlow(t *testing.T) {
 
 func TestCoalescerRejectsIPOptions(t *testing.T) {
 	w := &fakeTunWriter{gsoEnabled: true}
-	c := newTCPCoalescer(w)
+	c := NewTCPCoalescer(w)
 	pay := make([]byte, 500)
 	pkt := buildTCPv4(1000, tcpAck, pay)
 	// Bump IHL to 6 to simulate 4 bytes of IP options. Don't actually add
@@ -378,7 +378,7 @@ func TestCoalescerRejectsIPOptions(t *testing.T) {
 
 func TestCoalescerCapBySegments(t *testing.T) {
 	w := &fakeTunWriter{gsoEnabled: true}
-	c := newTCPCoalescer(w)
+	c := NewTCPCoalescer(w)
 	pay := make([]byte, 512)
 	seq := uint32(1000)
 	for i := 0; i < tcpCoalesceMaxSegs+5; i++ {
@@ -402,7 +402,7 @@ func TestCoalescerCapBySegments(t *testing.T) {
 // flows coalesce independently in a single Flush.
 func TestCoalescerMultipleFlowsInSameBatch(t *testing.T) {
 	w := &fakeTunWriter{gsoEnabled: true}
-	c := newTCPCoalescer(w)
+	c := NewTCPCoalescer(w)
 	pay := make([]byte, 1200)
 
 	// Flow A: sport 1000. Flow B: sport 3000.
@@ -459,7 +459,7 @@ func TestCoalescerMultipleFlowsInSameBatch(t *testing.T) {
 // writing passthrough packets synchronously.
 func TestCoalescerPreservesArrivalOrder(t *testing.T) {
 	w := &orderedFakeWriter{gsoEnabled: true}
-	c := newTCPCoalescer(w)
+	c := NewTCPCoalescer(w)
 	// Sequence: coalesceable TCP, ICMP (passthrough), coalesceable TCP on
 	// a different flow. Expected emit order: gso(X), plain(ICMP), gso(Y).
 	pay := make([]byte, 1200)
@@ -525,7 +525,7 @@ func stringSliceEq(a, b []string) bool {
 // packet (SYN) mid-flow only flushes its own flow, not others.
 func TestCoalescerInterleavedFlowsPreserveOrdering(t *testing.T) {
 	w := &fakeTunWriter{gsoEnabled: true}
-	c := newTCPCoalescer(w)
+	c := NewTCPCoalescer(w)
 	pay := make([]byte, 1200)
 
 	// Flow A two segments.

overlay/device.go

@@ -4,6 +4,7 @@ import (
 	"io"
 	"net/netip"
 
+	"github.com/slackhq/nebula/overlay/tio"
 	"github.com/slackhq/nebula/routing"
 )
 
@@ -11,59 +12,13 @@ import (
 // that don't do TSO segmentation. 65535 covers any single IP packet.
 const defaultBatchBufSize = 65535
 
-// Queue is a readable/writable tun queue. One Queue is driven by a single
-// read goroutine plus concurrent writers (see Write / WriteReject below).
-type Queue interface {
-	io.Closer
-
-	// Read returns one or more packets. The returned slices are borrowed
-	// from the Queue's internal buffer and are only valid until the next
-	// Read or Close on this Queue — callers must encrypt or copy each
-	// slice before the next call. Not safe for concurrent Reads; exactly
-	// one goroutine per Queue reads.
-	Read() ([][]byte, error)
-
-	// Write emits a single packet on the plaintext (outside→inside)
-	// delivery path. May run concurrently with WriteReject on the same
-	// Queue, but not with itself.
-	Write(p []byte) (int, error)
-
-	// WriteReject writes a single packet that originated from the inside
-	// path (reject replies or self-forward) using scratch state distinct
-	// from Write, so it can run concurrently with Write on the same Queue
-	// without a data race. On backends without a shared-scratch Write, a
-	// trivial delegation to Write is acceptable.
-	WriteReject(p []byte) (int, error)
-}
-
 type Device interface {
-	Queue
+	io.Closer
 	Activate() error
 	Networks() []netip.Prefix
 	Name() string
 	RoutesFor(netip.Addr) routing.Gateways
-	SupportsMultiqueue() bool
-	NewMultiQueueReader() (Queue, error)
-}
-
-// GSOWriter is implemented by Queues that can emit a TCP TSO superpacket
-// assembled from a header prefix plus one or more borrowed payload
-// fragments, in a single vectored write (writev with a leading
-// virtio_net_hdr). This lets the coalescer avoid copying payload bytes
-// between the caller's decrypt buffer and the TUN. Backends without GSO
-// support return false from GSOSupported and coalescing is skipped.
-//
-// hdr contains the IPv4/IPv6 + TCP header prefix (mutable — callers will
-// have filled in total length and pseudo-header partial). pays are
-// non-overlapping payload fragments whose concatenation is the full
-// superpacket payload; they are read-only from the writer's perspective
-// and must remain valid until the call returns. gsoSize is the MSS:
-// every segment except possibly the last is exactly that many bytes.
-// csumStart is the byte offset where the TCP header begins within hdr.
-//
-// hdr's TCP checksum field must already hold the pseudo-header partial
-// sum (single-fold, not inverted), per virtio NEEDS_CSUM semantics.
-type GSOWriter interface {
-	WriteGSO(hdr []byte, pays [][]byte, gsoSize uint16, isV6 bool, csumStart uint16) error
-	GSOSupported() bool
+	SupportsMultiqueue() bool //todo remove?
+	NewMultiQueueReader() error
+	Readers() []tio.Queue
 }

overlay/noop.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"net/netip"
 
+	"github.com/slackhq/nebula/overlay/tio"
 	"github.com/slackhq/nebula/routing"
 )
 
@@ -41,7 +42,7 @@ func (NoopTun) SupportsMultiqueue() bool {
 	return false
 }
 
-func (NoopTun) NewMultiQueueReader() (Queue, error) {
+func (NoopTun) NewMultiQueueReader() (tio.Queue, error) {
 	return nil, errors.New("unsupported")
 }
 

overlay/tio/container_gso_linux.go

@@ -0,0 +1,70 @@
+package tio
+
+import (
+	"encoding/binary"
+	"errors"
+	"fmt"
+
+	"golang.org/x/sys/unix"
+)
+
+type gsoContainer struct {
+	pq []*tunFile
+	// pqi is exactly the same as pq, but stored as the interface type
+	pqi        []Queue
+	shutdownFd int
+}
+
+func NewGSOContainer() (Container, error) {
+	shutdownFd, err := unix.Eventfd(0, unix.EFD_NONBLOCK|unix.EFD_CLOEXEC)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create eventfd: %w", err)
+	}
+
+	out := &gsoContainer{
+		pq:         []*tunFile{},
+		pqi:        []Queue{},
+		shutdownFd: shutdownFd,
+	}
+
+	return out, nil
+}
+
+func (c *gsoContainer) Queues() []Queue {
+	return c.pqi
+}
+
+func (c *gsoContainer) Add(fd int) error {
+	x, err := newTunFd(fd, c.shutdownFd)
+	if err != nil {
+		return err
+	}
+	c.pq = append(c.pq, x)
+	c.pqi = append(c.pqi, x)
+
+	return nil
+}
+
+func (c *gsoContainer) wakeForShutdown() error {
+	var buf [8]byte
+	binary.NativeEndian.PutUint64(buf[:], 1)
+	_, err := unix.Write(int(c.shutdownFd), buf[:])
+	return err
+}
+
+func (c *gsoContainer) Close() error {
+	errs := []error{}
+
+	// Signal all readers blocked in poll to wake up and exit
+	if err := c.wakeForShutdown(); err != nil {
+		errs = append(errs, err)
+	}
+
+	for _, x := range c.pq {
+		if err := x.Close(); err != nil {
+			errs = append(errs, err)
+		}
+	}
+
+	return errors.Join(errs...)
+}

overlay/tio/container_poll_linux.go

@@ -0,0 +1,69 @@
+package tio
+
+import (
+	"encoding/binary"
+	"errors"
+	"fmt"
+
+	"golang.org/x/sys/unix"
+)
+
+type pollContainer struct {
+	pq []*Poll
+	// pqi is exactly the same as pq, but stored as the interface type
+	pqi        []Queue
+	shutdownFd int
+}
+
+func NewPollContainer() (Container, error) {
+	shutdownFd, err := unix.Eventfd(0, unix.EFD_NONBLOCK|unix.EFD_CLOEXEC)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create eventfd: %w", err)
+	}
+
+	out := &pollContainer{
+		pq:         []*Poll{},
+		pqi:        []Queue{},
+		shutdownFd: shutdownFd,
+	}
+
+	return out, nil
+}
+
+func (c *pollContainer) Queues() []Queue {
+	return c.pqi
+}
+
+func (c *pollContainer) Add(fd int) error {
+	x, err := newPoll(fd, c.shutdownFd)
+	if err != nil {
+		return err
+	}
+	c.pq = append(c.pq, x)
+	c.pqi = append(c.pqi, x)
+
+	return nil
+}
+
+func (c *pollContainer) wakeForShutdown() error {
+	var buf [8]byte
+	binary.NativeEndian.PutUint64(buf[:], 1)
+	_, err := unix.Write(int(c.shutdownFd), buf[:])
+	return err
+}
+
+func (c *pollContainer) Close() error {
+	errs := []error{}
+
+	if err := c.wakeForShutdown(); err != nil {
+		errs = append(errs, err)
+	}
+
+	for _, x := range c.pq {
+		if err := x.Close(); err != nil {
+			errs = append(errs, err)
+		}
+	}
+
+	return errors.Join(errs...)
+}

overlay/tio/tio.go

@@ -0,0 +1,63 @@
+package tio
+
+import "io"
+
+// defaultBatchBufSize is the per-Queue scratch size for Read on backends
+// that don't do TSO segmentation. 65535 covers any single IP packet.
+const defaultBatchBufSize = 65535
+
+type Container interface {
+	Queues() []Queue
+	Add(fd int) error
+
+	io.Closer
+}
+
+// Queue is a readable/writable Poll queue. One Queue is driven by a single
+// read goroutine plus concurrent writers (see Write / WriteReject below).
+type Queue interface {
+	io.Closer
+
+	// Read returns one or more packets. The returned slices are borrowed
+	// from the Queue's internal buffer and are only valid until the next
+	// Read or Close on this Queue — callers must encrypt or copy each
+	// slice before the next call. Not safe for concurrent Reads; exactly
+	// one goroutine per Queue reads.
+	Read() ([][]byte, error)
+
+	// Write emits a single packet on the plaintext (outside→inside)
+	// delivery path. May run concurrently with WriteReject on the same
+	// Queue, but not with itself.
+	Write(p []byte) (int, error)
+
+	// WriteReject writes a single packet that originated from the inside
+	// path (reject replies or self-forward) using scratch state distinct
+	// from Write, so it can run concurrently with Write on the same Queue
+	// without a data race. On backends without a shared-scratch Write, a
+	// trivial delegation to Write is acceptable.
+	WriteReject(p []byte) (int, error)
+}
+
+// GSOWriter is implemented by Queues that can emit a TCP TSO superpacket
+// assembled from a header prefix plus one or more borrowed payload
+// fragments, in a single vectored write (writev with a leading
+// virtio_net_hdr). This lets the coalescer avoid copying payload bytes
+// between the caller's decrypt buffer and the TUN. Backends without GSO
+// support return false from GSOSupported and coalescing is skipped.
+//
+// hdr contains the IPv4/IPv6 + TCP header prefix (mutable — callers will
+// have filled in total length and pseudo-header partial). pays are
+// non-overlapping payload fragments whose concatenation is the full
+// superpacket payload; they are read-only from the writer's perspective
+// and must remain valid until the call returns. gsoSize is the MSS:
+// every segment except possibly the last is exactly that many bytes.
+// csumStart is the byte offset where the TCP header begins within hdr.
+//
+// # TODO fold into Queue
+//
+// hdr's TCP checksum field must already hold the pseudo-header partial
+// sum (single-fold, not inverted), per virtio NEEDS_CSUM semantics.
+type GSOWriter interface {
+	WriteGSO(hdr []byte, pays [][]byte, gsoSize uint16, isV6 bool, csumStart uint16) error
+	GSOSupported() bool
+}

overlay/tio/tio_gso_linux.go

@@ -0,0 +1,405 @@
+package tio
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"runtime"
+	"sync/atomic"
+	"syscall"
+	"unsafe"
+
+	"golang.org/x/sys/unix"
+)
+
+// Space for segmented output. Worst case is many small segments, each paying
+// an IP+TCP header. 128KiB comfortably covers the 64KiB payload ceiling.
+const tunSegBufSize = 131072
+
+// tunSegBufCap is the total size we allocate for the per-reader segment
+// buffer. It is sized as one worst-case TSO superpacket (tunSegBufSize) plus
+// the same again as drain headroom so a Read wake can accumulate
+// additional packets after an initial big read without overflowing.
+const tunSegBufCap = tunSegBufSize * 2
+
+// tunDrainCap caps how many packets a single Read will accumulate via
+// the post-wake drain loop. Sized to soak up a burst of small ACKs while
+// bounding how much work a single caller holds before handing off.
+const tunDrainCap = 64
+
+// gsoInitialPayIovs is the starting capacity (in payload fragments) of
+// tunFile.gsoIovs. Sized to cover the default coalesce segment cap without
+// any reallocations.
+const gsoInitialPayIovs = 66
+
+// validVnetHdr is the 10-byte virtio_net_hdr we prepend to every non-GSO TUN
+// write. Only flag set is VIRTIO_NET_HDR_F_DATA_VALID, which marks the skb
+// CHECKSUM_UNNECESSARY so the receiving network stack skips L4 checksum
+// verification. All packets that reach the plain Write / WriteReject paths
+// already carry a valid L4 checksum (either supplied by a remote peer whose
+// ciphertext we AEAD-authenticated, or produced by finishChecksum during TSO
+// segmentation, or built locally by CreateRejectPacket), so trusting them is
+// safe.
+var validVnetHdr = [virtioNetHdrLen]byte{unix.VIRTIO_NET_HDR_F_DATA_VALID}
+
+// tunFile wraps a TUN file descriptor with poll-based reads. The FD provided will be changed to non-blocking.
+// A shared eventfd allows Close to wake all readers blocked in poll.
+type tunFile struct { //todo rename GSO
+	fd         int
+	shutdownFd int
+	readPoll   [2]unix.PollFd
+	writePoll  [2]unix.PollFd
+	closed     atomic.Bool
+	readBuf    []byte        // scratch for a single raw read (virtio hdr + superpacket)
+	segBuf     []byte        // backing store for segmented output
+	segOff     int           // cursor into segBuf for the current Read drain
+	pending    [][]byte      // segments returned from the most recent Read
+	writeIovs  [2]unix.Iovec // preallocated iovecs for Write (coalescer passthrough); iovs[0] is fixed to validVnetHdr
+	// rejectIovs is a second preallocated iovec scratch used exclusively by
+	// WriteReject (reject + self-forward from the inside path). It mirrors
+	// writeIovs but lets listenIn goroutines emit reject packets without
+	// racing with the listenOut coalescer that owns writeIovs.
+	rejectIovs [2]unix.Iovec
+
+	// gsoHdrBuf is a per-queue 10-byte scratch for the virtio_net_hdr emitted
+	// by WriteGSO. Separate from validVnetHdr so a concurrent non-GSO Write on
+	// another queue never observes a half-written header.
+	gsoHdrBuf [virtioNetHdrLen]byte
+	// gsoIovs is the writev iovec scratch for WriteGSO. Sized to hold the
+	// virtio header + IP/TCP header + up to gsoInitialPayIovs payload
+	// fragments; grown on demand if a coalescer pushes more.
+	gsoIovs []unix.Iovec
+}
+
+func newTunFd(fd int, shutdownFd int) (*tunFile, error) {
+	if err := unix.SetNonblock(fd, true); err != nil {
+		return nil, fmt.Errorf("failed to set tun fd non-blocking: %w", err)
+	}
+
+	out := &tunFile{
+		fd:         fd,
+		shutdownFd: shutdownFd,
+		closed:     atomic.Bool{},
+		readBuf:    make([]byte, tunReadBufSize),
+		readPoll: [2]unix.PollFd{
+			{Fd: int32(fd), Events: unix.POLLIN},
+			{Fd: int32(shutdownFd), Events: unix.POLLIN},
+		},
+		writePoll: [2]unix.PollFd{
+			{Fd: int32(fd), Events: unix.POLLOUT},
+			{Fd: int32(shutdownFd), Events: unix.POLLIN},
+		},
+
+		segBuf:  make([]byte, tunSegBufSize),
+		gsoIovs: make([]unix.Iovec, 2, 2+gsoInitialPayIovs),
+	}
+
+	out.writeIovs[0].Base = &validVnetHdr[0]
+	out.writeIovs[0].SetLen(virtioNetHdrLen)
+	out.rejectIovs[0].Base = &validVnetHdr[0]
+	out.rejectIovs[0].SetLen(virtioNetHdrLen)
+	out.gsoIovs[0].Base = &out.gsoHdrBuf[0]
+	out.gsoIovs[0].SetLen(virtioNetHdrLen)
+
+	return out, nil
+}
+
+func (r *tunFile) blockOnRead() error {
+	const problemFlags = unix.POLLHUP | unix.POLLNVAL | unix.POLLERR
+	var err error
+	for {
+		_, err = unix.Poll(r.readPoll[:], -1)
+		if err != unix.EINTR {
+			break
+		}
+	}
+	//always reset these!
+	tunEvents := r.readPoll[0].Revents
+	shutdownEvents := r.readPoll[1].Revents
+	r.readPoll[0].Revents = 0
+	r.readPoll[1].Revents = 0
+	//do the err check before trusting the potentially bogus bits we just got
+	if err != nil {
+		return err
+	}
+	if shutdownEvents&(unix.POLLIN|problemFlags) != 0 {
+		return os.ErrClosed
+	} else if tunEvents&problemFlags != 0 {
+		return os.ErrClosed
+	}
+	return nil
+}
+
+func (r *tunFile) blockOnWrite() error {
+	const problemFlags = unix.POLLHUP | unix.POLLNVAL | unix.POLLERR
+	var err error
+	for {
+		_, err = unix.Poll(r.writePoll[:], -1)
+		if err != unix.EINTR {
+			break
+		}
+	}
+	//always reset these!
+	tunEvents := r.writePoll[0].Revents
+	shutdownEvents := r.writePoll[1].Revents
+	r.writePoll[0].Revents = 0
+	r.writePoll[1].Revents = 0
+	//do the err check before trusting the potentially bogus bits we just got
+	if err != nil {
+		return err
+	}
+	if shutdownEvents&(unix.POLLIN|problemFlags) != 0 {
+		return os.ErrClosed
+	} else if tunEvents&problemFlags != 0 {
+		return os.ErrClosed
+	}
+	return nil
+}
+
+func (r *tunFile) readRaw(buf []byte) (int, error) {
+	for {
+		if n, err := unix.Read(r.fd, buf); err == nil {
+			return n, nil
+		} else if err == unix.EAGAIN {
+			if err = r.blockOnRead(); err != nil {
+				return 0, err
+			}
+			continue
+		} else if err == unix.EINTR {
+			continue
+		} else if err == unix.EBADF {
+			return 0, os.ErrClosed
+		} else {
+			return 0, err
+		}
+	}
+}
+
+// Read reads one or more superpackets from the tun and returns the
+// resulting packets. The first read blocks via poll; once the fd is known
+// readable we drain additional packets non-blocking until the kernel queue
+// is empty (EAGAIN), we've collected tunDrainCap packets, or we're out of
+// segBuf headroom. This amortizes the poll wake over bursts of small
+// packets (e.g. TCP ACKs). Slices point into the tunFile's internal buffers
+// and are only valid until the next Read or Close on this Queue.
+func (r *tunFile) Read() ([][]byte, error) {
+	r.pending = r.pending[:0]
+	r.segOff = 0
+
+	// Initial (blocking) read. Retry on decode errors so a single bad
+	// packet does not stall the reader.
+	for {
+		n, err := r.readRaw(r.readBuf)
+		if err != nil {
+			return nil, err
+		}
+		if err := r.decodeRead(n); err != nil {
+			// Drop and read again — a bad packet should not kill the reader.
+			continue
+		}
+		break
+	}
+
+	// Drain: non-blocking reads until the kernel queue is empty, the drain
+	// cap is reached, or segBuf no longer has room for another worst-case
+	// superpacket.
+	for len(r.pending) < tunDrainCap && tunSegBufCap-r.segOff >= tunSegBufSize {
+		n, err := unix.Read(r.fd, r.readBuf)
+		if err != nil {
+			// EAGAIN / EINTR / anything else: stop draining. We already
+			// have a valid batch from the first read.
+			break
+		}
+		if n <= 0 {
+			break
+		}
+		if err := r.decodeRead(n); err != nil {
+			// Drop this packet and stop the drain; we'd rather hand off
+			// what we have than keep spinning here.
+			break
+		}
+	}
+
+	return r.pending, nil
+}
+
+// decodeRead decodes the virtio header plus payload in r.readBuf[:n], appends
+// the segments to r.pending, and advances r.segOff by the total scratch used.
+// Caller must have already ensured r.vnetHdr is true.
+func (r *tunFile) decodeRead(n int) error {
+	if n < virtioNetHdrLen {
+		return fmt.Errorf("short tun read: %d < %d", n, virtioNetHdrLen)
+	}
+	var hdr VirtioNetHdr
+	hdr.decode(r.readBuf[:virtioNetHdrLen])
+	before := len(r.pending)
+	if err := segmentInto(r.readBuf[virtioNetHdrLen:n], hdr, &r.pending, r.segBuf[r.segOff:]); err != nil {
+		return err
+	}
+	for k := before; k < len(r.pending); k++ {
+		r.segOff += len(r.pending[k])
+	}
+	return nil
+}
+
+func (r *tunFile) Write(buf []byte) (int, error) {
+	return r.writeWithScratch(buf, &r.writeIovs)
+}
+
+// WriteReject emits a packet using a dedicated iovec scratch (rejectIovs)
+// distinct from the one used by the coalescer's Write path. This avoids a
+// data race between the inside (listenIn) goroutine emitting reject or
+// self-forward packets and the outside (listenOut) goroutine flushing TCP
+// coalescer passthroughs on the same tunFile.
+func (r *tunFile) WriteReject(buf []byte) (int, error) {
+	return r.writeWithScratch(buf, &r.rejectIovs)
+}
+
+func (r *tunFile) writeWithScratch(buf []byte, iovs *[2]unix.Iovec) (int, error) {
+	if len(buf) == 0 {
+		return 0, nil
+	}
+	// Point the payload iovec at the caller's buffer. iovs[0] is pre-wired
+	// to validVnetHdr during tunFile construction so we don't rebuild it here.
+	iovs[1].Base = &buf[0]
+	iovs[1].SetLen(len(buf))
+	iovPtr := uintptr(unsafe.Pointer(&iovs[0]))
+	// The TUN fd is non-blocking (set in newTunFd / newFriend), so writev
+	// either completes promptly or returns EAGAIN — it cannot park the
+	// goroutine inside the kernel. That lets us use syscall.RawSyscall and
+	// skip the runtime.entersyscall / exitsyscall bookkeeping on every
+	// packet; we only pay that cost when we fall through to blockOnWrite.
+	for {
+		n, _, errno := syscall.RawSyscall(unix.SYS_WRITEV, uintptr(r.fd), iovPtr, 2)
+		if errno == 0 {
+			runtime.KeepAlive(buf)
+			if int(n) < virtioNetHdrLen {
+				return 0, io.ErrShortWrite
+			}
+			return int(n) - virtioNetHdrLen, nil
+		}
+		if errno == unix.EAGAIN {
+			runtime.KeepAlive(buf)
+			if err := r.blockOnWrite(); err != nil {
+				return 0, err
+			}
+			continue
+		}
+		if errno == unix.EINTR {
+			continue
+		}
+		if errno == unix.EBADF {
+			return 0, os.ErrClosed
+		}
+		runtime.KeepAlive(buf)
+		return 0, errno
+	}
+}
+
+// GSOSupported reports whether this queue was opened with IFF_VNET_HDR and
+// can accept WriteGSO. When false, callers should fall back to per-segment
+// Write calls.
+func (r *tunFile) GSOSupported() bool { return true }
+
+// WriteGSO emits a TCP TSO superpacket in a single writev. hdr is the
+// IPv4/IPv6 + TCP header prefix (already finalized — total length, IP csum,
+// and TCP pseudo-header partial set by the caller). pays are payload
+// fragments whose concatenation forms the full coalesced payload; each
+// slice is read-only and must stay valid until return. gsoSize is the MSS;
+// every segment except possibly the last is exactly gsoSize bytes.
+// csumStart is the byte offset where the TCP header begins within hdr.
+func (r *tunFile) WriteGSO(hdr []byte, pays [][]byte, gsoSize uint16, isV6 bool, csumStart uint16) error {
+	if len(hdr) == 0 || len(pays) == 0 {
+		return nil
+	}
+
+	// Build the virtio_net_hdr. When pays total to <= gsoSize the kernel
+	// would produce a single segment; keep NEEDS_CSUM semantics but skip
+	// the GSO type so the kernel doesn't spuriously mark this as TSO.
+	vhdr := VirtioNetHdr{
+		Flags:      unix.VIRTIO_NET_HDR_F_NEEDS_CSUM,
+		HdrLen:     uint16(len(hdr)),
+		GSOSize:    gsoSize,
+		CsumStart:  csumStart,
+		CsumOffset: 16, // TCP checksum field lives 16 bytes into the TCP header
+	}
+	var totalPay int
+	for _, p := range pays {
+		totalPay += len(p)
+	}
+	if totalPay > int(gsoSize) {
+		if isV6 {
+			vhdr.GSOType = unix.VIRTIO_NET_HDR_GSO_TCPV6
+		} else {
+			vhdr.GSOType = unix.VIRTIO_NET_HDR_GSO_TCPV4
+		}
+	} else {
+		vhdr.GSOType = unix.VIRTIO_NET_HDR_GSO_NONE
+		vhdr.GSOSize = 0
+	}
+	vhdr.encode(r.gsoHdrBuf[:])
+
+	// Build the iovec array: [virtio_hdr, hdr, pays...]. r.gsoIovs[0] is
+	// wired to gsoHdrBuf at construction and never changes.
+	need := 2 + len(pays)
+	if cap(r.gsoIovs) < need {
+		grown := make([]unix.Iovec, need)
+		grown[0] = r.gsoIovs[0]
+		r.gsoIovs = grown
+	} else {
+		r.gsoIovs = r.gsoIovs[:need]
+	}
+	r.gsoIovs[1].Base = &hdr[0]
+	r.gsoIovs[1].SetLen(len(hdr))
+	for i, p := range pays {
+		r.gsoIovs[2+i].Base = &p[0]
+		r.gsoIovs[2+i].SetLen(len(p))
+	}
+
+	iovPtr := uintptr(unsafe.Pointer(&r.gsoIovs[0]))
+	iovCnt := uintptr(len(r.gsoIovs))
+	for {
+		n, _, errno := syscall.RawSyscall(unix.SYS_WRITEV, uintptr(r.fd), iovPtr, iovCnt)
+		if errno == 0 {
+			runtime.KeepAlive(hdr)
+			runtime.KeepAlive(pays)
+			if int(n) < virtioNetHdrLen {
+				return io.ErrShortWrite
+			}
+			return nil
+		}
+		if errno == unix.EAGAIN {
+			runtime.KeepAlive(hdr)
+			runtime.KeepAlive(pays)
+			if err := r.blockOnWrite(); err != nil {
+				return err
+			}
+			continue
+		}
+		if errno == unix.EINTR {
+			continue
+		}
+		if errno == unix.EBADF {
+			return os.ErrClosed
+		}
+		runtime.KeepAlive(hdr)
+		runtime.KeepAlive(pays)
+		return errno
+	}
+}
+
+func (r *tunFile) Close() error {
+	if r.closed.Swap(true) {
+		return nil
+	}
+
+	//shutdownFd is owned by the container, so we should not close it
+
+	var err error
+	if r.fd >= 0 {
+		err = unix.Close(r.fd)
+		r.fd = -1
+	}
+
+	return err
+}

overlay/tio/tio_poll_linux.go

@@ -0,0 +1,205 @@
+package tio
+
+import (
+	"fmt"
+	"os"
+	"sync/atomic"
+	"syscall"
+	"unsafe"
+
+	"golang.org/x/sys/unix"
+)
+
+// Maximum size we accept for a single read from a TUN with IFF_VNET_HDR. A
+// TSO superpacket can be up to 64KiB of payload plus a single L2/L3/L4 header
+// prefix plus the virtio header.
+const tunReadBufSize = 65535
+
+type Poll struct {
+	fd int
+
+	readPoll  [2]unix.PollFd
+	writePoll [2]unix.PollFd
+	closed    atomic.Bool
+
+	readBuf  []byte
+	batchRet [1][]byte
+}
+
+func newPoll(fd int, shutdownFd int) (*Poll, error) {
+	if err := unix.SetNonblock(fd, true); err != nil {
+		_ = unix.Close(fd)
+		return nil, fmt.Errorf("failed to set Poll device as nonblocking: %w", err)
+	}
+
+	out := &Poll{
+		fd:      fd,
+		readBuf: make([]byte, tunReadBufSize),
+		readPoll: [2]unix.PollFd{
+			{Fd: int32(fd), Events: unix.POLLIN},
+			{Fd: int32(shutdownFd), Events: unix.POLLIN},
+		},
+		writePoll: [2]unix.PollFd{
+			{Fd: int32(fd), Events: unix.POLLOUT},
+			{Fd: int32(shutdownFd), Events: unix.POLLIN},
+		},
+	}
+	return out, nil
+}
+
+// blockOnRead waits until the Poll fd is readable or shutdown has been signaled.
+// Returns os.ErrClosed if Close was called.
+func (t *Poll) blockOnRead() error {
+	const problemFlags = unix.POLLHUP | unix.POLLNVAL | unix.POLLERR
+	var err error
+	for {
+		_, err = unix.Poll(t.readPoll[:], -1)
+		if err != unix.EINTR {
+			break
+		}
+	}
+	tunEvents := t.readPoll[0].Revents
+	shutdownEvents := t.readPoll[1].Revents
+	t.readPoll[0].Revents = 0
+	t.readPoll[1].Revents = 0
+	if err != nil {
+		return err
+	}
+	if shutdownEvents&(unix.POLLIN|problemFlags) != 0 {
+		return os.ErrClosed
+	}
+	if tunEvents&problemFlags != 0 {
+		return os.ErrClosed
+	}
+	return nil
+}
+
+func (t *Poll) blockOnWrite() error {
+	const problemFlags = unix.POLLHUP | unix.POLLNVAL | unix.POLLERR
+	var err error
+	for {
+		_, err = unix.Poll(t.writePoll[:], -1)
+		if err != unix.EINTR {
+			break
+		}
+	}
+	tunEvents := t.writePoll[0].Revents
+	shutdownEvents := t.writePoll[1].Revents
+	t.writePoll[0].Revents = 0
+	t.writePoll[1].Revents = 0
+	if err != nil {
+		return err
+	}
+	if shutdownEvents&(unix.POLLIN|problemFlags) != 0 {
+		return os.ErrClosed
+	}
+	if tunEvents&problemFlags != 0 {
+		return os.ErrClosed
+	}
+	return nil
+}
+
+func (t *Poll) Read() ([][]byte, error) {
+	if t.readBuf == nil {
+		t.readBuf = make([]byte, defaultBatchBufSize)
+	}
+	n, err := t.readOne(t.readBuf)
+	if err != nil {
+		return nil, err
+	}
+	t.batchRet[0] = t.readBuf[:n]
+	return t.batchRet[:], nil
+}
+
+func (t *Poll) readOne(to []byte) (int, error) {
+	// first 4 bytes is protocol family, in network byte order
+	var head [4]byte
+	iovecs := [2]syscall.Iovec{ //todo plat-specific
+		{&head[0], 4},
+		{&to[0], uint64(len(to))},
+	}
+	for {
+		n, _, errno := syscall.Syscall(syscall.SYS_READV, uintptr(t.fd), uintptr(unsafe.Pointer(&iovecs[0])), 2)
+		if errno == 0 {
+			bytesRead := int(n)
+			if bytesRead < 4 {
+				return 0, nil
+			}
+			return bytesRead - 4, nil
+		}
+		switch errno {
+		case unix.EAGAIN:
+			if err := t.blockOnRead(); err != nil {
+				return 0, err
+			}
+		case unix.EINTR:
+			// retry
+		case unix.EBADF:
+			return 0, os.ErrClosed
+		default:
+			return 0, errno
+		}
+	}
+}
+
+// Write is only valid for single threaded use
+func (t *Poll) Write(from []byte) (int, error) {
+	if len(from) <= 1 {
+		return 0, syscall.EIO
+	}
+
+	ipVer := from[0] >> 4
+	var head [4]byte
+	// first 4 bytes is protocol family, in network byte order
+	switch ipVer {
+	case 4:
+		head[3] = syscall.AF_INET
+	case 6:
+		head[3] = syscall.AF_INET6
+	default:
+		return 0, fmt.Errorf("unable to determine IP version from packet")
+	}
+
+	iovecs := [2]syscall.Iovec{ //todo plat specific
+		{&head[0], 4},
+		{&from[0], uint64(len(from))},
+	}
+	for {
+		n, _, errno := syscall.Syscall(syscall.SYS_WRITEV, uintptr(t.fd), uintptr(unsafe.Pointer(&iovecs[0])), 2)
+		if errno == 0 {
+			return int(n) - 4, nil
+		}
+		switch errno {
+		case unix.EAGAIN:
+			if err := t.blockOnWrite(); err != nil {
+				return 0, err
+			}
+		case unix.EINTR:
+			// retry
+		case unix.EBADF:
+			return 0, os.ErrClosed
+		default:
+			return 0, errno
+		}
+	}
+}
+
+func (t *Poll) Close() error {
+	if t.closed.Swap(true) {
+		return nil
+	}
+
+	//shutdownFd is owned by the container, so we should not close it
+
+	var err error
+	if t.fd >= 0 {
+		err = unix.Close(t.fd)
+		t.fd = -1
+	}
+
+	return err
+}
+
+func (t *Poll) WriteReject(p []byte) (int, error) {
+	return t.Write(p)
+}

overlay/tun_file_linux_test.go → overlay/tio/tun_file_linux_test.go

@@ -1,7 +1,7 @@
 //go:build linux && !android && !e2e_testing
 // +build linux,!android,!e2e_testing
 
-package overlay
+package tio
 
 import (
 	"errors"

overlay/tun_linux_offload.go → overlay/tio/tun_linux_offload.go

@@ -1,7 +1,7 @@
 //go:build linux && !android && !e2e_testing
 // +build linux,!android,!e2e_testing
 
-package overlay
+package tio
 
 import (
 	"encoding/binary"
@@ -10,66 +10,10 @@ import (
 	"golang.org/x/sys/unix"
 )
 
-// Size of the legacy struct virtio_net_hdr that the kernel prepends/expects on
-// a TUN opened with IFF_VNET_HDR (TUNSETVNETHDRSZ not set).
-const virtioNetHdrLen = 10
-
-// Maximum size we accept for a single read from a TUN with IFF_VNET_HDR. A
-// TSO superpacket can be up to 64KiB of payload plus a single L2/L3/L4 header
-// prefix plus the virtio header.
-const tunReadBufSize = 65535
-
-// Space for segmented output. Worst case is many small segments, each paying
-// an IP+TCP header. 128KiB comfortably covers the 64KiB payload ceiling.
-const tunSegBufSize = 131072
-
-// tunSegBufCap is the total size we allocate for the per-reader segment
-// buffer. It is sized as one worst-case TSO superpacket (tunSegBufSize) plus
-// the same again as drain headroom so a Read wake can accumulate
-// additional packets after an initial big read without overflowing.
-const tunSegBufCap = tunSegBufSize * 2
-
-// tunDrainCap caps how many packets a single Read will accumulate via
-// the post-wake drain loop. Sized to soak up a burst of small ACKs while
-// bounding how much work a single caller holds before handing off.
-const tunDrainCap = 64
-
-type virtioNetHdr struct {
-	Flags      uint8
-	GSOType    uint8
-	HdrLen     uint16
-	GSOSize    uint16
-	CsumStart  uint16
-	CsumOffset uint16
-}
-
-// decode reads a virtio_net_hdr in host byte order (TUN default; we never
-// call TUNSETVNETLE so the kernel matches our endianness).
-func (h *virtioNetHdr) decode(b []byte) {
-	h.Flags = b[0]
-	h.GSOType = b[1]
-	h.HdrLen = binary.NativeEndian.Uint16(b[2:4])
-	h.GSOSize = binary.NativeEndian.Uint16(b[4:6])
-	h.CsumStart = binary.NativeEndian.Uint16(b[6:8])
-	h.CsumOffset = binary.NativeEndian.Uint16(b[8:10])
-}
-
-// encode is the inverse of decode: writes the virtio_net_hdr fields into b
-// (must be at least virtioNetHdrLen bytes). Used to emit a TSO superpacket
-// on egress.
-func (h *virtioNetHdr) encode(b []byte) {
-	b[0] = h.Flags
-	b[1] = h.GSOType
-	binary.NativeEndian.PutUint16(b[2:4], h.HdrLen)
-	binary.NativeEndian.PutUint16(b[4:6], h.GSOSize)
-	binary.NativeEndian.PutUint16(b[6:8], h.CsumStart)
-	binary.NativeEndian.PutUint16(b[8:10], h.CsumOffset)
-}
-
 // segmentInto splits a TUN-side packet described by hdr into one or more
 // IP packets, each appended to *out as a slice of scratch. scratch must be
 // sized to hold every segment (including replicated headers).
-func segmentInto(pkt []byte, hdr virtioNetHdr, out *[][]byte, scratch []byte) error {
+func segmentInto(pkt []byte, hdr VirtioNetHdr, out *[][]byte, scratch []byte) error {
 	// When RSC_INFO is set the csum_start/csum_offset fields are repurposed to
 	// carry coalescing info rather than checksum offsets. A TUN writing via
 	// IFF_VNET_HDR should never emit this, but if it did we would silently
@@ -105,7 +49,7 @@ func segmentInto(pkt []byte, hdr virtioNetHdr, out *[][]byte, scratch []byte) er
 // handed us with NEEDS_CSUM set. csum_start / csum_offset point at the 16-bit
 // checksum field; we zero it, fold a full sum (the field was pre-loaded with
 // the pseudo-header partial sum by the kernel), and store the result.
-func finishChecksum(seg []byte, hdr virtioNetHdr) error {
+func finishChecksum(seg []byte, hdr VirtioNetHdr) error {
 	cs := int(hdr.CsumStart)
 	co := int(hdr.CsumOffset)
 	if cs+co+2 > len(seg) {
@@ -129,7 +73,7 @@ func finishChecksum(seg []byte, hdr virtioNetHdr) error {
 // are each summed once up front — every segment reuses those three pre-folded
 // uint32 values and combines them with small per-segment deltas (seq, flags,
 // tcpLen, ip_id, total_len) that are cheap to fold in.
-func segmentTCP(pkt []byte, hdr virtioNetHdr, out *[][]byte, scratch []byte) error {
+func segmentTCP(pkt []byte, hdr VirtioNetHdr, out *[][]byte, scratch []byte) error {
 	if hdr.GSOSize == 0 {
 		return fmt.Errorf("gso_size is zero")
 	}

overlay/tun_linux_offload_test.go → overlay/tio/tun_linux_offload_test.go

@@ -1,7 +1,7 @@
 //go:build linux && !android && !e2e_testing
 // +build linux,!android,!e2e_testing
 
-package overlay
+package tio
 
 import (
 	"encoding/binary"
@@ -23,7 +23,7 @@ func verifyChecksum(b []byte, pseudo uint32) bool {
 
 // buildTSOv4 builds a synthetic IPv4/TCP TSO superpacket with a payload of
 // `payLen` bytes split at `mss`.
-func buildTSOv4(t *testing.T, payLen, mss int) ([]byte, virtioNetHdr) {
+func buildTSOv4(t *testing.T, payLen, mss int) ([]byte, VirtioNetHdr) {
 	t.Helper()
 	const ipLen = 20
 	const tcpLen = 20
@@ -53,7 +53,7 @@ func buildTSOv4(t *testing.T, payLen, mss int) ([]byte, virtioNetHdr) {
 		pkt[ipLen+tcpLen+i] = byte(i & 0xff)
 	}
 
-	return pkt, virtioNetHdr{
+	return pkt, VirtioNetHdr{
 		Flags:      unix.VIRTIO_NET_HDR_F_NEEDS_CSUM,
 		GSOType:    unix.VIRTIO_NET_HDR_GSO_TCPV4,
 		HdrLen:     uint16(ipLen + tcpLen),
@@ -174,7 +174,7 @@ func TestSegmentTCPv6(t *testing.T) {
 		pkt[ipLen+tcpLen+i] = byte(i)
 	}
 
-	hdr := virtioNetHdr{
+	hdr := VirtioNetHdr{
 		Flags:      unix.VIRTIO_NET_HDR_F_NEEDS_CSUM,
 		GSOType:    unix.VIRTIO_NET_HDR_GSO_TCPV6,
 		HdrLen:     uint16(ipLen + tcpLen),
@@ -240,7 +240,7 @@ func TestSegmentGSONonePassesThrough(t *testing.T) {
 }
 
 func TestSegmentRejectsUDP(t *testing.T) {
-	hdr := virtioNetHdr{GSOType: unix.VIRTIO_NET_HDR_GSO_UDP}
+	hdr := VirtioNetHdr{GSOType: unix.VIRTIO_NET_HDR_GSO_UDP}
 	var out [][]byte
 	if err := segmentInto(nil, hdr, &out, nil); err == nil {
 		t.Fatalf("expected rejection for UDP GSO")
@@ -279,7 +279,7 @@ func BenchmarkSegmentTCPv4(b *testing.B) {
 			for i := 0; i < sz.payLen; i++ {
 				pkt[ipLen+tcpLen+i] = byte(i)
 			}
-			hdr := virtioNetHdr{
+			hdr := VirtioNetHdr{
 				Flags:      unix.VIRTIO_NET_HDR_F_NEEDS_CSUM,
 				GSOType:    unix.VIRTIO_NET_HDR_GSO_TCPV4,
 				HdrLen:     uint16(ipLen + tcpLen),
@@ -312,7 +312,7 @@ func TestTunFileWriteVnetHdrNoAlloc(t *testing.T) {
 	}
 	t.Cleanup(func() { _ = unix.Close(fd) })
 
-	tf := &tunFile{fd: fd, vnetHdr: true}
+	tf := &tunFile{fd: fd}
 	tf.writeIovs[0].Base = &validVnetHdr[0]
 	tf.writeIovs[0].SetLen(virtioNetHdrLen)
 

overlay/tio/tun_linux_test.go

@@ -0,0 +1,38 @@
+//go:build !e2e_testing
+// +build !e2e_testing
+
+package tio
+
+import (
+	"testing"
+
+	"github.com/slackhq/nebula/overlay"
+)
+
+var runAdvMSSTests = []struct {
+	name     string
+	tun      *overlay.tun
+	r        overlay.Route
+	expected int
+}{
+	// Standard case, default MTU is the device max MTU
+	{"default", &overlay.tun{DefaultMTU: 1440, MaxMTU: 1440}, overlay.Route{}, 0},
+	{"default-min", &overlay.tun{DefaultMTU: 1440, MaxMTU: 1440}, overlay.Route{MTU: 1440}, 0},
+	{"default-low", &overlay.tun{DefaultMTU: 1440, MaxMTU: 1440}, overlay.Route{MTU: 1200}, 1160},
+
+	// Case where we have a route MTU set higher than the default
+	{"route", &overlay.tun{DefaultMTU: 1440, MaxMTU: 8941}, overlay.Route{}, 1400},
+	{"route-min", &overlay.tun{DefaultMTU: 1440, MaxMTU: 8941}, overlay.Route{MTU: 1440}, 1400},
+	{"route-high", &overlay.tun{DefaultMTU: 1440, MaxMTU: 8941}, overlay.Route{MTU: 8941}, 0},
+}
+
+func TestTunAdvMSS(t *testing.T) {
+	for _, tt := range runAdvMSSTests {
+		t.Run(tt.name, func(t *testing.T) {
+			o := tt.tun.advMSS(tt.r)
+			if o != tt.expected {
+				t.Errorf("got %d, want %d", o, tt.expected)
+			}
+		})
+	}
+}

overlay/tio/vnethdr_linux.go

@@ -0,0 +1,39 @@
+package tio
+
+import "encoding/binary"
+
+// Size of the legacy struct virtio_net_hdr that the kernel prepends/expects on
+// a TUN opened with IFF_VNET_HDR (TUNSETVNETHDRSZ not set).
+const virtioNetHdrLen = 10
+
+type VirtioNetHdr struct {
+	Flags      uint8
+	GSOType    uint8
+	HdrLen     uint16
+	GSOSize    uint16
+	CsumStart  uint16
+	CsumOffset uint16
+}
+
+// decode reads a virtio_net_hdr in host byte order (TUN default; we never
+// call TUNSETVNETLE so the kernel matches our endianness).
+func (h *VirtioNetHdr) decode(b []byte) {
+	h.Flags = b[0]
+	h.GSOType = b[1]
+	h.HdrLen = binary.NativeEndian.Uint16(b[2:4])
+	h.GSOSize = binary.NativeEndian.Uint16(b[4:6])
+	h.CsumStart = binary.NativeEndian.Uint16(b[6:8])
+	h.CsumOffset = binary.NativeEndian.Uint16(b[8:10])
+}
+
+// encode is the inverse of decode: writes the virtio_net_hdr fields into b
+// (must be at least virtioNetHdrLen bytes). Used to emit a TSO superpacket
+// on egress.
+func (h *VirtioNetHdr) encode(b []byte) {
+	b[0] = h.Flags
+	b[1] = h.GSOType
+	binary.NativeEndian.PutUint16(b[2:4], h.HdrLen)
+	binary.NativeEndian.PutUint16(b[4:6], h.GSOSize)
+	binary.NativeEndian.PutUint16(b[6:8], h.CsumStart)
+	binary.NativeEndian.PutUint16(b[8:10], h.CsumOffset)
+}

overlay/tun_android.go

@@ -13,6 +13,7 @@ import (
 	"github.com/gaissmai/bart"
 	"github.com/sirupsen/logrus"
 	"github.com/slackhq/nebula/config"
+	"github.com/slackhq/nebula/overlay/tio"
 	"github.com/slackhq/nebula/routing"
 	"github.com/slackhq/nebula/util"
 )
@@ -126,6 +127,6 @@ func (t *tun) SupportsMultiqueue() bool {
 	return false
 }
 
-func (t *tun) NewMultiQueueReader() (Queue, error) {
+func (t *tun) NewMultiQueueReader() (tio.Queue, error) {
 	return nil, fmt.Errorf("TODO: multiqueue not implemented for android")
 }

overlay/tun_darwin.go

@@ -16,6 +16,7 @@ import (
 	"github.com/gaissmai/bart"
 	"github.com/sirupsen/logrus"
 	"github.com/slackhq/nebula/config"
+	"github.com/slackhq/nebula/overlay/tio"
 	"github.com/slackhq/nebula/routing"
 	"github.com/slackhq/nebula/util"
 	netroute "golang.org/x/net/route"
@@ -572,6 +573,6 @@ func (t *tun) SupportsMultiqueue() bool {
 	return false
 }
 
-func (t *tun) NewMultiQueueReader() (Queue, error) {
+func (t *tun) NewMultiQueueReader() (tio.Queue, error) {
 	return nil, fmt.Errorf("TODO: multiqueue not implemented for darwin")
 }

overlay/tun_disabled.go

@@ -9,6 +9,7 @@ import (
 	"github.com/rcrowley/go-metrics"
 	"github.com/sirupsen/logrus"
 	"github.com/slackhq/nebula/iputil"
+	"github.com/slackhq/nebula/overlay/tio"
 	"github.com/slackhq/nebula/routing"
 )
 
@@ -17,9 +18,10 @@ type disabledTun struct {
 	vpnNetworks []netip.Prefix
 
 	// Track these metrics since we don't have the tun device to do it for us
-	tx metrics.Counter
-	rx metrics.Counter
-	l  *logrus.Logger
+	tx         metrics.Counter
+	rx         metrics.Counter
+	l          *logrus.Logger
+	numReaders int
 
 	batchRet [1][]byte
 }
@@ -44,6 +46,7 @@ func newDisabledTun(vpnNetworks []netip.Prefix, queueLen int, metricsEnabled boo
 		vpnNetworks: vpnNetworks,
 		read:        make(chan []byte, queueLen),
 		l:           l,
+		numReaders:  1,
 	}
 
 	if metricsEnabled {
@@ -112,8 +115,17 @@ func (t *disabledTun) SupportsMultiqueue() bool {
 	return true
 }
 
-func (t *disabledTun) NewMultiQueueReader() (Queue, error) {
-	return t, nil
+func (t *disabledTun) NewMultiQueueReader() error {
+	t.numReaders++
+	return nil
+}
+
+func (t *disabledTun) Readers() []tio.Queue {
+	out := make([]tio.Queue, t.numReaders)
+	for i := range t.numReaders {
+		out[i] = t
+	}
+	return out
 }
 
 func (t *disabledTun) Close() error {

overlay/tun_freebsd.go

@@ -18,6 +18,7 @@ import (
 	"github.com/gaissmai/bart"
 	"github.com/sirupsen/logrus"
 	"github.com/slackhq/nebula/config"
+	"github.com/slackhq/nebula/overlay/tio"
 	"github.com/slackhq/nebula/routing"
 	"github.com/slackhq/nebula/util"
 	netroute "golang.org/x/net/route"
@@ -581,7 +582,7 @@ func (t *tun) SupportsMultiqueue() bool {
 	return false
 }
 
-func (t *tun) NewMultiQueueReader() (Queue, error) {
+func (t *tun) NewMultiQueueReader() (tio.Queue, error) {
 	return nil, fmt.Errorf("TODO: multiqueue not implemented for freebsd")
 }
 

overlay/tun_ios.go

@@ -16,6 +16,7 @@ import (
 	"github.com/gaissmai/bart"
 	"github.com/sirupsen/logrus"
 	"github.com/slackhq/nebula/config"
+	"github.com/slackhq/nebula/overlay/tio"
 	"github.com/slackhq/nebula/routing"
 	"github.com/slackhq/nebula/util"
 )
@@ -182,6 +183,6 @@ func (t *tun) SupportsMultiqueue() bool {
 	return false
 }
 
-func (t *tun) NewMultiQueueReader() (Queue, error) {
+func (t *tun) NewMultiQueueReader() (tio.Queue, error) {
 	return nil, fmt.Errorf("TODO: multiqueue not implemented for ios")
 }

overlay/tun_linux.go

@@ -4,478 +4,28 @@
 package overlay
 
 import (
-	"encoding/binary"
 	"fmt"
-	"io"
 	"net"
 	"net/netip"
 	"os"
-	"runtime"
 	"strings"
 	"sync"
 	"sync/atomic"
-	"syscall"
 	"time"
 	"unsafe"
 
 	"github.com/gaissmai/bart"
 	"github.com/sirupsen/logrus"
 	"github.com/slackhq/nebula/config"
+	"github.com/slackhq/nebula/overlay/tio"
 	"github.com/slackhq/nebula/routing"
 	"github.com/slackhq/nebula/util"
 	"github.com/vishvananda/netlink"
 	"golang.org/x/sys/unix"
 )
 
-// tunFile wraps a TUN file descriptor with poll-based reads. The FD provided will be changed to non-blocking.
-// A shared eventfd allows Close to wake all readers blocked in poll.
-type tunFile struct {
-	fd         int
-	shutdownFd int
-	lastOne    bool
-	readPoll   [2]unix.PollFd
-	writePoll  [2]unix.PollFd
-	closed     bool
-
-	// vnetHdr is true when this fd was opened with IFF_VNET_HDR and the
-	// kernel successfully accepted TUNSETOFFLOAD. Reads include a leading
-	// virtio_net_hdr and may carry a TSO superpacket we must segment;
-	// writes must prepend a zeroed virtio_net_hdr.
-	vnetHdr   bool
-	readBuf   []byte        // scratch for a single raw read (virtio hdr + superpacket)
-	segBuf    []byte        // backing store for segmented output
-	segOff    int           // cursor into segBuf for the current Read drain
-	pending   [][]byte      // segments returned from the most recent Read
-	writeIovs [2]unix.Iovec // preallocated iovecs for Write (coalescer passthrough); iovs[0] is fixed to validVnetHdr
-	// rejectIovs is a second preallocated iovec scratch used exclusively by
-	// WriteReject (reject + self-forward from the inside path). It mirrors
-	// writeIovs but lets listenIn goroutines emit reject packets without
-	// racing with the listenOut coalescer that owns writeIovs.
-	rejectIovs [2]unix.Iovec
-
-	// gsoHdrBuf is a per-queue 10-byte scratch for the virtio_net_hdr emitted
-	// by WriteGSO. Separate from validVnetHdr so a concurrent non-GSO Write on
-	// another queue never observes a half-written header.
-	gsoHdrBuf [virtioNetHdrLen]byte
-	// gsoIovs is the writev iovec scratch for WriteGSO. Sized to hold the
-	// virtio header + IP/TCP header + up to gsoInitialPayIovs payload
-	// fragments; grown on demand if a coalescer pushes more.
-	gsoIovs []unix.Iovec
-}
-
-// gsoInitialPayIovs is the starting capacity (in payload fragments) of
-// tunFile.gsoIovs. Sized to cover the default coalesce segment cap without
-// any reallocations.
-const gsoInitialPayIovs = 66
-
-// validVnetHdr is the 10-byte virtio_net_hdr we prepend to every non-GSO TUN
-// write. Only flag set is VIRTIO_NET_HDR_F_DATA_VALID, which marks the skb
-// CHECKSUM_UNNECESSARY so the receiving network stack skips L4 checksum
-// verification. All packets that reach the plain Write / WriteReject paths
-// already carry a valid L4 checksum (either supplied by a remote peer whose
-// ciphertext we AEAD-authenticated, or produced by finishChecksum during TSO
-// segmentation, or built locally by CreateRejectPacket), so trusting them is
-// safe.
-var validVnetHdr = [virtioNetHdrLen]byte{unix.VIRTIO_NET_HDR_F_DATA_VALID}
-
-// newFriend makes a tunFile for a MultiQueueReader that copies the shutdown eventfd from the parent tun
-func (r *tunFile) newFriend(fd int) (*tunFile, error) {
-	if err := unix.SetNonblock(fd, true); err != nil {
-		return nil, fmt.Errorf("failed to set tun fd non-blocking: %w", err)
-	}
-	out := &tunFile{
-		fd:         fd,
-		shutdownFd: r.shutdownFd,
-		vnetHdr:    r.vnetHdr,
-		readBuf:    make([]byte, tunReadBufSize),
-		readPoll: [2]unix.PollFd{
-			{Fd: int32(fd), Events: unix.POLLIN},
-			{Fd: int32(r.shutdownFd), Events: unix.POLLIN},
-		},
-		writePoll: [2]unix.PollFd{
-			{Fd: int32(fd), Events: unix.POLLOUT},
-			{Fd: int32(r.shutdownFd), Events: unix.POLLIN},
-		},
-	}
-	if r.vnetHdr {
-		out.segBuf = make([]byte, tunSegBufCap)
-		out.writeIovs[0].Base = &validVnetHdr[0]
-		out.writeIovs[0].SetLen(virtioNetHdrLen)
-		out.rejectIovs[0].Base = &validVnetHdr[0]
-		out.rejectIovs[0].SetLen(virtioNetHdrLen)
-		out.gsoIovs = make([]unix.Iovec, 2, 2+gsoInitialPayIovs)
-		out.gsoIovs[0].Base = &out.gsoHdrBuf[0]
-		out.gsoIovs[0].SetLen(virtioNetHdrLen)
-	}
-	return out, nil
-}
-
-func newTunFd(fd int, vnetHdr bool) (*tunFile, error) {
-	if err := unix.SetNonblock(fd, true); err != nil {
-		return nil, fmt.Errorf("failed to set tun fd non-blocking: %w", err)
-	}
-
-	shutdownFd, err := unix.Eventfd(0, unix.EFD_NONBLOCK|unix.EFD_CLOEXEC)
-	if err != nil {
-		return nil, fmt.Errorf("failed to create eventfd: %w", err)
-	}
-
-	out := &tunFile{
-		fd:         fd,
-		shutdownFd: shutdownFd,
-		lastOne:    true,
-		vnetHdr:    vnetHdr,
-		readBuf:    make([]byte, tunReadBufSize),
-		readPoll: [2]unix.PollFd{
-			{Fd: int32(fd), Events: unix.POLLIN},
-			{Fd: int32(shutdownFd), Events: unix.POLLIN},
-		},
-		writePoll: [2]unix.PollFd{
-			{Fd: int32(fd), Events: unix.POLLOUT},
-			{Fd: int32(shutdownFd), Events: unix.POLLIN},
-		},
-	}
-	if vnetHdr {
-		out.segBuf = make([]byte, tunSegBufCap)
-		out.writeIovs[0].Base = &validVnetHdr[0]
-		out.writeIovs[0].SetLen(virtioNetHdrLen)
-		out.rejectIovs[0].Base = &validVnetHdr[0]
-		out.rejectIovs[0].SetLen(virtioNetHdrLen)
-		out.gsoIovs = make([]unix.Iovec, 2, 2+gsoInitialPayIovs)
-		out.gsoIovs[0].Base = &out.gsoHdrBuf[0]
-		out.gsoIovs[0].SetLen(virtioNetHdrLen)
-	}
-
-	return out, nil
-}
-
-func (r *tunFile) blockOnRead() error {
-	const problemFlags = unix.POLLHUP | unix.POLLNVAL | unix.POLLERR
-	var err error
-	for {
-		_, err = unix.Poll(r.readPoll[:], -1)
-		if err != unix.EINTR {
-			break
-		}
-	}
-	//always reset these!
-	tunEvents := r.readPoll[0].Revents
-	shutdownEvents := r.readPoll[1].Revents
-	r.readPoll[0].Revents = 0
-	r.readPoll[1].Revents = 0
-	//do the err check before trusting the potentially bogus bits we just got
-	if err != nil {
-		return err
-	}
-	if shutdownEvents&(unix.POLLIN|problemFlags) != 0 {
-		return os.ErrClosed
-	} else if tunEvents&problemFlags != 0 {
-		return os.ErrClosed
-	}
-	return nil
-}
-
-func (r *tunFile) blockOnWrite() error {
-	const problemFlags = unix.POLLHUP | unix.POLLNVAL | unix.POLLERR
-	var err error
-	for {
-		_, err = unix.Poll(r.writePoll[:], -1)
-		if err != unix.EINTR {
-			break
-		}
-	}
-	//always reset these!
-	tunEvents := r.writePoll[0].Revents
-	shutdownEvents := r.writePoll[1].Revents
-	r.writePoll[0].Revents = 0
-	r.writePoll[1].Revents = 0
-	//do the err check before trusting the potentially bogus bits we just got
-	if err != nil {
-		return err
-	}
-	if shutdownEvents&(unix.POLLIN|problemFlags) != 0 {
-		return os.ErrClosed
-	} else if tunEvents&problemFlags != 0 {
-		return os.ErrClosed
-	}
-	return nil
-}
-
-func (r *tunFile) readRaw(buf []byte) (int, error) {
-	for {
-		if n, err := unix.Read(r.fd, buf); err == nil {
-			return n, nil
-		} else if err == unix.EAGAIN {
-			if err = r.blockOnRead(); err != nil {
-				return 0, err
-			}
-			continue
-		} else if err == unix.EINTR {
-			continue
-		} else if err == unix.EBADF {
-			return 0, os.ErrClosed
-		} else {
-			return 0, err
-		}
-	}
-}
-
-// Read reads one or more superpackets from the tun and returns the
-// resulting packets. The first read blocks via poll; once the fd is known
-// readable we drain additional packets non-blocking until the kernel queue
-// is empty (EAGAIN), we've collected tunDrainCap packets, or we're out of
-// segBuf headroom. This amortizes the poll wake over bursts of small
-// packets (e.g. TCP ACKs). Slices point into the tunFile's internal buffers
-// and are only valid until the next Read or Close on this Queue.
-func (r *tunFile) Read() ([][]byte, error) {
-	r.pending = r.pending[:0]
-	r.segOff = 0
-
-	// Initial (blocking) read. Retry on decode errors so a single bad
-	// packet does not stall the reader.
-	for {
-		n, err := r.readRaw(r.readBuf)
-		if err != nil {
-			return nil, err
-		}
-		if !r.vnetHdr {
-			r.pending = append(r.pending, r.readBuf[:n])
-			// Non-vnetHdr mode shares one readBuf so we can't drain safely
-			// without copying; return the single packet as before.
-			return r.pending, nil
-		}
-		if err := r.decodeRead(n); err != nil {
-			// Drop and read again — a bad packet should not kill the reader.
-			continue
-		}
-		break
-	}
-
-	// Drain: non-blocking reads until the kernel queue is empty, the drain
-	// cap is reached, or segBuf no longer has room for another worst-case
-	// superpacket.
-	for len(r.pending) < tunDrainCap && tunSegBufCap-r.segOff >= tunSegBufSize {
-		n, err := unix.Read(r.fd, r.readBuf)
-		if err != nil {
-			// EAGAIN / EINTR / anything else: stop draining. We already
-			// have a valid batch from the first read.
-			break
-		}
-		if n <= 0 {
-			break
-		}
-		if err := r.decodeRead(n); err != nil {
-			// Drop this packet and stop the drain; we'd rather hand off
-			// what we have than keep spinning here.
-			break
-		}
-	}
-
-	return r.pending, nil
-}
-
-// decodeRead decodes the virtio header plus payload in r.readBuf[:n], appends
-// the segments to r.pending, and advances r.segOff by the total scratch used.
-// Caller must have already ensured r.vnetHdr is true.
-func (r *tunFile) decodeRead(n int) error {
-	if n < virtioNetHdrLen {
-		return fmt.Errorf("short tun read: %d < %d", n, virtioNetHdrLen)
-	}
-	var hdr virtioNetHdr
-	hdr.decode(r.readBuf[:virtioNetHdrLen])
-	before := len(r.pending)
-	if err := segmentInto(r.readBuf[virtioNetHdrLen:n], hdr, &r.pending, r.segBuf[r.segOff:]); err != nil {
-		return err
-	}
-	for k := before; k < len(r.pending); k++ {
-		r.segOff += len(r.pending[k])
-	}
-	return nil
-}
-
-func (r *tunFile) Write(buf []byte) (int, error) {
-	return r.writeWithScratch(buf, &r.writeIovs)
-}
-
-// WriteReject emits a packet using a dedicated iovec scratch (rejectIovs)
-// distinct from the one used by the coalescer's Write path. This avoids a
-// data race between the inside (listenIn) goroutine emitting reject or
-// self-forward packets and the outside (listenOut) goroutine flushing TCP
-// coalescer passthroughs on the same tunFile.
-func (r *tunFile) WriteReject(buf []byte) (int, error) {
-	return r.writeWithScratch(buf, &r.rejectIovs)
-}
-
-func (r *tunFile) writeWithScratch(buf []byte, iovs *[2]unix.Iovec) (int, error) {
-	if !r.vnetHdr {
-		for {
-			if n, err := unix.Write(r.fd, buf); err == nil {
-				return n, nil
-			} else if err == unix.EAGAIN {
-				if err = r.blockOnWrite(); err != nil {
-					return 0, err
-				}
-				continue
-			} else if err == unix.EINTR {
-				continue
-			} else if err == unix.EBADF {
-				return 0, os.ErrClosed
-			} else {
-				return 0, err
-			}
-		}
-	}
-
-	if len(buf) == 0 {
-		return 0, nil
-	}
-	// Point the payload iovec at the caller's buffer. iovs[0] is pre-wired
-	// to validVnetHdr during tunFile construction so we don't rebuild it here.
-	iovs[1].Base = &buf[0]
-	iovs[1].SetLen(len(buf))
-	iovPtr := uintptr(unsafe.Pointer(&iovs[0]))
-	// The TUN fd is non-blocking (set in newTunFd / newFriend), so writev
-	// either completes promptly or returns EAGAIN — it cannot park the
-	// goroutine inside the kernel. That lets us use syscall.RawSyscall and
-	// skip the runtime.entersyscall / exitsyscall bookkeeping on every
-	// packet; we only pay that cost when we fall through to blockOnWrite.
-	for {
-		n, _, errno := syscall.RawSyscall(unix.SYS_WRITEV, uintptr(r.fd), iovPtr, 2)
-		if errno == 0 {
-			runtime.KeepAlive(buf)
-			if int(n) < virtioNetHdrLen {
-				return 0, io.ErrShortWrite
-			}
-			return int(n) - virtioNetHdrLen, nil
-		}
-		if errno == unix.EAGAIN {
-			runtime.KeepAlive(buf)
-			if err := r.blockOnWrite(); err != nil {
-				return 0, err
-			}
-			continue
-		}
-		if errno == unix.EINTR {
-			continue
-		}
-		runtime.KeepAlive(buf)
-		return 0, errno
-	}
-}
-
-// GSOSupported reports whether this queue was opened with IFF_VNET_HDR and
-// can accept WriteGSO. When false, callers should fall back to per-segment
-// Write calls.
-func (r *tunFile) GSOSupported() bool { return r.vnetHdr }
-
-// WriteGSO emits a TCP TSO superpacket in a single writev. hdr is the
-// IPv4/IPv6 + TCP header prefix (already finalized — total length, IP csum,
-// and TCP pseudo-header partial set by the caller). pays are payload
-// fragments whose concatenation forms the full coalesced payload; each
-// slice is read-only and must stay valid until return. gsoSize is the MSS;
-// every segment except possibly the last is exactly gsoSize bytes.
-// csumStart is the byte offset where the TCP header begins within hdr.
-func (r *tunFile) WriteGSO(hdr []byte, pays [][]byte, gsoSize uint16, isV6 bool, csumStart uint16) error {
-	if !r.vnetHdr {
-		return fmt.Errorf("WriteGSO called on tun without IFF_VNET_HDR")
-	}
-	if len(hdr) == 0 || len(pays) == 0 {
-		return nil
-	}
-
-	// Build the virtio_net_hdr. When pays total to <= gsoSize the kernel
-	// would produce a single segment; keep NEEDS_CSUM semantics but skip
-	// the GSO type so the kernel doesn't spuriously mark this as TSO.
-	vhdr := virtioNetHdr{
-		Flags:      unix.VIRTIO_NET_HDR_F_NEEDS_CSUM,
-		HdrLen:     uint16(len(hdr)),
-		GSOSize:    gsoSize,
-		CsumStart:  csumStart,
-		CsumOffset: 16, // TCP checksum field lives 16 bytes into the TCP header
-	}
-	var totalPay int
-	for _, p := range pays {
-		totalPay += len(p)
-	}
-	if totalPay > int(gsoSize) {
-		if isV6 {
-			vhdr.GSOType = unix.VIRTIO_NET_HDR_GSO_TCPV6
-		} else {
-			vhdr.GSOType = unix.VIRTIO_NET_HDR_GSO_TCPV4
-		}
-	} else {
-		vhdr.GSOType = unix.VIRTIO_NET_HDR_GSO_NONE
-		vhdr.GSOSize = 0
-	}
-	vhdr.encode(r.gsoHdrBuf[:])
-
-	// Build the iovec array: [virtio_hdr, hdr, pays...]. r.gsoIovs[0] is
-	// wired to gsoHdrBuf at construction and never changes.
-	need := 2 + len(pays)
-	if cap(r.gsoIovs) < need {
-		grown := make([]unix.Iovec, need)
-		grown[0] = r.gsoIovs[0]
-		r.gsoIovs = grown
-	} else {
-		r.gsoIovs = r.gsoIovs[:need]
-	}
-	r.gsoIovs[1].Base = &hdr[0]
-	r.gsoIovs[1].SetLen(len(hdr))
-	for i, p := range pays {
-		r.gsoIovs[2+i].Base = &p[0]
-		r.gsoIovs[2+i].SetLen(len(p))
-	}
-
-	iovPtr := uintptr(unsafe.Pointer(&r.gsoIovs[0]))
-	iovCnt := uintptr(len(r.gsoIovs))
-	for {
-		n, _, errno := syscall.RawSyscall(unix.SYS_WRITEV, uintptr(r.fd), iovPtr, iovCnt)
-		if errno == 0 {
-			runtime.KeepAlive(hdr)
-			runtime.KeepAlive(pays)
-			if int(n) < virtioNetHdrLen {
-				return io.ErrShortWrite
-			}
-			return nil
-		}
-		if errno == unix.EAGAIN {
-			runtime.KeepAlive(hdr)
-			runtime.KeepAlive(pays)
-			if err := r.blockOnWrite(); err != nil {
-				return err
-			}
-			continue
-		}
-		if errno == unix.EINTR {
-			continue
-		}
-		runtime.KeepAlive(hdr)
-		runtime.KeepAlive(pays)
-		return errno
-	}
-}
-
-func (r *tunFile) wakeForShutdown() error {
-	var buf [8]byte
-	binary.NativeEndian.PutUint64(buf[:], 1)
-	_, err := unix.Write(int(r.readPoll[1].Fd), buf[:])
-	return err
-}
-
-func (r *tunFile) Close() error {
-	if r.closed { // avoid closing more than once. Technically a fd could get re-used, which would be a problem
-		return nil
-	}
-	r.closed = true
-	if r.lastOne {
-		_ = unix.Close(r.shutdownFd)
-	}
-	return unix.Close(r.fd)
-}
-
 type tun struct {
-	*tunFile
-	readers     []*tunFile
+	readers     tio.Container
 	closeLock   sync.Mutex
 	Device      string
 	vpnNetworks []netip.Prefix
@@ -484,6 +34,7 @@ type tun struct {
 	TXQueueLen  int
 	deviceIndex int
 	ioctlFd     uintptr
+	vnetHdr     bool
 
 	Routes                    atomic.Pointer[[]Route]
 	routeTree                 atomic.Pointer[bart.Table[routing.Gateways]]
@@ -622,15 +173,28 @@ func newTun(c *config.C, l *logrus.Logger, vpnNetworks []netip.Prefix, multiqueu
 
 // newTunGeneric does all the stuff common to different tun initialization paths. It will close your files on error.
 func newTunGeneric(c *config.C, l *logrus.Logger, fd int, vnetHdr bool, vpnNetworks []netip.Prefix) (*tun, error) {
-	tfd, err := newTunFd(fd, vnetHdr)
+	var container tio.Container
+	var err error
+	if vnetHdr {
+		container, err = tio.NewGSOContainer()
+	} else {
+		container, err = tio.NewPollContainer()
+	}
+
 	if err != nil {
 		_ = unix.Close(fd)
 		return nil, err
 	}
+	err = container.Add(fd)
+	if err != nil {
+		_ = unix.Close(fd)
+		return nil, err
+	}
+
 	t := &tun{
-		tunFile:                   tfd,
-		readers:                   []*tunFile{tfd},
+		readers:                   container,
 		closeLock:                 sync.Mutex{},
+		vnetHdr:                   vnetHdr,
 		vpnNetworks:               vpnNetworks,
 		TXQueueLen:                c.GetInt("tun.tx_queue", 500),
 		useSystemRoutes:           c.GetBool("tun.use_system_route_table", false),
@@ -732,13 +296,13 @@ func (t *tun) SupportsMultiqueue() bool {
 	return true
 }
 
-func (t *tun) NewMultiQueueReader() (Queue, error) {
+func (t *tun) NewMultiQueueReader() error {
 	t.closeLock.Lock()
 	defer t.closeLock.Unlock()
 
 	fd, err := unix.Open("/dev/net/tun", os.O_RDWR, 0)
 	if err != nil {
-		return nil, err
+		return err
 	}
 
 	flags := uint16(unix.IFF_TUN | unix.IFF_NO_PI | unix.IFF_MULTI_QUEUE)
@@ -747,25 +311,23 @@ func (t *tun) NewMultiQueueReader() (Queue, error) {
 	}
 	if _, err = tunSetIff(fd, t.Device, flags); err != nil {
 		_ = unix.Close(fd)
-		return nil, err
+		return err
 	}
 
 	if t.vnetHdr {
 		if err = ioctl(uintptr(fd), unix.TUNSETOFFLOAD, uintptr(tsoOffloadFlags)); err != nil {
 			_ = unix.Close(fd)
-			return nil, fmt.Errorf("failed to enable offload on multiqueue tun fd: %w", err)
+			return fmt.Errorf("failed to enable offload on multiqueue tun fd: %w", err)
 		}
 	}
 
-	out, err := t.tunFile.newFriend(fd)
+	err = t.readers.Add(fd)
 	if err != nil {
 		_ = unix.Close(fd)
-		return nil, err
+		return err
 	}
 
-	t.readers = append(t.readers, out)
-
-	return out, nil
+	return nil
 }
 
 func (t *tun) RoutesFor(ip netip.Addr) routing.Gateways {
@@ -1195,6 +757,10 @@ func (t *tun) updateRoutes(r netlink.RouteUpdate) {
 	t.routeTree.Store(newTree)
 }
 
+func (t *tun) Readers() []tio.Queue {
+	return t.readers.Queues()
+}
+
 func (t *tun) Close() error {
 	t.closeLock.Lock()
 	defer t.closeLock.Unlock()
@@ -1204,32 +770,10 @@ func (t *tun) Close() error {
 		t.routeChan = nil
 	}
 
-	// Signal all readers blocked in poll to wake up and exit
-	_ = t.tunFile.wakeForShutdown()
-
 	if t.ioctlFd > 0 {
 		_ = unix.Close(int(t.ioctlFd))
 		t.ioctlFd = 0
 	}
 
-	for i := range t.readers {
-		if i == 0 {
-			continue //we want to close the zeroth reader last
-		}
-		err := t.readers[i].Close()
-		if err != nil {
-			t.l.WithField("reader", i).WithError(err).Error("error closing tun reader")
-		} else {
-			t.l.WithField("reader", i).Info("closed tun reader")
-		}
-	}
-
-	//this is t.readers[0] too
-	err := t.tunFile.Close()
-	if err != nil {
-		t.l.WithField("reader", 0).WithError(err).Error("error closing tun reader")
-	} else {
-		t.l.WithField("reader", 0).Info("closed tun reader")
-	}
-	return err
+	return t.readers.Close()
 }

overlay/tun_linux_test.go

@@ -1,34 +0,0 @@
-//go:build !e2e_testing
-// +build !e2e_testing
-
-package overlay
-
-import "testing"
-
-var runAdvMSSTests = []struct {
-	name     string
-	tun      *tun
-	r        Route
-	expected int
-}{
-	// Standard case, default MTU is the device max MTU
-	{"default", &tun{DefaultMTU: 1440, MaxMTU: 1440}, Route{}, 0},
-	{"default-min", &tun{DefaultMTU: 1440, MaxMTU: 1440}, Route{MTU: 1440}, 0},
-	{"default-low", &tun{DefaultMTU: 1440, MaxMTU: 1440}, Route{MTU: 1200}, 1160},
-
-	// Case where we have a route MTU set higher than the default
-	{"route", &tun{DefaultMTU: 1440, MaxMTU: 8941}, Route{}, 1400},
-	{"route-min", &tun{DefaultMTU: 1440, MaxMTU: 8941}, Route{MTU: 1440}, 1400},
-	{"route-high", &tun{DefaultMTU: 1440, MaxMTU: 8941}, Route{MTU: 8941}, 0},
-}
-
-func TestTunAdvMSS(t *testing.T) {
-	for _, tt := range runAdvMSSTests {
-		t.Run(tt.name, func(t *testing.T) {
-			o := tt.tun.advMSS(tt.r)
-			if o != tt.expected {
-				t.Errorf("got %d, want %d", o, tt.expected)
-			}
-		})
-	}
-}

overlay/tun_netbsd.go

@@ -16,6 +16,7 @@ import (
 	"github.com/gaissmai/bart"
 	"github.com/sirupsen/logrus"
 	"github.com/slackhq/nebula/config"
+	"github.com/slackhq/nebula/overlay/tio"
 	"github.com/slackhq/nebula/routing"
 	"github.com/slackhq/nebula/util"
 	netroute "golang.org/x/net/route"
@@ -412,7 +413,7 @@ func (t *tun) SupportsMultiqueue() bool {
 	return false
 }
 
-func (t *tun) NewMultiQueueReader() (Queue, error) {
+func (t *tun) NewMultiQueueReader() (tio.Queue, error) {
 	return nil, fmt.Errorf("TODO: multiqueue not implemented for netbsd")
 }
 

overlay/tun_openbsd.go

@@ -16,6 +16,7 @@ import (
 	"github.com/gaissmai/bart"
 	"github.com/sirupsen/logrus"
 	"github.com/slackhq/nebula/config"
+	"github.com/slackhq/nebula/overlay/tio"
 	"github.com/slackhq/nebula/routing"
 	"github.com/slackhq/nebula/util"
 	netroute "golang.org/x/net/route"
@@ -332,7 +333,7 @@ func (t *tun) SupportsMultiqueue() bool {
 	return false
 }
 
-func (t *tun) NewMultiQueueReader() (Queue, error) {
+func (t *tun) NewMultiQueueReader() (tio.Queue, error) {
 	return nil, fmt.Errorf("TODO: multiqueue not implemented for openbsd")
 }
 

overlay/tun_tester.go

@@ -13,6 +13,7 @@ import (
 	"github.com/gaissmai/bart"
 	"github.com/sirupsen/logrus"
 	"github.com/slackhq/nebula/config"
+	"github.com/slackhq/nebula/overlay/tio"
 	"github.com/slackhq/nebula/routing"
 )
 
@@ -142,6 +143,6 @@ func (t *TestTun) SupportsMultiqueue() bool {
 	return false
 }
 
-func (t *TestTun) NewMultiQueueReader() (Queue, error) {
+func (t *TestTun) NewMultiQueueReader() (tio.Queue, error) {
 	return nil, fmt.Errorf("TODO: multiqueue not implemented")
 }

overlay/tun_windows.go

@@ -17,6 +17,7 @@ import (
 	"github.com/gaissmai/bart"
 	"github.com/sirupsen/logrus"
 	"github.com/slackhq/nebula/config"
+	"github.com/slackhq/nebula/overlay/tio"
 	"github.com/slackhq/nebula/routing"
 	"github.com/slackhq/nebula/util"
 	"github.com/slackhq/nebula/wintun"
@@ -255,7 +256,7 @@ func (t *winTun) SupportsMultiqueue() bool {
 	return false
 }
 
-func (t *winTun) NewMultiQueueReader() (Queue, error) {
+func (t *winTun) NewMultiQueueReader() (tio.Queue, error) {
 	return nil, fmt.Errorf("TODO: multiqueue not implemented for windows")
 }
 

overlay/user.go

@@ -6,6 +6,7 @@ import (
 
 	"github.com/sirupsen/logrus"
 	"github.com/slackhq/nebula/config"
+	"github.com/slackhq/nebula/overlay/tio"
 	"github.com/slackhq/nebula/routing"
 )
 
@@ -28,6 +29,7 @@ func NewUserDevice(vpnNetworks []netip.Prefix) (Device, error) {
 
 type UserDevice struct {
 	vpnNetworks []netip.Prefix
+	numReaders  int
 
 	outboundReader *io.PipeReader
 	outboundWriter *io.PipeWriter
@@ -65,8 +67,17 @@ func (d *UserDevice) SupportsMultiqueue() bool {
 	return true
 }
 
-func (d *UserDevice) NewMultiQueueReader() (Queue, error) {
-	return d, nil
+func (d *UserDevice) NewMultiQueueReader() error {
+	d.numReaders++
+	return nil
+}
+
+func (d *UserDevice) Readers() []tio.Queue {
+	out := make([]tio.Queue, d.numReaders)
+	for i := range d.numReaders {
+		out[i] = d
+	}
+	return out
 }
 
 func (d *UserDevice) Pipe() (*io.PipeReader, *io.PipeWriter) {