net/tstun: reuse buffered packet from pool

We would call parsedPacketPool.Get() for all packets received in Read/Write.
This was wasteful and not necessary, fetch a single *packet.Parsed for
all packets.

Signed-off-by: Maisem Ali <maisem@tailscale.com>

net/tstun/wrap.go

@@ -559,9 +559,9 @@ func (t *Wrapper) Read(buffs [][]byte, sizes []int, offset int) (int, error) {
 	metricPacketOut.Add(int64(len(res.data)))
 
 	var buffsPos int
+	p := parsedPacketPool.Get().(*packet.Parsed)
+	defer parsedPacketPool.Put(p)
 	for _, data := range res.data {
-		p := parsedPacketPool.Get().(*packet.Parsed)
-		defer parsedPacketPool.Put(p)
 		p.Decode(data[res.dataOffset:])
 		if m := t.destIPActivity.Load(); m != nil {
 			if fn := m[p.Dst.Addr()]; fn != nil {
@@ -629,11 +629,7 @@ func (t *Wrapper) injectedRead(res tunInjectedRead, buf []byte, offset int) (int
 	return n, nil
 }
 
-func (t *Wrapper) filterIn(buf []byte) filter.Response {
-	p := parsedPacketPool.Get().(*packet.Parsed)
-	defer parsedPacketPool.Put(p)
-	p.Decode(buf)
-
+func (t *Wrapper) filterIn(p *packet.Parsed) filter.Response {
 	if p.IPProto == ipproto.TSMP {
 		if pingReq, ok := p.AsTSMPPing(); ok {
 			t.noteActivity()
@@ -732,8 +728,11 @@ func (t *Wrapper) Write(buffs [][]byte, offset int) (int, error) {
 	metricPacketIn.Add(int64(len(buffs)))
 	i := 0
 	if !t.disableFilter {
+		p := parsedPacketPool.Get().(*packet.Parsed)
+		defer parsedPacketPool.Put(p)
 		for _, buff := range buffs {
-			if t.filterIn(buff[offset:]) != filter.Accept {
+			p.Decode(buff[offset:])
+			if t.filterIn(p) != filter.Accept {
 				metricPacketInDrop.Add(1)
 			} else {
 				buffs[i] = buff

net/tstun/wrap_test.go

@@ -518,9 +518,11 @@ func TestPeerAPIBypass(t *testing.T) {
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
+			p := new(packet.Parsed)
+			p.Decode(tt.pkt)
 			tt.w.SetFilter(tt.filter)
 			tt.w.disableTSMPRejected = true
-			if got := tt.w.filterIn(tt.pkt); got != tt.want {
+			if got := tt.w.filterIn(p); got != tt.want {
 				t.Errorf("got = %v; want %v", got, tt.want)
 			}
 		})
@@ -548,7 +550,9 @@ func TestFilterDiscoLoop(t *testing.T) {
 	uh.Marshal(pkt)
 	copy(pkt[uh.Len():], discoPayload)
 
-	got := tw.filterIn(pkt)
+	p := new(packet.Parsed)
+	p.Decode(pkt)
+	got := tw.filterIn(p)
 	if got != filter.DropSilently {
 		t.Errorf("got %v; want DropSilently", got)
 	}