tweaks

interface.go

@@ -329,8 +329,7 @@ func (f *Interface) listenOut(i int) {
 	nb := make([]byte, 12, 12)
 
 	listener := func(fromUdpAddr netip.AddrPort, payload []byte, meta udp.RxMeta) {
-		plaintext := f.batchers[i].Reserve(len(payload))
-		f.readOutsidePackets(ViaSender{UdpAddr: fromUdpAddr}, plaintext[:0], payload, h, fwPacket, lhh, nb, i, ctCache.Get())
+		f.readOutsidePackets(ViaSender{UdpAddr: fromUdpAddr}, payload, h, fwPacket, lhh, nb, i, ctCache.Get())
 	}
 
 	flusher := func() {

noiseutil/cipher_state_test.go

@@ -147,6 +147,62 @@ func buildCipherStatesB(b *testing.B, c noise.CipherFunc) (*noise.CipherState, *
 	return eI, dR
 }
 
+// TestDecryptDangerRelayShapeNoAlloc covers the AD-only relay path used in
+// outside.go's handleOutsideRelayPacket: the body is AD, the trailing 16 bytes
+// are the AEAD tag, the plaintext is empty, and the caller passes nil as the
+// destination because it only needs the auth side-effect. The call must
+// succeed, return an empty plaintext, and not allocate on the hot path.
+func TestDecryptDangerRelayShapeNoAlloc(t *testing.T) {
+	cases := []struct {
+		name string
+		c    noise.CipherFunc
+		wrap func(*noise.CipherState) CipherState
+	}{
+		{"AESGCM", CipherAESGCM, func(cs *noise.CipherState) CipherState { return NewCipherStateAESGCM(cs) }},
+		{"ChaChaPoly", noise.CipherChaChaPoly, func(cs *noise.CipherState) CipherState { return NewCipherStateChaChaPoly(cs) }},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			encCS, decCS := buildCipherStates(t, tc.c)
+			enc, dec := tc.wrap(encCS), tc.wrap(decCS)
+
+			ad := make([]byte, 1200) // typical relay packet body size
+			for i := range ad {
+				ad[i] = byte(i)
+			}
+			nb := make([]byte, 12)
+
+			// Build the "signature value" the way handleOutsideRelayPacket sees it:
+			// empty plaintext encrypted with the body as AD yields just the 16-byte tag.
+			tag, err := enc.EncryptDanger(nil, ad, nil, 1, nb)
+			require.NoError(t, err)
+			require.Len(t, tag, dec.Overhead())
+
+			// Sanity: the relay-shaped call returns empty plaintext, no error.
+			out, err := dec.DecryptDanger(nil, ad, tag, 1, nb)
+			require.NoError(t, err)
+			assert.Empty(t, out)
+
+			// Tampering with the AD must fail authentication.
+			ad[0] ^= 0xff
+			_, err = dec.DecryptDanger(nil, ad, tag, 1, nb)
+			require.Error(t, err)
+			ad[0] ^= 0xff
+
+			// The hot path must not allocate. AllocsPerRun does a warm-up run, so any
+			// one-time setup is excluded. Counter has to advance so the AEAD nonce is
+			// unique per call, but we don't care whether the auth succeeds — we only
+			// care about whether the call path allocates.
+			var counter uint64 = 2
+			allocs := testing.AllocsPerRun(100, func() {
+				_, _ = dec.DecryptDanger(nil, ad, tag, counter, nb)
+				counter++
+			})
+			assert.Equal(t, 0.0, allocs, "DecryptDanger(nil, ...) must not allocate")
+		})
+	}
+}
+
 func TestCipherStateNilSafety(t *testing.T) {
 	var aes *CipherStateAESGCM
 	_, err := aes.EncryptDanger(nil, nil, nil, 0, make([]byte, 12))

outside.go

@@ -22,7 +22,7 @@ const (
 
 var ErrOutOfWindow = errors.New("out of window packet")
 
-func (f *Interface) readOutsidePackets(via ViaSender, out []byte, packet []byte, h *header.H, fwPacket *firewall.Packet, lhf *LightHouseHandler, nb []byte, q int, localCache firewall.ConntrackCache) {
+func (f *Interface) readOutsidePackets(via ViaSender, packet []byte, h *header.H, fwPacket *firewall.Packet, lhf *LightHouseHandler, nb []byte, q int, localCache firewall.ConntrackCache) {
 	err := h.Parse(packet)
 	if err != nil {
 		// Hole punch packets are 0 or 1 byte big, so lets ignore printing those errors
@@ -110,10 +110,11 @@ func (f *Interface) readOutsidePackets(via ViaSender, out []byte, packet []byte,
 
 	// Relay packets are special
 	if isMessageRelay {
-		f.handleOutsideRelayPacket(hostinfo, via, out, packet, h, fwPacket, lhf, nb, q, localCache)
+		f.handleOutsideRelayPacket(hostinfo, via, packet, h, fwPacket, lhf, nb, q, localCache)
 		return
 	}
 
+	out := f.batchers[q].Reserve(len(packet))
 	out, err = f.decrypt(hostinfo, h.MessageCounter, out, packet, h, nb)
 	if err != nil {
 		if f.l.Enabled(context.Background(), slog.LevelDebug) {
@@ -167,7 +168,7 @@ func (f *Interface) readOutsidePackets(via ViaSender, out []byte, packet []byte,
 	}
 }
 
-func (f *Interface) handleOutsideRelayPacket(hostinfo *HostInfo, via ViaSender, out []byte, packet []byte, h *header.H, fwPacket *firewall.Packet, lhf *LightHouseHandler, nb []byte, q int, localCache firewall.ConntrackCache) {
+func (f *Interface) handleOutsideRelayPacket(hostinfo *HostInfo, via ViaSender, packet []byte, h *header.H, fwPacket *firewall.Packet, lhf *LightHouseHandler, nb []byte, q int, localCache firewall.ConntrackCache) {
 	// The entire body is sent as AD, not encrypted.
 	// The packet consists of a 16-byte parsed Nebula header, Associated Data-protected payload, and a trailing 16-byte AEAD signature value.
 	// The packet is guaranteed to be at least 16 bytes at this point, b/c it got past the h.Parse() call above. If it's
@@ -175,9 +176,10 @@ func (f *Interface) handleOutsideRelayPacket(hostinfo *HostInfo, via ViaSender,
 	// which will gracefully fail in the DecryptDanger call.
 	signedPayload := packet[:len(packet)-hostinfo.ConnectionState.dKey.Overhead()]
 	signatureValue := packet[len(packet)-hostinfo.ConnectionState.dKey.Overhead():]
-	var err error
-	out, err = hostinfo.ConnectionState.dKey.DecryptDanger(out, signedPayload, signatureValue, h.MessageCounter, nb)
-	if err != nil {
+	// The decrypted output is empty (relay packets carry their payload as AD) and unused.
+	// The recursive readOutsidePackets call below operates on signedPayload. Passing
+	// nil avoids reserving an arena slot.
+	if _, err := hostinfo.ConnectionState.dKey.DecryptDanger(nil, signedPayload, signatureValue, h.MessageCounter, nb); err != nil {
 		return
 	}
 	// Successfully validated the thing. Get rid of the Relay header.
@@ -210,7 +212,7 @@ func (f *Interface) handleOutsideRelayPacket(hostinfo *HostInfo, via ViaSender,
 			relay:     relay,
 			IsRelayed: true,
 		}
-		f.readOutsidePackets(via, out[:0], signedPayload, h, fwPacket, lhf, nb, q, localCache)
+		f.readOutsidePackets(via, signedPayload, h, fwPacket, lhf, nb, q, localCache)
 	case ForwardingType:
 		// Find the target HostInfo relay object
 		targetHI, targetRelay, err := f.hostMap.QueryVpnAddrsRelayFor(hostinfo.vpnAddrs, relay.PeerAddr)
@@ -229,6 +231,7 @@ func (f *Interface) handleOutsideRelayPacket(hostinfo *HostInfo, via ViaSender,
 			case ForwardingType:
 				// Forward this packet through the relay tunnel
 				// Find the target HostInfo //todo it would potentially be nice to batch these
+				out := f.batchers[q].Reserve(len(packet) + header.Len + hostinfo.ConnectionState.dKey.Overhead())[:0]
 				f.SendVia(targetHI, targetRelay, signedPayload, nb, out, false)
 			case TerminalType:
 				hostinfo.logger(f.l).Error("Unexpected Relay Type of Terminal")