Eliminate some bounds check

internal/multiplex/obfs.go

@@ -43,35 +43,35 @@ func MakeObfs(salsaKey [32]byte, payloadCipher cipher.AEAD) Obfser {
 		// we need the encrypted data to be at least 8 bytes to be used as nonce for salsa20 stream header encryption
 		// this will be the case if the encryption method is an AEAD cipher, however for plain, it's well possible
 		// that the frame payload is smaller than 8 bytes, so we need to add on the difference
-		var extraLen uint8
+		var extraLen int
 		if payloadCipher == nil {
-			if len(f.Payload) < 8 {
-				extraLen = uint8(8 - len(f.Payload))
+			if extraLen = 8 - len(f.Payload); extraLen < 0 {
+				extraLen = 0
 			}
 		} else {
-			extraLen = uint8(payloadCipher.Overhead())
+			extraLen = payloadCipher.Overhead()
+			if extraLen < 8 {
+				return 0, errors.New("AEAD's Overhead cannot be fewer than 8 bytes")
+			}
 		}
-
 		// usefulLen is the amount of bytes that will be eventually sent off
-		usefulLen := HEADER_LEN + len(f.Payload) + int(extraLen)
-		if len(buf) < usefulLen {
+		usefulLen := HEADER_LEN + len(f.Payload) + extraLen
+		if usefulLen < HEADER_LEN || len(buf) < usefulLen { // compiler hint to eliminate bound check
 			return 0, io.ErrShortBuffer
-
 		}
 		// we do as much in-place as possible to save allocation
-		useful := buf[:usefulLen] // stream header + payload + potential overhead
-		header := useful[:HEADER_LEN]
-		encryptedPayloadWithExtra := useful[HEADER_LEN:]
+		header := buf[:HEADER_LEN]
+		encryptedPayloadWithExtra := buf[HEADER_LEN:usefulLen]
 
 		putU32(header[0:4], f.StreamID)
 		putU64(header[4:12], f.Seq)
 		header[12] = f.Closing
-		header[13] = extraLen
+		header[13] = byte(extraLen)
 
 		if payloadCipher == nil {
 			copy(encryptedPayloadWithExtra, f.Payload)
-			if extraLen != 0 {
-				util.CryptoRandRead(encryptedPayloadWithExtra[len(encryptedPayloadWithExtra)-int(extraLen):])
+			if extraLen != 0 { // read nonce
+				util.CryptoRandRead(encryptedPayloadWithExtra[len(encryptedPayloadWithExtra)-extraLen:])
 			}
 		} else {
 			ciphertext := payloadCipher.Seal(nil, header[:12], f.Payload, nil)
@@ -88,7 +88,7 @@ func MakeObfs(salsaKey [32]byte, payloadCipher cipher.AEAD) Obfser {
 
 func MakeDeobfs(salsaKey [32]byte, payloadCipher cipher.AEAD) Deobfser {
 	// stream header length + minimum data size (i.e. nonce size of salsa20)
-	minInputLen := HEADER_LEN + 8
+	const minInputLen = HEADER_LEN + 8
 	deobfs := func(in []byte) (*Frame, error) {
 		if len(in) < minInputLen {
 			return nil, fmt.Errorf("input size %v, but it cannot be shorter than %v bytes", len(in), minInputLen)
@@ -106,8 +106,8 @@ func MakeDeobfs(salsaKey [32]byte, payloadCipher cipher.AEAD) Deobfser {
 		extraLen := header[13]
 
 		usefulPayloadLen := len(pldWithOverHead) - int(extraLen)
-		if usefulPayloadLen < 0 {
-			return nil, errors.New("extra length is greater than total pldWithOverHead length")
+		if usefulPayloadLen < 0 || usefulPayloadLen > len(pldWithOverHead) {
+			return nil, errors.New("extra length is negative or extra length is greater than total pldWithOverHead length")
 		}
 
 		var outputPayload []byte