refactor: Improve parsing docs

Reveal intentions by: - extracting magic numbers into constants - changing function names with >1 responsibilities - documenting non-obvious behaviors.
2024-03-11 20:35:01 +01:00 · 2024-03-11 20:35:01 +01:00 · 3bd02ed46e
parent 4257788f33
commit 3bd02ed46e
3 changed files with 123 additions and 57 deletions
--- a/analyzer/internal/tls.go
+++ b/analyzer/internal/tls.go
@ -5,7 +5,26 @@ import (
 	"github.com/apernet/OpenGFW/analyzer/utils"
 )

-func ParseTLSClientHello(chBuf *utils.ByteBuffer) analyzer.PropMap {
+// TLS record types.
+const (
+	RecordTypeHandshake = 0x16
+)
+
+// TLS handshake message types.
+const (
+	TypeClientHello = 0x01
+	TypeServerHello = 0x02
+)
+
+// TLS extension numbers.
+const (
+	extServerName           = 0x0000
+	extALPN                 = 0x0010
+	extSupportedVersions    = 0x002b
+	extEncryptedClientHello = 0xfe0d
+)
+
+func ParseTLSClientHelloMsgData(chBuf *utils.ByteBuffer) analyzer.PropMap {
 	var ok bool
 	m := make(analyzer.PropMap)
 	// Version, random & session ID length combined are within 35 bytes,
@ -76,7 +95,7 @@ func ParseTLSClientHello(chBuf *utils.ByteBuffer) analyzer.PropMap {
 	return m
 }

-func ParseTLSServerHello(shBuf *utils.ByteBuffer) analyzer.PropMap {
+func ParseTLSServerHelloMsgData(shBuf *utils.ByteBuffer) analyzer.PropMap {
 	var ok bool
 	m := make(analyzer.PropMap)
 	// Version, random & session ID length combined are within 35 bytes,
@ -133,7 +152,7 @@ func ParseTLSServerHello(shBuf *utils.ByteBuffer) analyzer.PropMap {

 func parseTLSExtensions(extType uint16, extDataBuf *utils.ByteBuffer, m analyzer.PropMap) bool {
 	switch extType {
-	case 0x0000: // SNI
+	case extServerName:
 		ok := extDataBuf.Skip(2) // Ignore list length, we only care about the first entry for now
 		if !ok {
 			// Not enough data for list length
@ -154,7 +173,7 @@ func parseTLSExtensions(extType uint16, extDataBuf *utils.ByteBuffer, m analyzer
 			// Not enough data for SNI
 			return false
 		}
-	case 0x0010: // ALPN
+	case extALPN:
 		ok := extDataBuf.Skip(2) // Ignore list length, as we read until the end
 		if !ok {
 			// Not enough data for list length
@ -175,7 +194,7 @@ func parseTLSExtensions(extType uint16, extDataBuf *utils.ByteBuffer, m analyzer
 			alpnList = append(alpnList, alpn)
 		}
 		m["alpn"] = alpnList
-	case 0x002b: // Supported Versions
+	case extSupportedVersions:
 		if extDataBuf.Len() == 2 {
 			// Server only selects one version
 			m["supported_versions"], _ = extDataBuf.GetUint16(false, true)
@ -197,7 +216,7 @@ func parseTLSExtensions(extType uint16, extDataBuf *utils.ByteBuffer, m analyzer
 			}
 			m["supported_versions"] = versions
 		}
-	case 0xfe0d: // ECH
+	case extEncryptedClientHello:
 		// We can't parse ECH for now, just set a flag
 		m["ech"] = true
 	}
--- a/analyzer/tcp/tls.go
+++ b/analyzer/tcp/tls.go
@ -44,12 +44,12 @@ type tlsStream struct {
 func newTLSStream(logger analyzer.Logger) *tlsStream {
 	s := &tlsStream{logger: logger, reqBuf: &utils.ByteBuffer{}, respBuf: &utils.ByteBuffer{}}
 	s.reqLSM = utils.NewLinearStateMachine(
-		s.tlsClientHelloSanityCheck,
-		s.parseClientHello,
+		s.tlsClientHelloPreprocess,
+		s.parseClientHelloData,
 	)
 	s.respLSM = utils.NewLinearStateMachine(
-		s.tlsServerHelloSanityCheck,
-		s.parseServerHello,
+		s.tlsServerHelloPreprocess,
+		s.parseServerHelloData,
 	)
 	return s
 }
@ -89,61 +89,105 @@ func (s *tlsStream) Feed(rev, start, end bool, skip int, data []byte) (u *analyz
 	return update, cancelled || (s.reqDone && s.respDone)
 }

-func (s *tlsStream) tlsClientHelloSanityCheck() utils.LSMAction {
-	data, ok := s.reqBuf.Get(9, true)
+// tlsClientHelloPreprocess validates ClientHello message.
+//
+// During validation, message header and first handshake header may be removed
+// from `s.reqBuf`.
+func (s *tlsStream) tlsClientHelloPreprocess() utils.LSMAction {
+	// headers size: content type (1 byte) + legacy protocol version (2 bytes) +
+	//   + content length (2 bytes) + message type (1 byte) +
+	//   + handshake length (3 bytes)
+	const headersSize = 9
+
+	// minimal data size: protocol version (2 bytes) + random (32 bytes) +
+	//   + session ID (1 byte) + cipher suites (4 bytes) +
+	//   + compression methods (2 bytes) + no extensions
+	const minDataSize = 41
+
+	header, ok := s.reqBuf.Get(headersSize, true)
 	if !ok {
+		// not a full header yet
 		return utils.LSMActionPause
 	}
-	if data[0] != 0x16 || data[5] != 0x01 {
-		// Not a TLS handshake, or not a client hello
+
+	if header[0] != internal.RecordTypeHandshake || header[5] != internal.TypeClientHello {
 		return utils.LSMActionCancel
 	}
-	s.clientHelloLen = int(data[6])<<16 | int(data[7])<<8 | int(data[8])
-	if s.clientHelloLen < 41 {
-		// 2 (Protocol Version) +
-		// 32 (Random) +
-		// 1 (Session ID Length) +
-		// 2 (Cipher Suites Length) +_ws.col.protocol == "TLSv1.3"
-		// 2 (Cipher Suite) +
-		// 1 (Compression Methods Length) +
-		// 1 (Compression Method) +
-		// No extensions
-		// This should be the bare minimum for a client hello
+
+	s.clientHelloLen = int(header[6])<<16 | int(header[7])<<8 | int(header[8])
+	if s.clientHelloLen < minDataSize {
 		return utils.LSMActionCancel
 	}
+
+	// TODO: something is missing. See:
+	//   const messageHeaderSize = 4
+	//   fullMessageLen := int(header[3])<<8 | int(header[4])
+	//   msgNo := fullMessageLen / int(messageHeaderSize+s.serverHelloLen)
+	//   if msgNo != 1 {
+	// 	   // what here?
+	//   }
+	//   if messageNo != int(messageNo) {
+	// 	   // what here?
+	//   }
+
 	return utils.LSMActionNext
 }

-func (s *tlsStream) tlsServerHelloSanityCheck() utils.LSMAction {
-	data, ok := s.respBuf.Get(9, true)
+// tlsServerHelloPreprocess validates ServerHello message.
+//
+// During validation, message header and first handshake header may be removed
+// from `s.reqBuf`.
+func (s *tlsStream) tlsServerHelloPreprocess() utils.LSMAction {
+	// header size: content type (1 byte) + legacy protocol version (2 byte) +
+	//   + content length (2 byte) + message type (1 byte) +
+	//   + handshake length (3 byte)
+	const headersSize = 9
+
+	// minimal data size: server version (2 byte) + random (32 byte) +
+	//	 + session ID (>=1 byte) + cipher suite (2 byte) +
+	//	 + compression method (1 byte) + no extensions
+	const minDataSize = 38
+
+	header, ok := s.respBuf.Get(headersSize, true)
 	if !ok {
+		// not a full header yet
 		return utils.LSMActionPause
 	}
-	if data[0] != 0x16 || data[5] != 0x02 {
-		// Not a TLS handshake, or not a server hello
+
+	if header[0] != internal.RecordTypeHandshake || header[5] != internal.TypeServerHello {
 		return utils.LSMActionCancel
 	}
-	s.serverHelloLen = int(data[6])<<16 | int(data[7])<<8 | int(data[8])
-	if s.serverHelloLen < 38 {
-		// 2 (Protocol Version) +
-		// 32 (Random) +
-		// 1 (Session ID Length) +
-		// 2 (Cipher Suite) +
-		// 1 (Compression Method) +
-		// No extensions
-		// This should be the bare minimum for a server hello
+
+	s.serverHelloLen = int(header[6])<<16 | int(header[7])<<8 | int(header[8])
+	if s.serverHelloLen < minDataSize {
 		return utils.LSMActionCancel
 	}
+
+	// TODO: something is missing. See example:
+	//   const messageHeaderSize = 4
+	//   fullMessageLen := int(header[3])<<8 | int(header[4])
+	//   msgNo := fullMessageLen / int(messageHeaderSize+s.serverHelloLen)
+	//   if msgNo != 1 {
+	// 	   // what here?
+	//   }
+	//   if messageNo != int(messageNo) {
+	// 	   // what here?
+	//   }
+
 	return utils.LSMActionNext
 }

-func (s *tlsStream) parseClientHello() utils.LSMAction {
+// parseClientHelloData converts valid ClientHello message data (without
+// headers) into `analyzer.PropMap`.
+//
+// Parsing error may leave `s.reqBuf` in an unusable state.
+func (s *tlsStream) parseClientHelloData() utils.LSMAction {
 	chBuf, ok := s.reqBuf.GetSubBuffer(s.clientHelloLen, true)
 	if !ok {
 		// Not a full client hello yet
 		return utils.LSMActionPause
 	}
-	m := internal.ParseTLSClientHello(chBuf)
+	m := internal.ParseTLSClientHelloMsgData(chBuf)
 	if m == nil {
 		return utils.LSMActionCancel
 	} else {
@ -153,13 +197,17 @@ func (s *tlsStream) parseClientHello() utils.LSMAction {
 	}
 }

-func (s *tlsStream) parseServerHello() utils.LSMAction {
+// parseServerHelloData converts valid ServerHello message data (without
+// headers) into `analyzer.PropMap`.
+//
+// Parsing error may leave `s.respBuf` in an unusable state.
+func (s *tlsStream) parseServerHelloData() utils.LSMAction {
 	shBuf, ok := s.respBuf.GetSubBuffer(s.serverHelloLen, true)
 	if !ok {
 		// Not a full server hello yet
 		return utils.LSMActionPause
 	}
-	m := internal.ParseTLSServerHello(shBuf)
+	m := internal.ParseTLSServerHelloMsgData(shBuf)
 	if m == nil {
 		return utils.LSMActionCancel
 	} else {
--- a/analyzer/udp/quic.go
+++ b/analyzer/udp/quic.go
@ -36,41 +36,40 @@ type quicStream struct {
 }

 func (s *quicStream) Feed(rev bool, data []byte) (u *analyzer.PropUpdate, done bool) {
+	// minimal data size: protocol version (2 bytes) + random (32 bytes) +
+	//   + session ID (1 byte) + cipher suites (4 bytes) +
+	//   + compression methods (2 bytes) + no extensions
+	const minDataSize = 41
+
 	if rev {
 		// We don't support server direction for now
 		s.invalidCount++
 		return nil, s.invalidCount >= quicInvalidCountThreshold
 	}
+
 	pl, err := quic.ReadCryptoPayload(data)
-	if err != nil || len(pl) < 4 {
+	if err != nil || len(pl) < 4 { // FIXME: isn't length checked inside quic.ReadCryptoPayload? Also, what about error handling?
 		s.invalidCount++
 		return nil, s.invalidCount >= quicInvalidCountThreshold
 	}
-	// Should be a TLS client hello
-	if pl[0] != 0x01 {
-		// Not a client hello
+
+	if pl[0] != internal.TypeClientHello {
 		s.invalidCount++
 		return nil, s.invalidCount >= quicInvalidCountThreshold
 	}
+
 	chLen := int(pl[1])<<16 | int(pl[2])<<8 | int(pl[3])
-	if chLen < 41 {
-		// 2 (Protocol Version) +
-		// 32 (Random) +
-		// 1 (Session ID Length) +
-		// 2 (Cipher Suites Length) +_ws.col.protocol == "TLSv1.3"
-		// 2 (Cipher Suite) +
-		// 1 (Compression Methods Length) +
-		// 1 (Compression Method) +
-		// No extensions
-		// This should be the bare minimum for a client hello
+	if chLen < minDataSize {
 		s.invalidCount++
 		return nil, s.invalidCount >= quicInvalidCountThreshold
 	}
-	m := internal.ParseTLSClientHello(&utils.ByteBuffer{Buf: pl[4:]})
+
+	m := internal.ParseTLSClientHelloMsgData(&utils.ByteBuffer{Buf: pl[4:]})
 	if m == nil {
 		s.invalidCount++
 		return nil, s.invalidCount >= quicInvalidCountThreshold
 	}
+
 	return &analyzer.PropUpdate{
 		Type: analyzer.PropUpdateMerge,
 		M:    analyzer.PropMap{"req": m},