Handle truncated start login frame (#400)

Author: itzg

mcproto/decode.go

@@ -55,12 +55,12 @@ func DecodeLoginStart(data interface{}) (*LoginStart, error) {
 
 	loginStart.Name, err = ReadString(buffer)
 	if err != nil {
-		return nil, errors.Wrap(err, "failed to read username")
+		return loginStart, errors.Wrap(err, "failed to read username")
 	}
 
 	loginStart.PlayerUuid, err = ReadUuid(buffer)
 	if err != nil {
-		return nil, errors.Wrap(err, "failed to read player uuid")
+		return loginStart, errors.Wrap(err, "failed to read player uuid")
 	}
 
 	return loginStart, nil

mcproto/read.go

@@ -1,3 +1,5 @@
+// Package mcproto provides functions to read types and decode frames declared
+// at https://minecraft.wiki/w/Java_Edition_protocol
 package mcproto
 
 import (
@@ -27,7 +29,7 @@ const MaxFrameLength = 2097151
 func ReadPacket(reader *bufio.Reader, addr net.Addr, state State) (*Packet, error) {
 	logrus.
 		WithField("client", addr).
-		Debug("Reading packet")
+		Trace("Reading packet")
 
 	if state == StateHandshaking {
 		data, err := reader.Peek(1)
@@ -158,7 +160,7 @@ func ReadUTF16BEString(reader io.Reader, symbolLen uint16) (string, error) {
 func ReadFrame(reader io.Reader, addr net.Addr) (*Frame, error) {
 	logrus.
 		WithField("client", addr).
-		Debug("Reading frame")
+		Trace("Reading frame")
 
 	var err error
 	frame := &Frame{}

server/connector.go

@@ -4,6 +4,7 @@ import (
 	"bufio"
 	"bytes"
 	"context"
+	"errors"
 	"fmt"
 	"github.com/google/uuid"
 	"io"
@@ -227,7 +228,7 @@ func (c *Connector) HandleConnection(ctx context.Context, frontendConn net.Conn)
 
 	logrus.
 		WithField("client", clientAddr).
-		Info("Got connection")
+		Debug("Got connection")
 	defer logrus.WithField("client", clientAddr).Debug("Closing frontend connection")
 
 	// Tee-off the inspected content to a buffer so that we can retransmit it to the backend connection
@@ -273,14 +274,22 @@ func (c *Connector) HandleConnection(ctx context.Context, frontendConn net.Conn)
 
 		var playerInfo *PlayerInfo = nil
 		if handshake.NextState == mcproto.StateLogin {
-			playerInfo, err = c.readUserInfo(bufferedReader, clientAddr, handshake.NextState)
+			playerInfo, err = c.readPlayerInfo(bufferedReader, clientAddr, handshake.NextState)
 			if err != nil {
-				logrus.
-					WithError(err).
-					WithField("clientAddr", clientAddr).
-					Error("Failed to read user info")
-				c.metrics.Errors.With("type", "read").Add(1)
-				return
+				if errors.Is(err, io.EOF) {
+					logrus.
+						WithError(err).
+						WithField("clientAddr", clientAddr).
+						WithField("player", playerInfo).
+						Warn("Truncated buffer while reading player info")
+				} else {
+					logrus.
+						WithError(err).
+						WithField("clientAddr", clientAddr).
+						Error("Failed to read user info")
+					c.metrics.Errors.With("type", "read").Add(1)
+					return
+				}
 			}
 			logrus.
 				WithField("client", clientAddr).
@@ -319,7 +328,7 @@ func (c *Connector) HandleConnection(ctx context.Context, frontendConn net.Conn)
 	}
 }
 
-func (c *Connector) readUserInfo(bufferedReader *bufio.Reader, clientAddr net.Addr, state mcproto.State) (*PlayerInfo, error) {
+func (c *Connector) readPlayerInfo(bufferedReader *bufio.Reader, clientAddr net.Addr, state mcproto.State) (*PlayerInfo, error) {
 	loginPacket, err := mcproto.ReadPacket(bufferedReader, clientAddr, state)
 	if err != nil {
 		return nil, fmt.Errorf("failed to read login packet: %w", err)