Add TCP checksum verification.

PiperOrigin-RevId: 242704699
Change-Id: I87db368ca343b3b4bf4f969b17d3aa4ce2f8bd4f

1 files changed, 32 insertions, 36 deletions

diff --git a/pkg/tcpip/header/tcp.go b/pkg/tcpip/header/tcp.go
index 6e3ee2e50..e656ebb15 100644
--- a/pkg/tcpip/header/tcp.go
+++ b/pkg/tcpip/header/tcp.go
@@ -22,16 +22,17 @@ import (
 	"gvisor.googlesource.com/gvisor/pkg/tcpip/seqnum"
 )
 
+// These constants are the offsets of the respective fields in the TCP header.
 const (
-	srcPort     = 0
-	dstPort     = 2
-	seqNum      = 4
-	ackNum      = 8
-	dataOffset  = 12
-	tcpFlags    = 13
-	winSize     = 14
-	tcpChecksum = 16
-	urgentPtr   = 18
+	TCPSrcPortOffset   = 0
+	TCPDstPortOffset   = 2
+	TCPSeqNumOffset    = 4
+	TCPAckNumOffset    = 8
+	TCPDataOffset      = 12
+	TCPFlagsOffset     = 13
+	TCPWinSizeOffset   = 14
+	TCPChecksumOffset  = 16
+	TCPUrgentPtrOffset = 18
 )
 
 const (
@@ -179,27 +180,27 @@ const (
 
 // SourcePort returns the "source port" field of the tcp header.
 func (b TCP) SourcePort() uint16 {
-	return binary.BigEndian.Uint16(b[srcPort:])
+	return binary.BigEndian.Uint16(b[TCPSrcPortOffset:])
 }
 
 // DestinationPort returns the "destination port" field of the tcp header.
 func (b TCP) DestinationPort() uint16 {
-	return binary.BigEndian.Uint16(b[dstPort:])
+	return binary.BigEndian.Uint16(b[TCPDstPortOffset:])
 }
 
 // SequenceNumber returns the "sequence number" field of the tcp header.
 func (b TCP) SequenceNumber() uint32 {
-	return binary.BigEndian.Uint32(b[seqNum:])
+	return binary.BigEndian.Uint32(b[TCPSeqNumOffset:])
 }
 
 // AckNumber returns the "ack number" field of the tcp header.
 func (b TCP) AckNumber() uint32 {
-	return binary.BigEndian.Uint32(b[ackNum:])
+	return binary.BigEndian.Uint32(b[TCPAckNumOffset:])
 }
 
 // DataOffset returns the "data offset" field of the tcp header.
 func (b TCP) DataOffset() uint8 {
-	return (b[dataOffset] >> 4) * 4
+	return (b[TCPDataOffset] >> 4) * 4
 }
 
 // Payload returns the data in the tcp packet.
@@ -209,32 +210,32 @@ func (b TCP) Payload() []byte {
 
 // Flags returns the flags field of the tcp header.
 func (b TCP) Flags() uint8 {
-	return b[tcpFlags]
+	return b[TCPFlagsOffset]
 }
 
 // WindowSize returns the "window size" field of the tcp header.
 func (b TCP) WindowSize() uint16 {
-	return binary.BigEndian.Uint16(b[winSize:])
+	return binary.BigEndian.Uint16(b[TCPWinSizeOffset:])
 }
 
 // Checksum returns the "checksum" field of the tcp header.
 func (b TCP) Checksum() uint16 {
-	return binary.BigEndian.Uint16(b[tcpChecksum:])
+	return binary.BigEndian.Uint16(b[TCPChecksumOffset:])
 }
 
 // SetSourcePort sets the "source port" field of the tcp header.
 func (b TCP) SetSourcePort(port uint16) {
-	binary.BigEndian.PutUint16(b[srcPort:], port)
+	binary.BigEndian.PutUint16(b[TCPSrcPortOffset:], port)
 }
 
 // SetDestinationPort sets the "destination port" field of the tcp header.
 func (b TCP) SetDestinationPort(port uint16) {
-	binary.BigEndian.PutUint16(b[dstPort:], port)
+	binary.BigEndian.PutUint16(b[TCPDstPortOffset:], port)
 }
 
 // SetChecksum sets the checksum field of the tcp header.
 func (b TCP) SetChecksum(checksum uint16) {
-	binary.BigEndian.PutUint16(b[tcpChecksum:], checksum)
+	binary.BigEndian.PutUint16(b[TCPChecksumOffset:], checksum)
 }
 
 // CalculateChecksum calculates the checksum of the tcp segment.
@@ -258,20 +259,20 @@ func (b TCP) ParsedOptions() TCPOptions {
 }
 
 func (b TCP) encodeSubset(seq, ack uint32, flags uint8, rcvwnd uint16) {
-	binary.BigEndian.PutUint32(b[seqNum:], seq)
-	binary.BigEndian.PutUint32(b[ackNum:], ack)
-	b[tcpFlags] = flags
-	binary.BigEndian.PutUint16(b[winSize:], rcvwnd)
+	binary.BigEndian.PutUint32(b[TCPSeqNumOffset:], seq)
+	binary.BigEndian.PutUint32(b[TCPAckNumOffset:], ack)
+	b[TCPFlagsOffset] = flags
+	binary.BigEndian.PutUint16(b[TCPWinSizeOffset:], rcvwnd)
 }
 
 // Encode encodes all the fields of the tcp header.
 func (b TCP) Encode(t *TCPFields) {
 	b.encodeSubset(t.SeqNum, t.AckNum, t.Flags, t.WindowSize)
-	binary.BigEndian.PutUint16(b[srcPort:], t.SrcPort)
-	binary.BigEndian.PutUint16(b[dstPort:], t.DstPort)
-	b[dataOffset] = (t.DataOffset / 4) << 4
-	binary.BigEndian.PutUint16(b[tcpChecksum:], t.Checksum)
-	binary.BigEndian.PutUint16(b[urgentPtr:], t.UrgentPointer)
+	binary.BigEndian.PutUint16(b[TCPSrcPortOffset:], t.SrcPort)
+	binary.BigEndian.PutUint16(b[TCPDstPortOffset:], t.DstPort)
+	b[TCPDataOffset] = (t.DataOffset / 4) << 4
+	binary.BigEndian.PutUint16(b[TCPChecksumOffset:], t.Checksum)
+	binary.BigEndian.PutUint16(b[TCPUrgentPtrOffset:], t.UrgentPointer)
 }
 
 // EncodePartial updates a subset of the fields of the tcp header. It is useful
@@ -290,18 +291,13 @@ func (b TCP) EncodePartial(partialChecksum, length uint16, seqnum, acknum uint32
 	b.encodeSubset(seqnum, acknum, flags, rcvwnd)
 
 	// Add the contributions of the passed-in fields to the checksum.
-	checksum = Checksum(b[seqNum:seqNum+8], checksum)
-	checksum = Checksum(b[winSize:winSize+2], checksum)
+	checksum = Checksum(b[TCPSeqNumOffset:TCPSeqNumOffset+8], checksum)
+	checksum = Checksum(b[TCPWinSizeOffset:TCPWinSizeOffset+2], checksum)
 
 	// Encode the checksum.
 	b.SetChecksum(^checksum)
 }
 
-// TCPChecksumOffset returns offset of the checksum field.
-func TCPChecksumOffset() uint16 {
-	return tcpChecksum
-}
-
 // ParseSynOptions parses the options received in a SYN segment and returns the
 // relevant ones. opts should point to the option part of the TCP Header.
 func ParseSynOptions(opts []byte, isAck bool) TCPSynOptions {