netstack: Don't exclude length when a pseudo-header checksum is calculated

This is a preparation for GSO changes (cl/234508902).

RELNOTES[gofers]: Refactor checksum code to include length, which
it already did, but in a convoluted way. Should be a no-op.

PiperOrigin-RevId: 240460794
Change-Id: I537381bc670b5a9f5d70a87aa3eb7252e8f5ace2

1 files changed, 12 insertions, 5 deletions

diff --git a/pkg/tcpip/header/checksum.go b/pkg/tcpip/header/checksum.go
index 2c2fcbd9b..2e8c65fac 100644
--- a/pkg/tcpip/header/checksum.go
+++ b/pkg/tcpip/header/checksum.go
@@ -17,6 +17,8 @@
 package header
 
 import (
+	"encoding/binary"
+
 	"gvisor.googlesource.com/gvisor/pkg/tcpip"
 	"gvisor.googlesource.com/gvisor/pkg/tcpip/buffer"
 )
@@ -76,12 +78,17 @@ func ChecksumCombine(a, b uint16) uint16 {
 	return uint16(v + v>>16)
 }
 
-// PseudoHeaderChecksum calculates the pseudo-header checksum for the
-// given destination protocol and network address, ignoring the length
-// field. Pseudo-headers are needed by transport layers when calculating
-// their own checksum.
-func PseudoHeaderChecksum(protocol tcpip.TransportProtocolNumber, srcAddr tcpip.Address, dstAddr tcpip.Address) uint16 {
+// PseudoHeaderChecksum calculates the pseudo-header checksum for the given
+// destination protocol and network address. Pseudo-headers are needed by
+// transport layers when calculating their own checksum.
+func PseudoHeaderChecksum(protocol tcpip.TransportProtocolNumber, srcAddr tcpip.Address, dstAddr tcpip.Address, totalLen uint16) uint16 {
 	xsum := Checksum([]byte(srcAddr), 0)
 	xsum = Checksum([]byte(dstAddr), xsum)
+
+	// Add the length portion of the checksum to the pseudo-checksum.
+	tmp := make([]byte, 2)
+	binary.BigEndian.PutUint16(tmp, totalLen)
+	xsum = Checksum(tmp, xsum)
+
 	return Checksum([]byte{0, uint8(protocol)}, xsum)
 }