Fix netlink TLV parsing for FOU

This fixes a bug that netlink messages for FOU are not correctly deserialized

Fix unused var

shift

Fix incorrect test case

Author: martin31821

fou_linux.go

@@ -1,3 +1,4 @@
+//go:build linux
 // +build linux
 
 package netlink
@@ -171,36 +172,40 @@ func (h *Handle) FouList(fam int) ([]Fou, error) {
 func deserializeFouMsg(msg []byte) (Fou, error) {
 	// we'll skip to byte 4 to first attribute
 	msg = msg[3:]
-	var shift int
 	fou := Fou{}
 
 	for {
-		// attribute header is at least 16 bits
+		// attribute header is at least 32 bits (16 bit type + 16 bit length)
 		if len(msg) < 4 {
 			return fou, ErrAttrHeaderTruncated
 		}
 
 		lgt := int(binary.BigEndian.Uint16(msg[0:2]))
-		if len(msg) < lgt+4 {
+		lgt4 := lgt & (^0x3)
+
+		// Padding to 4 bytes according to netlink man7
+		lgtPad := lgt4 + 4
+		if lgt4 == lgt {
+			lgtPad = lgt
+		}
+
+		if len(msg) < lgtPad {
 			return fou, ErrAttrBodyTruncated
 		}
 		attr := binary.BigEndian.Uint16(msg[2:4])
 
-		shift = lgt + 3
 		switch attr {
 		case FOU_ATTR_AF:
 			fou.Family = int(msg[5])
 		case FOU_ATTR_PORT:
 			fou.Port = int(binary.BigEndian.Uint16(msg[5:7]))
-			// port is 2 bytes
-			shift = lgt + 2
 		case FOU_ATTR_IPPROTO:
 			fou.Protocol = int(msg[5])
 		case FOU_ATTR_TYPE:
 			fou.EncapType = int(msg[5])
 		}
 
-		msg = msg[shift:]
+		msg = msg[lgtPad:]
 
 		if len(msg) < 4 {
 			break

fou_test.go

@@ -1,3 +1,4 @@
+//go:build linux
 // +build linux
 
 package netlink
@@ -42,7 +43,7 @@ func TestFouDeserializeMsg(t *testing.T) {
 	}
 
 	// deserialize truncated attribute header
-	msg = []byte{3, 1, 0, 0, 5, 0, 2, 0, 2, 0, 0}
+	msg = []byte{3, 1, 0, 0, 5, 0, 2, 0, 2, 0}
 	if _, err := deserializeFouMsg(msg); err == nil {
 		t.Error("expected attribute body truncated error")
 	} else if err != ErrAttrBodyTruncated {