Correct validation on bitlist Marshalling

encode.go

@@ -233,6 +233,26 @@ func ValidateBitlist(buf []byte, bitLimit uint64) error {
 	return nil
 }
 
+// BitlistLen provides the bitlist length of a byte array
+func BitlistLen(b []byte) int {
+	if len(b) == 0 {
+		return 0
+	}
+	// The most significant bit is present in the last byte in the array.
+	last := b[len(b)-1]
+
+	// Determine the position of the most significant bit.
+	msb := bits.Len8(last)
+	if msb == 0 {
+		return 0
+	}
+
+	// The absolute position of the most significant bit will be the number of
+	// bits in the preceding bytes plus the position of the most significant
+	// bit. Subtract this value by 1 to determine the length of the bitlist.
+	return 8*(len(b)-1) + msb - 1
+}
+
 // DecodeDynamicLength decodes the length from the dynamic input
 func DecodeDynamicLength(buf []byte, maxSize int) (int, error) {
 	if len(buf) == 0 {

sszgen/generator/validate.go

@@ -2,7 +2,7 @@ package generator
 
 func (v *Value) validate() string {
 	switch v.t {
-	case TypeBitList, TypeBytes:
+	case TypeBytes:
 		// this is a fixed-length array, not a slice, so it's size is a constant we don't need to check
 		if v.c {
 			return ""
@@ -25,6 +25,29 @@ func (v *Value) validate() string {
 			"size": v.s,
 		})
 
+	case TypeBitList:
+		// this is a fixed-length array, not a slice, so it's size is a constant we don't need to check
+		if v.c {
+			return ""
+		}
+		// for fixed size collections, we need to ensure the size is an exact match
+		cmp := "!="
+		// for variable size values, we want to ensure it doesn't exceed max size bound
+		if !v.isFixed() {
+			cmp = ">"
+		}
+
+		tmpl := `if size := ssz.BitlistLen(::.{{.name}}); size {{.cmp}} {{.size}} {
+			err = ssz.ErrBytesLengthFn("--.{{.name}}", size, {{.size}})
+			return
+		}
+		`
+		return execTmpl(tmpl, map[string]interface{}{
+			"cmp":  cmp,
+			"name": v.name,
+			"size": v.s,
+		})
+
 	case TypeVector:
 		// this is a fixed-length array, not a slice, so it's size is a constant we don't need to check
 		if v.c {