Restrict global and function references to memory size

gen/operand.go

@@ -12,10 +12,10 @@ func randOperand(t vm.OperandNumericType) *vm.Operand {
 
 		switch op.Type {
 		case vm.GlobalMemoryIndex:
-			op.Value = RandBiasedUint64()
+			op.Value = RandBiasedUint64n(vm.GlobalMemorySize)
 
 		case vm.FunctionMemoryIndex:
-			op.Value = RandBiasedUint64()
+			op.Value = RandBiasedUint64n(vm.FunctionMemorySize)
 
 		case vm.Literal:
 			switch t {

gen/rand.go

@@ -9,6 +9,16 @@ func RandBiasedUint64() uint64 {
 	return (rand.Uint64() | 0x8000000000000000) >> rand.Intn(65)
 }
 
+// Like RandBiasedUint64() but always returns < n
+func RandBiasedUint64n(n uint64) uint64 {
+	for {
+		ret := RandBiasedUint64()
+		if ret < n {
+			return ret
+		}
+	}
+}
+
 // Generate a random int64 with an even distribution of the tuple
 // {sign, bits.Len64(math.Abs())}
 func RandBiasedInt64() uint64 {