FLP04-J. Check floating point inputs for exceptional values

Floating-point numbers can take on two kinds of exceptional values, infinity and NaN (not-a-number). These values are produced as a result of exceptional or otherwise unresolvable floating point operations. Additionally, they may be obtained directly from user input through methods like Double.valueOf(String s). Failure to detect and handle such values can result in inconsistent behavior.

NaN values are particularly problematic because the expression NaN == NaN always returns false (See FLP02-J. Do not attempt comparisons with NaN). In general, any comparisons with NaN return false, and all arithmetic functions on NaN inputs simply propagate the taint throughout the code. Just one occurrence of a NaN value can effectuate regressions within other code segments.

The method Double.valueOf(String s) can return NaN or an infinite double as specified by its contract. Programs should install checks to ensure that all floating point inputs (especially those obtained from the user) do not contain either of these values before proceeding to operate on them. The methods Double.isNaN(double d) and Double.isInfinite(double d) can be used for this purpose.

Noncompliant Code Example

This noncompliant code example accepts user data without validating it.

double currentBalance; // User's cash balance

void doDeposit(String userInput){
  double val;
  try {
    val = Double.valueOf(userInput);
  }
  catch(NumberFormatException e) {
    // Handle input format error
  }

  if(val >= Double.MAX_VALUE - currentBalance) {
    // Handle range error
  }

  currentBalance += val;
}

This can be a problem if an invalid value is entered for val and subsequently used in calculations or as control values. The user could, for example, input the strings infinity or NaN on the command line, which would be parsed by Double.valueOf(String s) into the floating-point representations of either infinity or NaN. All subsequent calculations using these values would be invalid, possibly causing runtime exceptions or enabling denial of service (DoS) attacks.

In this compliant solution, entering NaN for val would force currentBalance to also equal NaN, corrupting its value. If this value is used in other expressions, every resulting value will become NaN, possibly corrupting important data.

Compliant Solution

This compliant solution validates the floating point input before using it. The value is tested to ensure that it is neither infinity, negative infinity nor NaN.

double currentBalance; // User's cash balance

void doDeposit(String s){
  double val;
  try {
    val = Double.valueOf(userInput);
  }
  catch(NumberFormatException e) {
    // Handle input format error
  }

  if (Double.isInfinite(val)){
    // Handle infinity error
  }

  if (Double.isNaN(val)) {
    // Handle NaN error
  }

  if(val >= Double.MAX_VALUE - currentBalance) {
    // Handle range error
  }
  currentBalance += val;
}

Exceptions

EX1: Occasionally, NaN or infinity may be acceptable as expected inputs to a program. In such cases, explicit checks may not be necessary. However, such programs must be prepared to handle these inputs gracefully and should not allow the propagation of taint to other values by using them in mathematical expressions where they are inappropriate.

Risk Assessment

Incorrect or missing validation of floating point input can result in miscalculations and unexpected results, possibly leading to inconsistent program behavior and Denial of Service (DoS).