FLP03-C. Detect and handle floating point errors

Errors during floating-point operations are often neglected by programmers who instead focus on validating operands before an operation. Errors occurring during floating-point operations are admittedly difficult to determine and diagnose, but the benefits of doing so often outweighs the costs. This recommendation suggests ways to capture errors during floating-point operations.

The following code exhibits undefined behavior:

int j = 0;
int iResult = 1 / j;

On most implementations, integer division by zero is a terminal error, commonly printing a diagnostic message and aborting the program.

double x = 0.0;
double dResult = 1 / x;

Floating-point division by zero also results in undefined behavior, although most implementations do not treat this as a terminal error. If additional precautions are not taken, this results in a silent error.

The most portable way of determining a floating-point exceptional condition has occurred is to use the floating-point exception facilities provided by C99 in fenv.h [[ISO/IEC 9899:1999]].

However, the C99 floating-point exception functions are not without problems. The following caveats exist regarding the interaction between floating-point exceptions and conversions:

• Conversion from floating-point to integer may cause an "invalid" floating-point exception. If this occurs, the value of that integer is undefined and should not be used.
• Most implementations fail to raise "invalid" for conversions from any negative or "large" positive floating-point values to unsigned integer types or to signed char (see tflt2int.c).
• When a noninteger floating-point value is converted to an integer the "inexact" floating-point exception is raised.

For information regarding floating-point number conversions, see FLP34-C. Ensure that floating point conversions are within range of the new type.

C99 does not require all implementations to support floating-point exceptions. Each exception macro in fenv.h is defined if and only if the corresponding exception is supported. Only implementations that use IEC 60559 (formerly IEEE-754) floating-point arithmetic are required to support all five exceptions defined by C99. Nevertheless, these functions are the most portable solution for handling floating-point exceptions.

A less portable but potentially more secure solution is to use the capabilities provided by the underlying implementation. If this approach is taken, the caveats of that system need to be well understood. Table 6-1 can serve as a starting point for some common operating systems:

Noncompliant Code Example

In this noncompliant code example, floating-point operations are performed without checking for errors. Note that range checking has been intentionally omitted because the intent is to detect errors following the floating-point operation.

void fpOper_noErrorChecking(void) {
  /* ... */
  double a = 1e-40, b, c = 0.1;
  float x = 0, y;
  /* inexact and underflows */
  y = a;
  /* divide by zero operation */
  b = y / x;
  /* inexact (loss of precision) */
  c = sin(30) * a;
  /* ... */
}

However, exceptional conditions (as indicated by the comments)occur that may lead to unexpected arithmetic results.

Compliant Solution (C99)

This compliant solution uses C99 standard functions to handle floating-point errors.

#include <fenv.h>
#pragma STDC FENV_ACCESS ON

void fpOper_fenv(void) {
  double a = 1e-40, b, c = 0.1;
  float x = 0, y;
  int fpeRaised;
  /* ... */

  feclearexcept(FE_ALL_EXCEPT);
  /* Store a into y is inexact and underflows: */
  y = a;
  fpeRaised = fetestexcept(FE_ALL_EXCEPT);
  /* fpeRaised  has FE_INEXACT and FE_UNDERFLOW */

  feclearexcept(FE_ALL_EXCEPT);

  /* divide by zero operation */
  b = y / x;
  fpeRaised = fetestexcept(FE_ALL_EXCEPT);
  /* fpeRaised has FE_DIVBYZERO */

  feclearexcept(FE_ALL_EXCEPT);

  c = sin(30) * a;
  fpeRaised = fetestexcept(FE_ALL_EXCEPT);
  /* fpeRaised has FE_INEXACT */

  feclearexcept(FE_ALL_EXCEPT);
  /* ... */
}

Compliant Solution (Windows)

Microsoft Visual Studio 2008 and earlier versions do not support C99 functions to handle floating-point errors. Windows provides an alternative method for handling floating-point errors using _statusfp(), _statusfp2(), and _clearfp().

void fpOper_usingStatus(void) {
  /* ... */
  double a = 1e-40, b, c;
  float x = 0, y;
  unsigned int rv = _clearfp();

  /* Store into y is inexact and underflows: */
  y = a;
  rv = _clearfp();  /* rv has _SW_INEXACT and _SW_UNDERFLOW */

  /* zero-divide */
  b = y / x; rv = _clearfp(); /* rv has _SW_ZERODIVIDE */

  /* inexact */
  c = sin(30) * a; rv = _clearfp(); /* rv has _SW_INEXACT */
  /* ... */
}

Compliant Solution (Windows SEH)

Microsoft Visual Studio 2008 also uses structured exception handling (SEH) to handle floating-point operation. SEH provides more information about the error and allows the programmer to change the results of the floating-point operation that caused the error condition.

void fp_usingSEH(void) {
  /* ... */
  double a = 1e-40, b, c = 0.1;
  float x = 0, y;
  unsigned int rv ;

  unmask_fpsr();

  _try {
    /* Store into y is inexact and underflows */
    y = a;

    /* divide by zero operation */
    b = y / x;

    /* inexact */
    c = sin(30) * a;
  }

  _except (_fpieee_flt(
             GetExceptionCode(),
             GetExceptionInformation(),
             fpieee_handler)) {
  {
	printf ("fpieee_handler: EXCEPTION_EXECUTE_HANDLER");
  }

  /* ... */
}

void unmask_fpsr(void) {
  unsigned int u;
  unsigned int control_word;
  _controlfp_s(&control_word, 0, 0);
  u = control_word & ~(_EM_INVALID
                     | _EM_DENORMAL
                     | _EM_ZERODIVIDE
                     | _EM_OVERFLOW
                     | _EM_UNDERFLOW
                     | _EM_INEXACT);
  _controlfp_s( &control_word, u, _MCW_EM);
  return ;
}

int fpieee_handler(_FPIEEE_RECORD *ieee) {
  /* ... */

  switch (ieee->RoundingMode) {
    case _FpRoundNearest:
      /* ... */
      break;

      /* Other RMs include _FpRoundMinusInfinity,
       * _FpRoundPlusInfinity, _FpRoundChopped */

      /* ... */
    }

  switch (ieee->Precision) {
    case _FpPrecision24:
      /* ... */
      break;

      /* Other Ps include _FpPrecision53*/
      /* ... */
    }

   switch (ieee->Operation) {
     case _FpCodeAdd:
       /* ... */
       break;

       /* Other Ops include _FpCodeSubtract, _FpCodeMultiply,
        * _FpCodeDivide, _FpCodeSquareRoot, _FpCodeCompare,
        * _FpCodeConvert, _FpCodeConvertTrunc */
       /* ... */
    }

  /* 
   * process the bitmap ieee->Cause.
   * process the bitmap ieee->Enable.
   * process the bitmap ieee->Status.
   * process the Operand ieee->Operand1, 
   * evaluate format and Value.
   * process the Operand ieee->Operand2, 
   * evaluate format and Value.
   * process the Result ieee->Result, 
   * evaluate format and Value .
   * The result should be set according to the operation 
   * specified in ieee->Cause and the result format as 
   * specified in ieee->Result.
   */

  /* ... */
}

Risk Assessment

Undetected floating-point exceptions may result in lower program efficiency, inaccurate results, or software vulnerabilities. Most processors stall for a significant duration (sometimes up to a second or even more on 32-bit desktop processors) when an operation incurs a NaN (not a number) value.

Automated Detection

Compass/ROSE could detect violations of this rule, by ensuring that floating point operations are surrounded by feclearexcept() and fetestexcept(). It would need to look for type conversions to float or double, divisions (by a number not known to be nonzero) and multiplication. It may be wisest to apply this to all floating point operations in general.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[IEEE 754]]
[[Intel 01]]
[[Keil 08]]
[[MITRE 07]] CWE ID 369, "Divide By Zero"
[[MSDN]] "fpieee_flt (CRT)"
[[Open Group 04]] "fenv.h - Floating-point environment"
[[SecurityFocus 07]]

FLP02-C. Consider avoiding floating point numbers when precise computation is needed      05. Floating Point (FLP)       FLP30-C. Do not use floating point variables as loop counters