An attempt to do a division or modulo operation using zero as the divisor causes a runtime error. Division by zero defects often occur due to ineffective error handling or race conditions, and typically cause abnormal program termination. Before a value is used as the divisor of a division or modulo operation in C/C++ code, it must be checked to confirm that it is not equal to zero.

The DBZ checkers look for instances in which a zero constant value is used as the divisor of a division or modulo operation.

The DBZ.CONST.CALL checker flags situations in which an explicit zero constant value is passed directly to a function call and might be used as a divisor of a division or modulo operation without checking it for the zero value.

Vulnerability and risk

Integer division by zero usually result in the failure of the process or an exception. It can also result in success of the operation, but gives an erroneous answer. Floating-point division by zero is more subtle. It depends on the implementation of the compiler. If the compiler is following the IEEE floating-point standard (IEEE 754), then the result of the floating-point division by zero has a well-defined result. However, the C and C++ standards do not enforce compliance to IEEE 754. Thus, floating-point division by zero has an undefined behavior in C and C++ and might result in the failure of the process or an exception.

Division by zero issues typically occur due to ineffective exception handling. To avoid this vulnerability, check for a zero value before using it as the divisor of a division or modulo operation.

Vulnerable code example

1  int compute_mean(int array[], size_t size)
2  {
3      int sum = 0;
4      for (size_t i = 0; i < size; ++i) {
5          sum += array[i];
6      }
7      return sum / size;
8  }
9
10 void use_mean()
11 {     
12     int mean = compute_mean(0, 0);
13 }

Klocwork produces an issue report at line 11 indicating that the zero constant value is passed directly to the function ‘compute_mean’ and it is used in this function as the divisor of the division operation on line 7. A division by zero can produce unexpected and unintended results.

Fixed code example

1  int compute_mean(int array[], size_t size)
2  {
3      if (size == 0) {
4          return 0;   // or exceptional case.
5      }
6
7      int sum = 0;
8      for (size_t i = 0; i < size; ++i) {
9          sum += array[i];
10     }    
11     return sum / size;
12 } 
13
14 void use_mean()
15 {
16     int mean = compute_mean(0, 0);
17 }

The problem from the previous snippet is fixed: the input variable ‘size’ is checked for the exceptional case of the zero constant value in line 3 and prevents the division from happening in this specific case.

Related checkers

External guidance

CWE-369: Divide By Zero

Extension

This checker can be extended through the Klocwork knowledge base. See Tuning C/C++ analysis for more information.

Limitations

This checker only deals with integral types, not floating-point types. This means that defects involving floating-point types will not be detected by this checker. For example:

1  void do_dbz(double divisor)
2  {    
3      double doit = 23 / divisor;     // Divide by zero will not be detected here since it is a floating-point type.
4  }
5
6  void call_do_dbz()
7  {   
8      do_dbz(0.0);    // Divide by zero not detected by this checker.
9  }