CERT C: Rule INT33-C

Integer division by zero occurs when the denominator of a division or modulo operation can be a zero-valued integer.

A division by zero can result in a program crash.

The fix depends on the root cause of the defect. Often the result details show a sequence of events that led to the defect. Use this event list to determine how the denominator variable acquires a zero value. You can implement the fix on any event in the sequence. If the result details do not show the event history, you can trace back using right-click options in the source code and see previous related events. See also Interpret Bug Finder Results in Polyspace Desktop User Interface.

It is a good practice to check for zero values of a denominator before division and handle the error. Instead of performing the division directly:

res = num/den;

res = div(num, den);

See examples of fixes below.

If you do not want to fix the issue, add comments to your result or code to avoid another review. See:

int fraction(int num)
{
    int denom = 0;
    int result = 0;

    result = num/denom; //Noncompliant

    return result;
}

A division by zero error occurs at num/denom because denom is zero.

int fraction(int num)
{
    int denom = 0;
    int result = 0;

    if (denom != 0)
        result = num/denom;

    return result;
}

Before dividing, add a test to see if the denominator is zero, checking before division occurs. If denom is always zero, this correction can produce a dead code defect in your Polyspace^® results.

One possible correction is to change the denominator value so that denom is not zero.

int fraction(int num)
{
    int denom = 2;
    int result = 0;

    result = num/denom;

    return result;
}

int mod_arr(int input)
{
    int arr[5];
    for(int i = 0; i < 5; i++)
    {
        arr[i] = input % i; //Noncompliant
    }

    return arr[0]+arr[1]+arr[2]+arr[3]+arr[4];
}

In this example, Polyspace flags the modulo operation as a division by zero. Because modulo is inherently a division operation, the divisor (right hand argument) cannot be zero. The modulo operation uses the for loop index as the divisor. However, the for loop starts at zero, which cannot be an iterator.

One possible correction is checking the divisor before the modulo operation. In this example, see if the index i is zero before the modulo operation.

int mod_arr(int input)
{
    int arr[5];
    for(int i = 0; i < 5; i++)
    {
        if(i != 0)
        {
             arr[i] = input % i;
        }
        else
        {
             arr[i] = input;
        }
    }

    return arr[0]+arr[1]+arr[2]+arr[3]+arr[4];
}

Another possible correction is changing the divisor to a nonzero integer. In this example, add one to the index before the % operation to avoid dividing by zero.

int mod_arr(int input)
{
    int arr[5];
    for(int i = 0; i < 5; i++)
    {
         arr[i] = input % (i+1);
    }

    return arr[0]+arr[1]+arr[2]+arr[3]+arr[4];
}

Tainted division operand detects division operations where one or both of the integer operands is from an unsecure source.

These risks can be used to execute arbitrary code. This code is usually outside the scope of a program's implicit security policy.

Before performing the division, validate the values of the operands. Check for denominators of 0 or -1, and numerators of the minimum integer value.

#include<stdio.h>
extern void print_int(int);

int taintedintdivision(void) {
    int num, den;
	scanf("%lf %lf",&num, &den);
	int r =  num/den;  //Noncompliant
    print_int(r);
    return r;
}

This example function divides two argument variables, then prints and returns the result. The argument values are unknown and can cause division by zero or integer overflow.

One possible correction is to check the values of the numerator and denominator before performing the division.

#include<stdio.h>
#include <limits.h> 
extern void print_int(int);

int taintedintdivision(void) {
	int num, den;
	scanf("%lf %lf",&num, &den);
	int r = 0;
	if (den!=0 && !(num=INT_MIN && den==-1)){
		r =  num/den; 	
	}
	print_int(r);
	return r;
}

Tainted modulo operand checks the operands of remainder % operations. Bug Finder flags modulo operations with one or more tainted operands.

These risks can be exploited by attackers to gain access to your program or the target in general.

Before performing the modulo operation, validate the values of the operands. Check the second operand for values of 0 and -1. Check both operands for negative values.

#include<stdio.h>
extern void print_int(int);

int taintedintmod(void) {
	int userden;
	scanf("%d",&userden);
    int rem =  128%userden;  //Noncompliant
    print_int(rem);
    return rem;
}

In this example, the function performs a modulo operation by using a user input. The input is not checked before calculating the remainder for values that can crash the program, such as 0 and -1.

One possible correction is to check the values of the operands before performing the modulo operation. In this corrected example, the modulo operation continues only if the second operand is greater than zero.

#include<stdio.h>
extern void print_int(int);

int taintedintmod(void) {
	int userden;
	scanf("%d",&userden);
	int rem = 0;
    if (userden > 0) { 
        rem = 128 % userden; 
    }
    print_int(rem);
    return rem;
}