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Wiki MarkupThe operation of the remainder operator in Java is defined in the Java Language Specification \[[JLS 2005|AA. Bibliography#JLS 05]\], Section 15 (JLS), §15.17.3 , "Remainder Operator %": [JLS 2013], states,

The remainder operation for operands that are integers after binary numeric promotion (§5.6.2) produces a result value such that (a/b)*b+(a%b) is equal to a. This identity holds even in the special case that the dividend is the negative integer of largest possible magnitude for its type and the divisor is -1 (the remainder is 0). It follows from this rule that the result of the remainder operation can be negative only if the dividend is negative, and can be positive only if the dividend is positive; moreover, the magnitude of the result is always less than the magnitude of the divisor.

Although clearly defined in the Java specification, the behavior of the remainder operator's equivalent is undefined in several early C implementations. Programmers who are unaware of this distinction might always expect a positive remainder and code accordingly. This can result in vulnerabilities.

The result of the remainder operator implies the following behaviorhas the same sign as the dividend (the first operand in the expression):

5 % 3 produces 2
5 % (-3) produces 2
(-5) % 3 produces -2
(-5) % (-3) produces  -2

The result has the same sign as the dividend (the first operand in the expression)As a result, code that depends on the remainder operation to always return a positive result is erroneous.

Noncompliant Code Example

In this This noncompliant code example , uses the integer hashKey references as an element of index into the hash array. However, as the hash key is not guaranteed to be positive, the lookup function may fail, triggering a java.lang.ArrayIndexOutOfBoundsException on all negative inputs. 

private int SIZE = 16;	
public int[] hash = new int[SIZE];
	
public int lookup(int hashKey) {
  return hash[hashKey % SIZE];
}

A negative hash key produces a negative result from the remainder operator, causing the lookup() method to throw java.lang.ArrayIndexOutOfBoundsException.

Compliant Solution

This compliant solution calls a method that the imod() method, which always returns a modulus that is always positive.positive remainder:

// methodMethod imod() gives non-negativenonnegative result
private int SIZE = 16;
public int[] hash = new int[SIZE];

private int imod(int i, int j) {
  returnint (itemp == Integer.MIN_VALUE) ? 0 :i % j;
       return  (itemp < 0) ? ((-i) % j) : (i % j);
}
	
public int lookup(int hashKey) {
  return hash[imod(hashKey, size)];
}

Note that Integer.MIN_VALUE is a psecial corner case, because it can not be negated. (Mathematically, - Integer.MIN_VALUE == 1 + Integer.MAX_VALUE.)

Compliant Solution

Alternatively, an explicit range check must be performed on the numerator at every susceptible point as demonstrated in this compliant solution.

-temp : temp; 
  // Unary minus will succeed without overflow  
  // because temp cannot be Integer.MIN_VALUE
}
	
public int lookup(int hashKey) {
  if (hashKey < 0)
    return hash[imod(-hashKey), % SIZE)];
  return hash[hashKey % SIZE];
}

Note that providing a well-documented imod method is a better choice as it improves readability and makes it clear that its sole purpose is to return positive values when required and not to "fix" the unintuitive behavior of the remainder operator, as defined by the specification.

Risk Assessment

Assuming a positive remainder when using the remainder operator can result in incorrect computations.

Other Languages

This guideline appears in the C Secure Coding Standard as INT10-C. Do not assume a positive remainder when using the % operator.

This guideline appears in the C++ Secure Coding Standard as INT10-CPP. Do not assume a positive remainder when using the % operator,

Related Vulnerabilities

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

Bibliography

\[[JLS 2005|AA. Bibliography#JLS 05]\] [§15.17.3 Remainder Operators|http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.17.3]

Applicability

Incorrectly assuming a positive remainder from a remainder operation can result in erroneous code.

Automated Detection

Bibliography

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Image Added Image Added Image AddedINT01-J. Range check before casting integers to narrower types      06. Integers (INT)      INT03-J. Do not cast numeric types to wider floating-point types without range checking