CodeQL documentation

Comparison of narrow type with wide type in loop condition

ID: java/comparison-with-wider-type
Kind: problem
Severity: warning
Precision: medium
Tags:
   - reliability
   - security
   - external/cwe/cwe-190
   - external/cwe/cwe-197
Query suites:
   - java-security-extended.qls
   - java-security-and-quality.qls

Click to see the query in the CodeQL repository

In a loop condition, comparison of a value of a narrow type with a value of a wide type may always evaluate to true if the wider value is sufficiently large (or small). This is because the narrower value may overflow. This can lead to an infinite loop.

Recommendation

Change the types of the compared values so that the value on the narrower side of the comparison is at least as wide as the value it is being compared with.

Example

In this example, bytesReceived is compared against MAXGET in a while loop. However, bytesReceived is a short, and MAXGET is a long. Because MAXGET is larger than Short.MAX_VALUE, the loop condition is always true, so the loop never terminates.

This problem is avoided in the ‘GOOD’ case because bytesReceived2 is a long, which is as wide as the type of MAXGET.

class Test {
	public static void main(String[] args) {
		
		{		
			int BIGNUM = Integer.MAX_VALUE;
			long MAXGET = Short.MAX_VALUE + 1;
			
			char[] buf = new char[BIGNUM];

			short bytesReceived = 0;
			
			// BAD: 'bytesReceived' is compared with a value of wider type.
			// 'bytesReceived' overflows before reaching MAXGET,
			// causing an infinite loop.
			while (bytesReceived < MAXGET) {
				bytesReceived += getFromInput(buf, bytesReceived);
			}
		}
		
		{
			long bytesReceived2 = 0;
			
			// GOOD: 'bytesReceived2' has a type at least as wide as MAXGET.
			while (bytesReceived2 < MAXGET) {
				bytesReceived2 += getFromInput(buf, bytesReceived2);
			}
		}
		
	}
	
	public static int getFromInput(char[] buf, short pos) {
		// write to buf
		// ...
		return 1;
	}
}

References