CodeQL documentation

Comparison of narrow type with wide type in loop condition

ID: cpp/comparison-with-wider-type
Kind: problem
Severity: warning
Precision: high
Tags:
   - reliability
   - security
   - external/cwe/cwe-190
   - external/cwe/cwe-197
   - external/cwe/cwe-835
Query suites:
   - cpp-code-scanning.qls
   - cpp-security-extended.qls
   - cpp-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 result in unexpected behavior 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, bytes_received is compared against max_get in a while loop. However, bytes_received is an int16_t, and max_get is an int32_t. Because max_get is larger than INT16_MAX, the loop condition is always true, so the loop never terminates.

This problem is avoided in the ‘GOOD’ case because bytes_received2 is an int32_t, which is as wide as the type of max_get.

void main(int argc, char **argv) {
	uint32_t big_num = INT32_MAX;
	char buf[big_num];
	int16_t bytes_received = 0;
	int max_get = INT16_MAX + 1;

	// BAD: 'bytes_received' is compared with a value of a wider type.
	// 'bytes_received' overflows before  reaching 'max_get',
	// causing an infinite loop
	while (bytes_received < max_get)
		bytes_received += get_from_input(buf, bytes_received);
	}

	uint32_t bytes_received = 0;

	// GOOD: 'bytes_received2' has a type  at least as wide as 'max_get'
	while (bytes_received < max_get) {
		bytes_received += get_from_input(buf, bytes_received);
	}

}


int getFromInput(char *buf, short pos) {
	// write to buf
	// ...
	return 1;
}

References