CodeQL documentation

Suspicious add with sizeof

ID: cpp/suspicious-add-sizeof
Kind: problem
Severity: warning
Precision: high
Tags:
   - security
   - external/cwe/cwe-468
Query suites:
   - cpp-code-scanning.qls
   - cpp-security-extended.qls
   - cpp-security-and-quality.qls

Click to see the query in the CodeQL repository

Pointer arithmetic in C and C++ is automatically scaled according to the size of the data type. For example, if the type of p is T* and sizeof(T) == 4 then the expression p+1 adds 4 bytes to p.

This query finds code of the form p + k*sizeof(T). Such code is usually a mistake because there is no need to manually scale the offset by sizeof(T).

Recommendation

  1. Whenever possible, use the array subscript operator rather than pointer arithmetic. For example, replace *(p+k) with p[k].
  2. Cast to the correct type before using pointer arithmetic. For example, if the type of p is char* but it really points to an array of type double[] then use the syntax (double*)p + k to get a pointer to the k’th element of the array.

Example

int example1(int i) {
  int intArray[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
  int *intPointer = intArray;
  // BAD: the offset is already automatically scaled by sizeof(int),
  // so this code will compute the wrong offset.
  return *(intPointer + (i * sizeof(int)));
}

int example2(int i) {
  int intArray[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
  int *intPointer = intArray;
  // GOOD: the offset is automatically scaled by sizeof(int).
  return *(intPointer + i);
}

References

  • Common Weakness Enumeration: CWE-468.