CodeQL documentation

Suspicious pointer scaling to void

ID: cpp/suspicious-pointer-scaling-void
Kind: problem
Severity: warning
Precision: medium
Tags:
   - security
   - external/cwe/cwe-468
Query suites:
   - cpp-security-extended.qls
   - cpp-security-and-quality.qls

Click to see the query in the CodeQL repository

Casting arbitrary pointers into void* and then accessing their contents should be done with care. The results may not be portable.

This query finds pointer arithmetic expressions where a pointer to void (or similar) is then cast to another type and dereferenced.

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 void* 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.
  3. If pointer arithmetic must be done with a single-byte width, prefer char * to void *, as pointer arithmetic on void * is a nonstandard GNU extension.

Example

char example1(int i) {
  int intArray[5] = { 1, 2, 3, 4, 5 };
  void *voidPointer = (void *)intArray;
  // BAD: the pointer arithmetic uses type void*, so the offset
  // is not scaled by sizeof(int).
  return *(voidPointer + i);
}

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.