All Integer operations must be guaranteed not to result in overflow, truncation, or sign error

Non-compliant Code Example 1

In the following non-compliant code example, cBlocks is multiplied by 16 and the result is stored in the unsigned long long int alloc.  The result of this multiplication can overflow because it is a 32 bit operation and the resulting value stored in alloc invalid.

void* AllocBlocks(size_t cBlocks) {
  if (cBlocks == 0) return NULL;
  unsigned long long alloc = cBlocks * 16;
  return (alloc < UINT_MAX)
    ? malloc(cBlocks * 16)
    : NULL;
}

Compliant Solution 1

On architectures where unsigned long long int is guaranteed to have 2x the number of bits as size_tupcast the variable used in the multiplication to a 64-bit value.  This ensures the multiplication operation is performed

void* AllocBlocks(size_t cBlocks) {
  if (cBlocks == 0) return NULL;
  unsigned long long alloc =
           (unsigned long long)cBlocks*16;
  return (alloc < UINT_MAX)
    ? malloc(cBlocks * 16)
    : NULL;
}

The assumption concerning the relationship of unsigned long long int and size_t must be document in the header for each file that depends upon this assumption for correct execution.

Exceptions

Unsigned integers can be allowed to exhibit modulo behavior if and only if

1. the variable declaration is clearly commented as supporting modulo behavior
2. each operation on that integer is also clearly commented as supporting modulo behavior
3. if the integer exhibiting modulo behavior contributes to the value of an integer not marked as exhibiting modulo behavior, the resulting integer must obey this rule.

Consequences

Improper range checking can lead to buffer overflows and the execution of arbitary code by an attacker.

References

• Seacord 05 Chapter 5 Integers