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This code example is non-compliant on systems where size_t is an unsigned 32-bit value and long long is a 64-bit value. In this example, the programmer tests for integer overflow by assigning the value UINTcomparing SIZE_MAX to max and testing if length + BLOCK_HEADER_SIZE > max. Because length is declared as size_t, however, the addition is performed as a 32-bit operation and can result in an integer overflow. The comparison with max SIZE_MAX in this example will always test false. If an overflow occurs, malloc() will allocate insufficient space for mBlock, which could lead to a subsequent buffer overflow.

enum { BLOCK_HEADER_SIZE = 16 };
unsigned long long max = UINT_MAX;

void *AllocateBlock(size_t length) {
  struct memBlock *mBlock;

  if (length + BLOCK_HEADER_SIZE > max(unsigned long long) SIZE_MAX) return NULL;
  mBlock 
    = (struct memBlock *)malloc(length + BLOCK_HEADER_SIZE);
  if (!mBlock) return NULL;

  /* fill in block header and return data portion */

  return mBlock;
}

GCC Compiler Version 3.4.4 produces a warning for this non compliant code example.

Compliant Solution

In this compliant solution, the length operand is upcast to unsigned long long, ensuring that the addition takes place in this size.

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This test for integer overflow is only effective when the sizeof(unsigned long long) > sizeof(size_t). If both size_t and unsigned long long types are represented as a 64-bit unsigned value, the result of the addition operation may not be representable as an unsigned long long value.

Compliant Solution

In this compliant solution, the length operand is subtracted from SIZE_MAX, this ensures that no overflow can occur (see INT30-C. Ensure that unsigned integer operations do not wrap).

enum { BLOCK_HEADER_SIZE = 16 };

void *AllocateBlock(size_t length) {
  struct memBlock *mBlock;

  if (SIZE_MAX - length > BLOCK_HEADER_SIZE) return NULL;
  mBlock 
    = (struct memBlock *)malloc(length + BLOCK_HEADER_SIZE);
  if (!mBlock) return NULL;

  /* fill in block header and return data portion */

  return mBlock;
}

Non-Compliant Code Example

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There are two separate problems with this non-compliant code example. The first problem is that this code assumes an implementation where unsigned long long has a least four more bits than size_t. The second problem, assuming an implementation where size_t is a 32-bit value and unsigned long long is represented by a 64-bit value, is that to be compliant with C99, multiplying two 32-bit numbers in this context must yield a 32-bit result. Any integer overflow resulting from this multiplication will remain undetected by this code, and the expression alloc < UINT_MAX will always be true.

Compliant Solution

In this compliant solution, the cBlocks operand is upcast to unsigned long long, ensuring that the multiplication takes place in this size.

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