The calloc() function takes two arguments: the number of elements to allocate and the storage size of those elements. The calloc() function multiples these arguments together and allocates the resulting quantity of memory. However, if the result of multiplying the number of elements to allocate and the storage size cannot be represented properly as a size_t, an arithmetic overflow will occur. Therefore, it is necessary to check the product of the arguments to calloc() for an arithmetic overflow. If an overflow occurs, the program should detect and handle it appropriately.
Non-compliant Code Example 1
In this example, the user defined function get_size() (not shown) is used to calculate the size requirements for a dynamic array of unsigned long long int and stored in the variable num_elements. When calloc() is called to allocate the buffer, num_elements is multiplied by the sizeof(unsigned long long) to compute the overall size requirements. If the number of elements multiplied by the size can not be represented as a size_t calloc() may allocate a buffer of insufficient size. When data is copied to that buffer, a buffer overflow may occur.
size_t num_elements = get_size();
long *buffer = calloc(num_elements, sizeof(long));
if (buffer == NULL) {
/* handle error condition */
}
Compliant Solution 1
To correct this, a test is performed on the product of num_elements and sizeof(long) before the call to calloc(). It is assumed in the following example that the call to multsize_t() does not return if the multiplication operation results in an overflow and that, if the function returns succesfully, the multiplication performed by calloc() and can also be performed successfully.
long *buffer;
size_t num_elements = calc_size();
(void) multsize_t(num_elements, sizeof(long));
if(errno) {
/* multsize_t overflowed and error handler returned*/
}
buffer = calloc(num_elements, sizeof(long));
if (buffer == NULL) {
/* handle error condition */
}
Note that the maximum amount of allocatable memory is typically limited to a value less than SIZE_MAX (the maximum value of size_t).
References
- ISO/IEC 9899-1999 7.18.3 Limits of other integer types
- Seacord 05 Chapter 4 Dynamic Memory Management
- RUS-CERT Advisory 2002-08:02 http://cert.uni-stuttgart.de/advisories/calloc.php
- Secunia Advisory SA10635 http://secunia.com/advisories/10635/