ARR00-A. Understand how arrays work

The incorrect use of arrays has traditionally been a source of exploitable vulnerabilities. Elements referenced within an array using the subscript operator [] are not checked unless the programmer provides adequate bounds checking. As a result, the expression array [pos] = value can be used by an attacker to transfer control to arbitrary code.
As a result, if the attacker can control the values of both pos and value in the expression array [pos] = value, he can perform an arbitrary write (overwrite other storage locations with contents of his choice). The consequences range from changing a variable used to determine what permissions the program grants to executing arbitrary code with the permissions of the vulnerable process. Arrays are also a common source of buffer overflows when iterators exceed the dimensions of the array.

An array is a series of objects, all of which are the same size and type. Each object in an array is called an array element. The entire array is stored contiguously in memory (that is, there are no gaps between elements). Arrays are commonly used to represent a sequence of elements where random access is important but there is little or no need to insert new elements into the sequence (which can be an expensive operation with arrays).

Arrays containing a constant number of elements can be declared as follows

enum { ARRAY_SIZE = 12 };
int dis[ARRAY_SIZE];

These statements allocate storage for an array of twelve integers referenced by dis. Arrays are indexed from 0..n-1 (where n represents an array dimension). Arrays can also be declared as follows:

int ita[];

This is called an incomplete type because the size is unknown. If an array of unknown size is initialized, its size is determined by the largest indexed element with an explicit initializer. At the end of its initializer list, the array no longer has incomplete type.

int ita[] = { 1, 2 };

While these declarations work fine when the size of the array is known at compilation time, it is not possible to declare an array in this fashion when the size can be determined only at runtime. The C99 standard adds support for variable length arrays or arrays whose size is determined at runtime. Before the introduction of variable length arrays in C99, however, these "arrays" were typically implemented as pointers to their respective element types allocated using malloc(), as shown in this example.

int *dat = (int *) malloc(ARRAY_SIZE * sizeof(int));

Both dis and dat arrays can then be initialized as follows:

for(i = 0; i < ARRAY_SIZE; i++) {
   dis[i] = 42; /* Assigns 42 to each element; */
   /* ... */
}

The dat array can also be initialized as follows:

for (i = 0; i < ARRAY_SIZE; i++) {
   *dat = 42;
   dat++;
}
dat -= ARRAY_SIZE;

It is important to retain any value returned by malloc() so that it may be later sent to free(). One possible way of preserving such a value would be to use a constant pointer.

int * const dat 
  = (int * const) malloc(ARRAY_SIZE * sizeof(int));
/* ... */
free(dat);
dat = NULL; 

The dis identifier cannot be incremented, so the expression dis++ results in a fatal compilation error. Both arrays can be initialized as follows:

int *p = dis;
for (i = 0; i < ARRAY_SIZE; i++)  {
  *p = 42; // Assigns 42 to each element;
  p++;
}

The variable p is declared as a pointer to an integer and then incremented in the loop. This technique can be used to initialize both arrays and is a better style of programming than incrementing the pointer to the array because it does not change the pointer to the start of the array.

Obviously, there is a relationship between array subscripts [] and pointers. The expression dis[i] is equivalent to *(dis+i). In other words, if dis is an array object (equivalently, a pointer to the initial element of an array object) and i is an integer, dis[i] designates the ith element of dis (counting from zero). In fact, because *(dis+i) can be expressed as *(i+dis), the expression dis[i] can be represented as i[dis], although doing so is not encouraged.

The initial element of an array is accessed using an index of zero; for example, dat[0] references the first element of dat array. The dat identifier points to the start of the array, so adding zero is inconsequential in that *(dat+i) is equivalent to *(dat+0), which is equivalent to *(dat).

Risk Assessment

Arrays are a common source of vulnerabilities in C language programs, because they are frequently used but always fully understood.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[ISO/IEC 9899:1999]] Section 6.7.5.2, "Array declarators"

06. Arrays (ARR)      06. Arrays (ARR)       ARR01-A. Do not apply the sizeof operator to a pointer when taking the size of an array