When performing pointer arithmetic, the size of the computation is automatically scaled to the size of the pointer type. For instance, a pointer to a four-byte integer will be scaled by four bytes at a time.
Improper use of pointer arithmetic can lead to miscalculations that result in subtle and hard to spot coding errors.
Non-Compliant Code Example
In this example, taken from Dowd, buf_ptr is used to insert new integers into buf, which is an array of 1024 integers. If there is data to be inserted into buf (which is indicated by havedata()) and buf_ptr has not been incremented past buf + sizeof(buf), then an integer is inserted into buf via buf_ptr. However, the sizeof operator returns the total number of bytes in buf, which, assuming four-byte integers, is 4096 bytes. This value is then scaled to the size of an integer and added to buf. As a result, it is possible to write integers past the end of buf and cause a buffer overflow.
int buf[1024];
int *buf_ptr = buf;
while (havedata() && buf_ptr < buf + sizeof(buf))
{
*buf_ptr++ = parseint(getdata());
}
Compliant Code Example
To correct this example, the size of buf can be directly added to buf and used as an upper bound. The integer literal is scaled to the size of an integer and the upper bound of buf is appropriately checked.
int buf[1024];
int *b = buf;
while (havedata() && b < buf+1024)
{
*b++ = parseint(getdata());
}
Risk Analysis
Failure to understand and properly use pointer arithmetic can allow an attacker to execute arbitrary code.
Reference
[[Dowd]] Chapter 6, "C Language Issues" (Vulnerabilities)