When performing pointer arithmetic, the size of the value to add to a pointer is automatically scaled to the size of the pointer's type. For instance, when adding a value to a pointer to a four-byte integer, the value is scaled by a factor of four and then added to the pointer. Failing to understand how pointer arithmetic works can lead to miscalculations that result in serious errors, such as buffer overflows.
Non-Compliant Code Example
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In this non-compliant code example, integer values returned by {{parseint(getdata())}} are stored into an array of {{INTBUFSIZE}} elements of type {{int}} called {{buf}} \[[Dowd 06|AA. C References#Dowd 06]\]. If data is available for insertion into {{buf}} (which is indicated by {{havedata()}}) and {{buf_ptr}} has not been incremented past {{buf + sizeof(buf)}}, an integer value is stored at the address referenced by {{buf_ptr}}. However, the {{sizeof}} operator returns the total number of bytes in {{buf}}, which is typically a multiple of the number of elements in {{buf}}. This value is scaled to the size of an integer and added to {{buf}}. As a result, the check to make sure integers are not written past the end of {{buf}} is incorrect and a buffer overflow is possible. |
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int buf[INTBUFSIZE];
int *buf_ptr = buf;
while (havedata() && buf_ptr < buf + sizeof(buf)) {
*buf_ptr++ = parseint(getdata());
}
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Compliant Solution
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In this compliant solution, the size of buf is added directly 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 checked correctly.
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int buf[INTBUFSIZE];
int *buf_ptr = buf;
while (havedata() && buf_ptr < (buf + INTBUFSIZE)) {
*buf_ptr++ = parseint(getdata());
}
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Non-Compliant Code Example
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The following example is based on a flaw in the OpenBSD operating system. An integer, skip, is added as an offset to a pointer of type struct big. The adjusted pointer is then used as a destination address in a call to memset(). However, when skip is added to the struct big pointer, it is automatically scaled by the size of struct big, which is 32 bytes (assuming 4 byte integers, 8 byte long long integers, and no structure padding). This results in the call to memset() writing to unintended memory.
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A similar situation occurred in OpenBSD's {{make}} command \[[Murenin 07|AA. C References#Murenin 07]\]. |
Compliant Solution
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To correct this example, the struct big pointer is cast as a char *. This causes skip_member to be scaled by a factor of 1.
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