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Non-Compliant Code Example 2

Wiki MarkupThe following example is based on a flaw discovered in the OpenBsd OpenBSD operating system \[[ref]\]. An integer, {{skip}}, is added as an offset to a pointer of type {{struct big}} and the sum 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.

struct big {
    unsigned long long ull_1; /* typically 8 bytes */
    unsigned long long ull_2; /* typically 8 bytes */
    unsigned long long ull_3; /* typically 8 bytes */
    int si_4; /* typically 4 bytes */
    int si_5; /* typically 4 bytes */
};
/* ... */
size_t skip = sizeof(unsigned long long);
struct big *s = malloc(sizeof(struct big));
if (!s) {
   /* Handle malloc() error */
}

memset(s + skip, 0, sizeof(struct big) - skip);
/* ... */
free(s);

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