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The example below, is inspired by Dowd, assuming assumes that the size of struct buffer is equal to the sizeof(size_t) + (sizeof(char) * 50), which (assuming sizeof(size_t) is 4 bytes) is 54 may be incorrect. The size of struct buffer may actually be a larger due to structure padding.
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struct buffer {
size_t size;
char buffer[50];
};
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void func(struct buffer *buf) {
struct buffer *buf_cpy = malloc((sizeof(size_t)+(sizeof(char)*50))); /* assuming sizeof(size_t) is 4, this equals 54 */
if (buf_cpy == NULL) {
/* Handle malloc() error */
}
...
memcpy(buf_cpy, buf, sizeof(struct buffer)); /* May result with padding, sizeof(struct buffer) may equal 56, causing in a small buffer overflow */
}
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