kbloom is a bitmask with 'size2' bits, where 'size2' is the next power of 2 of 'size'. Thus, if 'size' is smaller than 4, 'size2' is smaller than 8, so that size2 / 8 == 0 and calloc allocates 0 bytes. This causes heap corruption when kbloom is subsequently written to. See discussion on https://groups.google.com/g/librsync/c/vmqzQS1QjIw.