void dns_packet_rewind(DnsPacket *p, size_t idx) {
        assert(p);
        assert(idx <= p->size);
        assert(idx >= DNS_PACKET_HEADER_SIZE);

        p->rindex = idx;
}

The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.