tcf_get_base_ptr
unsigned char *ptr = tcf_get_base_ptr(skb, cmp->layer);
unsigned char *ptr = tcf_get_base_ptr(skb, nbyte->hdr.layer);
ptr = tcf_get_base_ptr(skb, tm->from_layer);
ptr = tcf_get_base_ptr(skb, tm->to_layer);