question about hash function

correct if I'm wrong:
in function  lb_get_skb_hash()
the hash is calculated from length returned from filter run:
         lhash = SK_RUN_FILTER(fp, skb);
         c = (char *) &lhash;
         return c[0] ^ c[1] ^ c[2] ^ c[3];

this results that hash for same length packets will have the same hash (e.g tcp handshake)
?

what I'm actually looking for is to have similar hash as in bonding-mode XOR with
layer-3-and-4 hash (I can't use mac.. I have wireless interfaces).

So if I just pass as an option to set filter (fprog) e.g ip && (udp || tcp) hash
will not be from udp/tcp headers, but again from packet length after filter run.