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The UDP dissector calls sub-dissectors with the length of the UDP payload. From packet-udp.c:

decode_udp_ports(tvb, offset, pinfo, tree, udph->uh_sport, udph->uh_dport, udph->uh_ulen);

Whether there's Ethernet padding present or not, your dissector won't know about it. In fact, your dissector won't even know if the UDP datagram was transmitted over Ethernet or some other link layer protocol.