How to attach ground wire for ceiling fixture, if mounting bracket does not have a green ground screw, and no ground wire coming from junction box?
Although 2 whites and a black would be unusual.
In the US code allows for a grounding conductor to be pulled from the service, or any point fed from that service panel.
2 wire fixtures did not have grounds for the first 75 years of electrical code so many fixtures were not grounded.
If fed by conduit and individual wires the box may be grounded through the conduit.