There are occasions when officers, patrolling a crowd prior to an unlawful assembly or as one is about to erupt, will come across individuals with masks, large groups of masks, for no legitimate reason. There is that fine line between encouraging someone not to wear a mask to then their engaging in an unlawful act where you can act.
But you can use certain provisions of the Criminal Code to prevent offences. If officers were to come across a knapsack, which happened in Vancouver, full of masks—well known, he was part of a group—ink-filled eggs, marbles, all the tools of a riotous individual, the officers would be permitted to act where they couldn't under existing law, in my view.