I have plenty of fist hand experience with Canadian boarder guards.
Summary: The less you say the better. You are attending a Linux conference. the end.
You are not "working" with or for it or anything that might sound like work. "volunteering" is work. If you hint that you might help out the reg desk, it is their job to get things evaluated by the labor dept that understands the labor market etc and that takes a minimum of 3 days. As long as you don't leave the country with a bag of money they don't care what you did at the conference.
My wife spoke at PyCon in Montreal. That fact got her a round of questions, mostly "are you being paid to speak?" - I would avoid saying anything other than "attending."