Can I use brake pads designed for linear-pull brakes on my side-pull cantilever brakes? I have side-pull (dual-pivot) cantilever brakes. My friend gave me brake pads that are 'for all linear-pull brakes'. 

Can I use brake pads that are advertised for linear-pull brake systems with my side-pull cantilever brake system?
Linear pull brake blocks come with a stack of washers designed to provide toe-in (a gap at the back of the brake block to reduce squeal) and different thickness washers to get the cantilever arm vertical to the rim.

Side pull brakes have none of that, just a simple washer for the outside surface of the brake arm.

Put the linear pull brakes in to your side-pull brakes with the whole washer stack on the outside, tighten it up and you should be good to go. You can experiment with putting the extra washers in for that toe in or omit the concave/convex toe-in washers entirely. But do use at least one washer on the outside.

Just make sure that when you have tightened them up that you cannot twist the brake block around with your hand and that the brake blocks meet parallel with the rim neither touching the tyre or falling off the bottom of the rim. Test before you go down any big hills.

Hope that helps!