I meeeeean... plenty of functions allocate internally and don't let the user pass in an allocator. So it's not clear to me how to do this at least somewhat universally. You could try to integrate it into the global allocator, I suppose, but then how do you know which allocations to wipe? Should anything allocated in the secret mode be zeroed on free? Or should anything be zeroed if the deallocation happens while in secret mode? Or are both of these necessary conditions? It seems tricky to define rigidly.

And stack's the main problem, yeah. It's kind of the main reason why zeroing registers is not enough. That and inter-procedural optimizations.