Re: Two issues with stack overflow protection

[Paul Eggert]

Daniel Colascione wrote:
> > > What's wrong with just mprotecting a guard page at the end of the stack,
> > > >>and on overflow, giving that region normal protection, unwinding as
> > > >>normal, then, at top level, restoring the guard page?
> > >
> > >Unwinding can grow the stack.
> Sure. That's why you open up more stack to do the unwinding. Having done
> that, if you still overflow, just abort.

Yes, that was my point.  Emacs already does the business about the guard page, 
and opening up more stack, and so forth.  The tricky part is the "if you still 
overflow, just abort", something that's easy enough to describe at the high 
level but perhaps not so easy to actually write the code.  Part of the issue is 
that the guard page business is done under the covers by the OS, not by Emacs 
directly -- in general Emacs doesn't know where the guard page(s) are.

I'm sure there are other issues that won't get discovered until someone actually 
writes and tests something.  For example, here's something I just thought of: 
the conservative marking phase of the Emacs garbage collector may need to be 
taught about the split stack (currently it assumes the C stack is contiguous).