bug#18410: Use SAFE_ALLOCA etc. to avoid unbounded stack allocation.

[Demetrios Obenour]

This is crucial. Otherwise, a security vulnerability could result.

MAX_ALLOCA should not be larger than the page size for the target architecture.

Demetrios Obenour

-----Original Message-----
From: bug-gnu-emacs-bounces+demetriobenour=gmail.com@gnu.org 
[mailto:bug-gnu-emacs-bounces+demetriobenour=gmail.com@gnu.org] On Behalf Of 
Paul Eggert
Sent: Sunday, September 7, 2014 10:38 PM
To: Stefan Monnier
Cc: 18410@debbugs.gnu.org
Subject: bug#18410: Use SAFE_ALLOCA etc. to avoid unbounded stack allocation.

Stefan Monnier wrote:
> MAX_ALLOCA is chosen small so that we can allocate several/many 
> objects of size MAX_ALLOCA.

That's one reason, but another is that stack-overflow checking often relies on 
guard pages.  If we blindly increase MAX_ALLOCA (or some variant of it, just 
for call-process) Emacs could bypass stack-overflow checking, resulting in 
behavior that could be worse than simply dumping core.

If I understand things correctly, Dmitry's recent stack-overflow changes don't 
affect this, as they don't deal with the guard-page region size.