Re: Stack size

[Chuck Swiger <chuck () codefab com>]

Cat Okita wrote:
> In the process of doing some trivial (and admittedly sloppy) programming
> recently, it became apparent that OSX has a surprisingly low stack size
> of 512K (measured via getrlimit, to show the current (soft) limit, and
> the hard limit).

512K probably still represents a thousand or so PPC stack frames, which 
is arguably too small-- certainly if the program uses alloca() or local 
variables to any extent.

> Checking other (albeit not the same hardware architecture) systems for
> curiousity, it seems that it's much more common to have a stack size
> of 8Mb.

8 MB is common, yes.  See /etc/login.conf on FreeBSD for how to change 
the defaults, or /etc/profile on most systems for a place to adjust the 
defaults limits to something you find more reasonable.  Something like 
this, and/or with -H instead of -S to adjust the hard limits:

# Set reasonable soft process limits.
ulimit -Sc 0
ulimit -Sd 512000
ulimit -Ss 16384
ulimit -Sn 256
ulimit -Su 100

> Is there any particular reason why it would be advantageous to have
> such a small stack size?

In terms of security?  No, not really, although the Mach kernel used by 
OS X has long made the distinction between VM pages which are readable, 
writable, or executable, if the underlying hardware supports it.  So it 
already supports prevention of executing code on the stack if the G4 CPU 
can, mitigating the risk of some exploits.

If you're asking why OS X has such a small default, it's possibly 
because that's the amount of stack space per thread per task, and some 
Mach apps are heavily threaded.  If you reserve 16 MB of space per stack 
with an app that forks a few dozen threads, you can overrun the chunk of 
VM address space that the system expects the stack(s) to fit into.  More 
of a legacy setting from the days of MachO fixed-VM shared libraries 
(from NEXTSTEP)...

-Chuck

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