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From: Robert Watson <ratson@FreeBSD.org>
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To: Suleiman Souhlal <ssouhlal@FreeBSD.org>
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Subject: Re: [PATCH] Stackgap
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On Fri, 27 May 2005, Suleiman Souhlal wrote:

> You can find an implementation of stackgap from OpenBSD at http:// 
> people.freebsd.org/~ssouhlal/testing/stackgap-20050527.diff
>
> You can control the range of the random stack gap with the 
> kern.stackgap_random sysctl. A value of 0 disables it. Otherwise, it has 
> to be a power of 2 and not too large. The default value is 64K.
>
> I've only had the chance to test this on i386. Could anyone test it on 
> other architectures as well?
>
> Any comments/objections?

In the past, substantial performance hits have been measured due to poor 
stack alignment.  Specifically, in combination with less optimal compiler 
behavior, the results have been pretty nasty.  Have you tried 
micro-benchmarking a series of runs with this stack offset randomness 
using floating point on stack arguments to see if there's a measurable 
cost to moving the stack around?  Hopefull if all is well, there will be 
little or no difference, but a small error here could result in a 
substantial performance hit...

Robert N M Watson