linux-kernel - Re: [PATCH] x86: Don't randomize stack unless current->personality permits it

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Date:	Sat, 15 Jul 2006 20:39:58 +0300
From:	Al Boldi <a1426z@...ab.com>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Frank van Maarseveen <frankvm@...nkvm.com>,
	linux-kernel@...r.kernel.org, Andi Kleen <ak@...e.de>
Subject: Re: [PATCH] x86: Don't randomize stack unless current->personality permits it

Arjan van de Ven wrote:
> On Sat, 2006-07-15 at 17:09 +0300, Al Boldi wrote:
> > Arjan van de Ven wrote:
> > > On Sat, 2006-07-15 at 14:29 +0300, Al Boldi wrote:
> > > > Arjan van de Ven wrote:
> > > > > > BTW, why does randomize_stack_top() mod against (8192*1024)
> > > > > > instead of (8192) like arch_align_stack()?
> > > > >
> > > > >  because it wants to randomize for 8Mb, unlike arch_align_stack
> > > > > which wants to randomize the last 8Kb within this 8Mb ;)
> > > >
> > > > Randomizing twice?
> > >
> > > a VMA can only be randomized in 4Kb (well page size) granularity, so
> > > the 8Mb randomization can only work in that 4Kb unit, the "second"
> > > randomization can work in 16 byte granularity.
> > >
> > > > There is even a case where a mere rename or running through an extra
> > > > shell causes a slowdown.  And that's with randomization turned off.
> > >
> > > randomization off will slow stuff down yes... you get cache alias
> > > contention that way.
>
> a question.. do you have prelink installed/active on your system? that
> may very well mess with timings...

No, not that I am aware of.  How can I find out?
I confirmed this on a standard rhel4 virgin-install.  Does rhel4 have prelink 
enabled by default?

But even so, I don't think this is a timing issue, as the slowdowns are so 
huge it is obvious even without timing.

And 2.4.31 doesn't have this problem.

Did you try it?

Thanks!

--
Al

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