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

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Date:	Mon, 24 Jul 2006 18:57:46 +0300
From:	Al Boldi <a1426z@...ab.com>
To:	Chuck Ebbert <76306.1226@...puserve.com>
Cc:	Paulo Marques <pmarques@...popie.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Frank van Maarseveen <frankvm@...nkvm.com>
Subject: Re: [PATCH] x86: Don't randomize stack unless current->personality permits it

Al Boldi wrote:
> Chuck Ebbert wrote:
> > On Tue, 18 Jul 2006 08:21:45 +0300. Al Boldi wrote:
> > > Going one step further,
> > > with #define arch_stack_align(x) (x)
> > > all blips/hits/weirdness are gone
> > >
> > > Which means that either arch_stack_align isn't necessary at all, or
> > > randomization isn't working as intended.
> > >
> > > Can somebody prove me wrong here?
> >
> > Your program seems highly sensitive to any changes,
>
> Extremely sensitive.  You may have noticed the strange repetitions in main
> instead of a for loop.  It's like that for a reason:  compile semantics.
>
> > $ ./tst.ex
> > &x = 0xbfb32d90, &y = 0xbfb32d98
> >   10    6   10   10    6   10    7   10   10   10   10   10   10   10  
>
> With your changes on:
>
> stock kernel, randomize_va_space=0, gcc.322 -Os tstExec.c,
> while :; do ./a.out; done
> &x = 0xbffff874, &y = 0xbffff86c   28   28
> &x = 0xbffff874, &y = 0xbffff86c   27   27  
> &x = 0xbffff874, &y = 0xbffff86c   27   27
> &x = 0xbffff874, &y = 0xbffff86c   28   27
> &x = 0xbffff874, &y = 0xbffff86c   27   30
> &x = 0xbffff874, &y = 0xbffff86c   27   29
>
> stock kernel, randomize_va_space=1, gcc.322 -Os tstExec.c,
> while :; do ./a.out; done
> &x = 0xbfe2e614, &y = 0xbfe2e60c   29   28
> &x = 0xbfd6a104, &y = 0xbfd6a0fc   55   56  
> &x = 0xbf91d454, &y = 0xbf91d44c   27   27
> &x = 0xbf941e84, &y = 0xbf941e7c   55   56
> &x = 0xbfa75834, &y = 0xbfa7582c   28   27
> &x = 0xbfb58634, &y = 0xbfb5862c   27   30

After closer inspection, it looks like addresses ending with 3c,7c,bc,fc 
cause a slowdown on P4, while addresses ending with 1c,3c,5c,7c,9c,bc,dc,fc 
cause a slowdown on P2.

Any easy way to instruct the kernel to skip those addresses?

Thanks!

--
Al

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