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

[Al Boldi <a1426z@gawab.com>]

Chuck Ebbert wrote:
> On Mon, 24 Jul 2006 18:57:46 +0300, Al Boldi wrote:
> > > 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.
>
> Those addresses cause 'y' to span a cacheline (P4 = 64 bytes, P2 = 32.)
> Even when the kernel aligns to 128 bytes this could happen depending
> on how deeply you nest functions.
>
> > Any easy way to instruct the kernel to skip those addresses?
>
> First, I think you need to define locals in order of decreasing size.
> IOW 'x' and 'y' need to be first inside fn(), but that may not work
> when things get inlined.  So using the '-malign-double' GCC option,
> or forcing alignment with '__attribute__ ((aligned(8)))' for each variable
> might be better.
>
> Then you have to make sure the stack is aligned. See
> '-mpreferred-stack-boundary'.

This would imply a recompile, what about precompiled dists?  Do they compile 
the sources this way?

> I still think the kernel should be aligning the stack to 128 bytes anyway.

I think so too, but can you see how randomization aggravates the situation?


Thanks!

--
Al

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