linux-kernel - Re: [tip:x86/asm] x86/i386: Make sure stack-protector segment base is cache aligned

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Date:	Thu, 03 Sep 2009 14:15:10 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	mingo@...hat.com, linux-kernel@...r.kernel.org,
	jeremy.fitzhardinge@...rix.com, stable@...nel.org,
	tglx@...utronix.de, mingo@...e.hu,
	linux-tip-commits@...r.kernel.org, Tejun Heo <tj@...nel.org>
Subject: Re: [tip:x86/asm] x86/i386: Make sure stack-protector segment base
 is cache aligned

On 09/03/2009 01:45 PM, Jeremy Fitzhardinge wrote:
> 
> Two problems:
> 
>     * gcc generates %gs: references for stack-protector, but we use %fs
>       for percpu data (because restoring %fs is faster if it's a null
>       selector; TLS uses %gs).  I guess we could use %fs if
>       !CONFIG_CC_STACKPROTECTOR, or %gs if we are using it (though that
>       has some fiddly ramifications for things like ptrace).

Well, by touching two segments we're getting the worst of both worlds,
so at least assuming some significant number of real-world deployments
use CC_STACKPROTECTOR, we really don't want to pessimize that case too much.

>     * The i386 percpu %fs base is offset by -__per_cpu_start from the
>       percpu variables, so we can directly refer to %fs:per_cpu__foo. 
>       I'm not sure what it would take to unify i386 to use the same
>       scheme as x86-64.

OK, I was under the impression that that had already been done (and no,
I didn't bother to look at the code.)  I guess I was wrong (and yes,
this is an absolute precondition.)

> Neither looks insoluble.

Agreed.  Looks like something that can and probably should be done but
is a bit further out.

	-hpa
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