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

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Message-ID: <4AA080A0.7010804@kernel.org>
Date:	Fri, 04 Sep 2009 11:51:12 +0900
From:	Tejun Heo <tj@...nel.org>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>, 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
Subject: Re: [tip:x86/asm] x86/i386: Make sure stack-protector segment base
 is cache aligned

Hello,

H. Peter Anvin wrote:
> 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.

Yes, this one definitely seems doable.  BTW, how much performance does
CC_STACKPROTECTOR cost?  That's an ambiguous question but really any
number would help to develop a general sense.  Considering fedora is
doing it by default, I assume it isn't too high?

>>     * 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.)

I tried this a while ago but hit an obstacle which I don't remember
what exactly was now and decided the conversion wasn't worth the
trouble.  IIRC, it was something substantial.  I'll dig through my
trees.

Thanks.

-- 
tejun
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