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

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Message-ID: <4AA034FB.1030400@goop.org>
Date:	Thu, 03 Sep 2009 14:28:27 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"H. Peter Anvin" <hpa@...or.com>
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 09/03/09 14:15, 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.
>   

I'm assuming that stack-protector has fairly serious performance impact
anyway, so a bit of extra entry/exit cost is acceptable.  But I agree
that there's no point in making it gratuitously bad.

    J
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