Re: [tip:x86/asm] x86/i386: Make sure stack-protector segment baseis cache aligned

From: Jeremy Fitzhardinge
Date: Fri Sep 04 2009 - 11:59:52 EST

On 09/04/09 07:15, Arjan van de Ven wrote:
> On Thu, 3 Sep 2009 23:18:05 +0200
> Ingo Molnar <mingo@xxxxxxx> wrote:
>> * H. Peter Anvin <hpa@xxxxxxxxx> 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.
>> Fedora has stackprotector enabled so it's used in a widespread way.
>> Ingo
> the other issue is that afaik we want the kernel to use the other
> register than userspace does...

We do for percpu (%fs), but gcc always generates %gs references for
stack-protector. The difference between "pop %seg" for a null vs
non-null selector was fairly small (a couple of cycles), so using %gs
when stack-protector is enabled isn't a huge deal. To put it another
way, calling one stack-protected function in kernel mode would probably
make up the difference between using %fs vs %gs.


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