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

From: H. Peter Anvin
Date: Thu Sep 03 2009 - 17:16:44 EST

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.

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