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 13:45:54 -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 13:26, H. Peter Anvin wrote:
> With the new zero-based percpu segment, it seems we should be able to
> subsume the stack protector into the percpu segment and reference both
> via %gs -- we just have to reserve the first 24 bytes of the segment,
> and being able to reduce the number of segments we need in the kernel is
> good for multiple reasons.
>
> Tejun - am I missing something why that would be hard or impossible?
>   

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).
    * 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.

Neither looks insoluble.

    J
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