Date: Wed, 9 Jul 2008 13:14:05 -0700
From: Arjan van de Ven <arjan@infradead.org>
To: ebiederm@xmission.com (Eric W. Biederman)
Cc: Mike Travis <travis@sgi.com>, Jeremy Fitzhardinge <jeremy@goop.org>,
       Ingo Molnar <mingo@elte.hu>, Andrew Morton <akpm@linux-foundation.org>,
       "H. Peter Anvin" <hpa@zytor.com>,
       Christoph Lameter <cl@linux-foundation.org>,
       Jack Steiner <steiner@sgi.com>, linux-kernel@vger.kernel.org
Subject: Re: [RFC 00/15] x86_64: Optimize percpu accesses
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	<m1abgqzv3g.fsf@frodo.ebiederm.org>
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On Wed, 09 Jul 2008 13:00:19 -0700
ebiederm@xmission.com (Eric W. Biederman) wrote:

> 
> I just took a quick look at how stack_protector works on x86_64.
> Unless there is some deep kernel magic that changes the segment
> register to %gs from the ABI specified %fs CC_STACKPROTECTOR is
> totally broken on x86_64.  We access our pda through %gs.

and so does gcc in kernel mode.

> 
> Further -fstack-protector-all only seems to detect against buffer
> overflows and thus corruption of the stack.  Not stack overflows.  So
> it doesn't appear especially useful.

stopping buffer overflows and other return address corruption is not
useful? Excuse me?

> 
> So we don't we kill the broken CONFIG_CC_STACKPROTECTOR.  Stop trying
> to figure out how to use a zero based percpu area.

So why don't we NOT do that and fix instead what you're trying to do?

> 
> That should allow us to make the current pda a per cpu variable, and
> use %gs with a large offset to access the per cpu area. 

and what does that gain us?


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