linux-kernel - Re: [RFC 00/15] x86_64: Optimize percpu accesses

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Message-ID: <20080709131405.54f9d49b@infradead.org>
Date:	Wed, 9 Jul 2008 13:14:05 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	ebiederm@...ssion.com (Eric W. Biederman)
Cc:	Mike Travis <travis@....com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Jack Steiner <steiner@....com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC 00/15] x86_64: Optimize percpu accesses

On Wed, 09 Jul 2008 13:00:19 -0700
ebiederm@...ssion.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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