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

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Date:	Wed, 09 Jul 2008 13:00:19 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Mike Travis <travis@....com>
Cc:	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

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.

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.

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

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 since it is only the per cpu accesses
and the pda accesses that will change we should not need to fight toolchain issues
and other weirdness.  The linked binary can remain the same.

Eric

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