linux-kernel - Re: _proxy_pda still makes linking modules fail

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Message-Id: <1173838623.5443.20.camel@localhost.localdomain>
Date:	Wed, 14 Mar 2007 13:17:03 +1100
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Marcin 'Qrczak' Kowalczyk <qrczak@....org.pl>,
	linux-kernel@...r.kernel.org
Subject: Re: _proxy_pda still makes linking modules fail

On Tue, 2007-03-13 at 16:57 +0100, Andi Kleen wrote:
> On Tue, Mar 13, 2007 at 05:23:52PM +1100, Rusty Russell wrote:
> > In particular, it's been put in GCC 4.1 for
> > CONFIG_CC_STACKPROTECTOR, which assumes %gs:40 will give the stack
> > canary.
> 
> Yes that was always ugly, but I don't know a better way.

Well, "%gs:__gcc_stack_protector" would have been better.  We could have
defined __gcc_stack_protector as an absolute symbol (0x40) at the
moment, and made it a real per-cpu var later.

> > For the record: the PDA should never have existed, that's what percpu
> > vars were supposed to be for.  Something went wrong here 8(
> 
> PDA predates per cpu.

Indeed, but I should have converted it over back in 2003 (?) when the
per-cpu stuff went in 8(

> > The ideal solution has always been to use __thread, but no architecture
> > has yet managed it (I tried for i386, and it quickly caused unbearable
> 
> I tried it too, but __thread is hopeless for kernel code
> 
> > pain).  On x86-64 that uses "%fs" on x86-64, not "%gs" as the kernel
> > does, but I might try that if I feel particularly masochistic soon...
> 
> Then swapgs wouldn't work anymore (there is no swapfs)

Good point.

Thanks,
Rusty.

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