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Date: Wed, 31 May 2017 09:09:16 -0700
From: Kees Cook <keescook@...omium.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Christoph Hellwig <hch@...radead.org>, 
	"Eric W. Biederman" <ebiederm@...ssion.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Josh Poimboeuf <jpoimboe@...hat.com>, PaX Team <pageexec@...email.hu>, Jann Horn <jannh@...gle.com>, 
	Eric Biggers <ebiggers3@...il.com>, Elena Reshetova <elena.reshetova@...el.com>, 
	Hans Liljestrand <ishkamiel@...il.com>, David Windsor <dwindsor@...il.com>, 
	Greg KH <gregkh@...uxfoundation.org>, Ingo Molnar <mingo@...hat.com>, 
	Alexey Dobriyan <adobriyan@...il.com>, "Serge E. Hallyn" <serge@...lyn.com>, arozansk@...hat.com, 
	Davidlohr Bueso <dave@...olabs.net>, Manfred Spraul <manfred@...orfullife.com>, 
	"axboe@...nel.dk" <axboe@...nel.dk>, James Bottomley <James.Bottomley@...senpartnership.com>, 
	"x86@...nel.org" <x86@...nel.org>, Ingo Molnar <mingo@...nel.org>, Arnd Bergmann <arnd@...db.de>, 
	"David S. Miller" <davem@...emloft.net>, Rik van Riel <riel@...hat.com>, 
	linux-arch <linux-arch@...r.kernel.org>, 
	"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v5 2/3] x86/asm: Add suffix macro for GEN_*_RMWcc()

On Wed, May 31, 2017 at 7:03 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, May 31, 2017 at 06:17:16AM -0700, Kees Cook wrote:
>> On Wed, May 31, 2017 at 4:13 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>> > On Tue, May 30, 2017 at 02:39:51PM -0700, Kees Cook wrote:
>> >> The coming x86 refcount protection needs to be able to add trailing
>> >> instructions to the GEN_*_RMWcc() operations. This extracts the
>> >> difference between the goto/non-goto cases so the helper macros
>> >> can be defined outside the #ifdef cases. Additionally adds argument
>> >> naming to the resulting asm for referencing from suffixed
>> >> instructions, and adds clobbers for "cc", and "cx" to let suffixes
>> >> use _ASM_CX, and retain any set flags.
>> >
>> > Another option is to simply require __GCC_ASM_FLAG_OUTPUT__ for the fast
>> > refcount stuff. That would result in simpler and more readable code.
>>
>> What versions of GCC support that?
>
> IIRC 6+

Given how many folks are still using 4.9 (and lower, see the thread
with Arnd[1]), I'd like to just keep this as I have it. It's not much
less readable, IMO (It was already pretty complex). I cleaned it up a
little before making it more ugly, so I think on sum, it's only a
little more weird. I think that's better than making this
compiler-specific or copy/pasting.

-Kees

[1] https://lkml.org/lkml/2017/4/25/66

-- 
Kees Cook
Pixel Security

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