linux-kernel - Re: [PATCH v5 2/3] x86/asm: Add suffix macro for GEN_*

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Message-ID: <CAGXu5jKV-NjD5=tFsfeik-Egh4sctzPnQnZtuv9D7-WSkkREVw@mail.gmail.com>
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