linux-kernel - Re: x86-64: Maintain 16-byte stack alignment

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Message-ID: <CALCETrW_+NT5b3ZqRKsJeoRvy5Y+nF1G9rPoiTvbVijyg_vzWg@mail.gmail.com>
Date:   Wed, 11 Jan 2017 10:20:45 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: x86-64: Maintain 16-byte stack alignment

On Wed, Jan 11, 2017 at 12:09 AM, Herbert Xu
<herbert@...dor.apana.org.au> wrote:
> On Wed, Jan 11, 2017 at 08:06:54AM +0000, Ard Biesheuvel wrote:
>>
>> Couldn't we update the __aligned(x) macro to emit 32 if arch == x86
>> and x == 16? All other cases should work just fine afaict
>
> Not everyone uses that macro.  You'd also need to add some checks
> to stop people from using the gcc __attribute__ directly.
>

You'd also have to audit things to make sure that __aligned__(16)
isn't being used for non-stack purposes.  After all, __aligned__(16)
in static data is fine, and it's also fine as a promise to GCC that
some object is 16-byte aligned.

--Andy