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

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Message-ID: <CALCETrXygy1ACDyeWRE++hLF=b4q17mTYOmWWY94wKN1Fm7xGQ@mail.gmail.com>
Date:   Tue, 10 Jan 2017 09:09:40 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: x86-64: Maintain 16-byte stack alignment

On Tue, Jan 10, 2017 at 9:05 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Tue, Jan 10, 2017 at 6:39 AM, Herbert Xu <herbert@...dor.apana.org.au> wrote:
>>
>> BTW this is with Debian gcc 4.7.2 which does not allow an 8-byte
>> stack alignment as attempted by the Makefile:
>
> I'm pretty sure we have random asm code that may not maintain a
> sus16-byte stack alignment when it calls other code (including, in some
> cases, calling C code).

I suspect so.

If we change this, changing pt_regs might make sense but is kind of
weird.  It also needs to be tested with and without frame pointers.

>
> So I'm not at all convinced that this is a good idea. We shouldn't
> expect 16-byte alignment to be something trustworthy.
>
>                  Linus



-- 
Andy Lutomirski
AMA Capital Management, LLC