From: Andy Lutomirski <luto@amacapital.net>
Subject: Re: x86-64: Maintain 16-byte stack alignment
Date: Thu, 12 Jan 2017 12:08:07 -0800
Message-ID: <CALCETrVz-wEFVUwrpS8-Ln9SWnsF5KxkqJC-Br6wJ+e0LGM9UA@mail.gmail.com>
References: <20170110143340.GA3787@gondor.apana.org.au> <20170110143913.GA3822@gondor.apana.org.au>
 <CA+55aFyuUCr3aUBhFt6T3vKyZOskLyH8Kd_2jeFkPYxxvEaXEg@mail.gmail.com>
 <20170111031124.GA4515@gondor.apana.org.au> <CA+55aFxJOzMim_d-O2E2yip8JWo0NdYs_72sNwFKSkTjy8q0Sw@mail.gmail.com>
 <CA+55aFzypqpMog8aTczWYok0MqOcMi4gGzCor6k-NdJzBoLWyQ@mail.gmail.com>
 <20170111043541.GA4944@gondor.apana.org.au> <CALCETrX_vWTWu=kh4Zawn2OuhPnLip8xTqBGjYU-BvpJ9=ruTg@mail.gmail.com>
 <CALCETrVb-NyKJajGEsLOrQqQrQ7cRm4sqLRxNr_yVE2-mjiDrg@mail.gmail.com>
 <20170112140215.rh247gwk55fjzmg7@treble> <CA+55aFxP+V6Wbq5Xw_NOksiWouEMg4gjBJgeGa-qFyxDMnTmcA@mail.gmail.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Cc: Josh Poimboeuf <jpoimboe@redhat.com>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <CA+55aFxP+V6Wbq5Xw_NOksiWouEMg4gjBJgeGa-qFyxDMnTmcA@mail.gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: linux-crypto.vger.kernel.org

On Thu, Jan 12, 2017 at 11:51 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Thu, Jan 12, 2017 at 6:02 AM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>>
>> Just to clarify, I think you're asking if, for versions of gcc which
>> don't support -mpreferred-stack-boundary=3, objtool can analyze all C
>> functions to ensure their stacks are 16-byte aligned.
>>
>> It's certainly possible, but I don't see how that solves the problem.
>> The stack will still be misaligned by entry code.  Or am I missing
>> something?
>
> I think the argument is that we *could* try to align things, if we
> just had some tool that actually then verified that we aren't missing
> anything.
>
> I'm not entirely happy with checking the generated code, though,
> because as Ingo says, you have a 50:50 chance of just getting it right
> by mistake. So I'd much rather have some static tool that checks
> things at a code level (ie coccinelle or sparse).

What I meant was checking the entry code to see if it aligns stack
frames, and good luck getting sparse to do that.  Hmm, getting 16-byte
alignment for real may actually be entirely a lost cause.  After all,
I think we have some inline functions that do asm volatile ("call
..."), and I don't see any credible way of forcing alignment short of
generating an entirely new stack frame and aligning that.  Ick.  This
whole situation stinks, and I wish that the gcc developers had been
less daft here in the first place or that we'd noticed and gotten it
fixed much longer ago.

Can we come up with a macro like STACK_ALIGN_16 that turns into
__aligned__(32) on bad gcc versions and combine that with your sparse
patch?