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

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Date:   Thu, 12 Jan 2017 09:18:40 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.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

* Herbert Xu <herbert@...dor.apana.org.au> wrote:

> > But if we can't do this with automatic verification, then I'm not sure
> > I want to do it at all.  The asm is already more precarious than I'd
> > like, and having a code path that is misaligned is asking for obscure
> > bugs down the road.
> 
> I understand the need for automated checks at this point in time.
>  But longer term this is just part of the calling ABI.  After all,
> we don't add checks everywhere to ensure people preserve rbx.

The intelligent and responsible way to introduce such post facto ABI changes is 
via a smarter assembler: which would detect the obvious cases where assembly code 
generates a misaligned stack, at build time.

Assembly code can obviously still mess up in a hard to detect fashion if it tries 
- but that's OK, as most assembly code doesn't try to go outside regular stack 
allocation patterns.

Such a static check is relatively straightforward to do in assembly tooling - and 
perhaps objtool could do this too, as it already tracks the instructions that 
change the stack offset.

( And yes, this is what the GCC guys should have done, instead of sloppily 
  introducing such silent breakages and making the whole application landscape 
  less robust ... )

Thanks,

	Ingo