From: Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: x86-64: Maintain 16-byte stack alignment
Date: Thu, 12 Jan 2017 14:55:04 -0600
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Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        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: Andy Lutomirski <luto@amacapital.net>
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On Thu, Jan 12, 2017 at 02:15:11PM -0600, Josh Poimboeuf wrote:
> On Thu, Jan 12, 2017 at 12:08:07PM -0800, Andy Lutomirski wrote:
> > 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.
> 
> Actually we already found all such cases and fixed them by forcing a new
> stack frame, thanks to objtool.  For example, see 55a76b59b5fe.
> 
> > 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?

This could work.  Only concerns I'd have are:

- Are there (or will there be in the future) any asm functions which
  assume a 16-byte aligned stack?  (Seems unlikely.  Stack alignment is
  common in the crypto code but they do the alignment manually.)

- Who's going to run sparse all the time to catch unauthorized users of
  __aligned__(16)?

-- 
Josh