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

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Message-ID: <20170111032622.GD4515@gondor.apana.org.au>
Date:   Wed, 11 Jan 2017 11:26:22 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: x86-64: Maintain 16-byte stack alignment

On Tue, Jan 10, 2017 at 11:22:15AM -0800, Andy Lutomirski wrote:
>
> > Actually, the breakage is introduced by the patch Herbert refers to
> >
> > https://patchwork.kernel.org/patch/9468391/
> >
> > where the state is replaced by a simple
> >
> > u32 state[16] __aligned(CHACHA20_STATE_ALIGN);
> >
> > which seemed harmless enough to me. So the code above works fine.
> 
> So how about just the one-line patch of adding the
> force_align_arg_pointer?  Would that solve the problem?

It probably does.  However, this is too error-prone.  Surely
you can't expect random kernel developers to know to add this
force_align_arg_pointer every time they try to align a stack
variable to 16 bytes?

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt