netdev - Re: [PATCH 2/3] sha512: reduce stack usage to safe number

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Date:	Sat, 14 Jan 2012 13:14:33 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Alexey Dobriyan <adobriyan@...il.com>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	linux-crypto@...r.kernel.org, netdev@...r.kernel.org,
	ken@...elabs.ch, Steffen Klassert <steffen.klassert@...unet.com>,
	Eric Dumazet <eric.dumazet@...il.com>, security@...nel.org
Subject: Re: [PATCH 2/3] sha512: reduce stack usage to safe number

On Sat, Jan 14, 2012 at 12:41 PM, Alexey Dobriyan <adobriyan@...il.com> wrote:
>
> For the record, it generates "andl $15" here.

Ok. That means that gcc was able to prove that it never had any signed
values (which is certainly reasonable when you do things like "for
(i=0; i<X;i++)"). But it's better to simply not rely on gcc always
getting details like this right.

It's also better to use a model that simply doesn't even require you
as a programmer to have to even *think* about signed values.

It's easy to get "%" wrong by mistake (signed integer modulus didn't
even use to have well-defined semantics in traditional C), and there
is almost never any excuse for using it for powers-of-two.

> Here is updated patch which explicitly uses & (equally tested):

Thanks,

                     Linus
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