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

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Date:	Tue, 17 Jan 2012 14:03:09 +0200
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Laight <David.Laight@...lab.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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>,
	security@...nel.org
Subject: Re: [PATCH 2/3] sha512: reduce stack usage to safe number

On 1/16/12, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Le lundi 16 janvier 2012 à 09:56 +0000, David Laight a écrit :
>> Doesn't this badly overflow W[] ..
>>
>> > +#define SHA512_0_15(i, a, b, c, d, e, f, g, h) \
>> > +	t1 = h + e1(e) + Ch(e, f, g) + sha512_K[i] + W[i];	\
>> ...
>> > +	for (i = 0; i < 16; i += 8) {
>> ...
>> > +		SHA512_0_15(i + 7, b, c, d, e, f, g, h, a);
>> > +	}
>>
>> 	David
>>
>>
>
> No overflow since loop is done for only i=0 and i=8
>
> By the way, I suspect previous code was chosen years ago because this
> version uses less stack but adds much more code bloat.

I think W[80] was use because it's the most straightforward way to
write this code
by following spec.

All SHA definitions have full message schedule pseudocoded
before hash computation.

> size crypto/sha512_generic.o crypto/sha512_generic_old.o
>    text	   data	    bss	    dec	    hex	filename
>   17369	    704	      0	  18073	   4699	crypto/sha512_generic.o
>    8249	    704	      0	   8953	   22f9	crypto/sha512_generic_old.o

This is because SHA-512 is fundamentally 64-bit algorithm multiplied by
excessive unrolling.

Surprisingly, doing variable renaming by hand like in spec:
t1 = ...
t2 = ...
h = g;
g = f;
f = e;
e = d + T1;
d = c;
c = b;
b = a;
a = t1 + t2;

bring stack space on i386 under control too.
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