linux-kernel - Re: [PATCH] sha: prevent removal of memset as dead store in sha1

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Message-ID: <84144f021002250816o2c2cef0fke484c7e43256dba4@mail.gmail.com>
Date:	Thu, 25 Feb 2010 18:16:59 +0200
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Mikael Pettersson <mikpe@...uu.se>
Cc:	Roel Kluin <roel.kluin@...il.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S. Miller" <davem@...emloft.net>,
	linux-crypto@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sha: prevent removal of memset as dead store in 
	sha1_update()

On Thu, Feb 25, 2010 at 5:56 PM, Mikael Pettersson <mikpe@...uu.se> wrote:
> I fear that the only portable (across compiler versions) and safe
> solution is to invoke an assembly-coded dummy function with prototype
>
>        void use(void *p);
>
> and rewrite the code above as
>
>        {
>                u32 temp[...];
>                ...
>                memset(temp, 0, sizeof temp);
>                use(temp);
>        }
>
> This forces the compiler to consider the buffer live after the
> memset, so the memset cannot be eliminated.

So is there some "do not optimize" GCC magic that we could use for a
memzero_secret() helper function?

                        Pekka
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