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Date: Thu, 25 Feb 2010 17:29:16 +0100
From: Mikael Pettersson <mikpe@it.uu.se>
To: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Mikael Pettersson <mikpe@it.uu.se>, Roel Kluin <roel.kluin@gmail.com>,
       Herbert Xu <herbert@gondor.apana.org.au>,
       "David S. Miller" <davem@davemloft.net>, linux-crypto@vger.kernel.org,
       Andrew Morton <akpm@linux-foundation.org>,
       LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] sha: prevent removal of memset as dead store in 
	sha1_update()
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Pekka Enberg writes:
 > On Thu, Feb 25, 2010 at 5:56 PM, Mikael Pettersson <mikpe@it.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?

I guess there's some -fno-builtin-... that might achieve this effect,
but that would disable all memset optimizations, not just those affecting
sensitive data.

You'd want a function attribute or magic type annotation and apply it
only to the specific cases where it's needed. Alas, I know of no such
attribute or annotation. ('volatile' doesn't work, I tried that.)

Ask on gcc@gcc.gnu.org.