DomainKey-Signature: a=rsa-sha1; c=nofws;
        d=gmail.com; s=gamma;
        h=mime-version:in-reply-to:references:from:date:message-id:subject:to
         :cc:content-type:content-transfer-encoding;
        b=RdvpWOjAl4wAUgMLiQglYtGF131iKOxQfCoYQZgWD4p0waWfhN4cS5GrEXlc2SXYoy
         l/JOygUEQysWI1Q/8MAlo+mXr2gQeTpwlGHiwkqZDsHJL42wjQKjbyrO5L0twhHA/4Sp
         mbVReQnvY21QPdP8PXuqOWRkwb/r9EykcvPCw=
MIME-Version: 1.0
In-Reply-To: <84144f021002250816o2c2cef0fke484c7e43256dba4@mail.gmail.com>
References: <4B8692E3.9030509@gmail.com> <19334.40337.651079.440912@pilspetsen.it.uu.se> 
	<84144f021002250816o2c2cef0fke484c7e43256dba4@mail.gmail.com>
From: roel kluin <roel.kluin@gmail.com>
Date: Thu, 25 Feb 2010 17:33:03 +0100
Message-ID: <25e057c01002250833n1e13391drfcc806df369c5a55@mail.gmail.com>
Subject: Re: [PATCH] sha: prevent removal of memset as dead store in 
	sha1_update()
To: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Mikael Pettersson <mikpe@it.uu.se>,
       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>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8BIT
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1729
Lines: 61

On Thu, Feb 25, 2010 at 5:16 PM, Pekka Enberg <penberg@cs.helsinki.fi> wrote:
> 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?
>
>                        Pekka
>

        *(volatile char *)p = *(volatile char *)p;

appears to work when called after the memset:

---
inline void ensure_memset(void* p)
{
        *(volatile char *)p = *(volatile char *)p;
}

void foo()
{
        char password[] = "secret";
        password[0]='S';
        printf ("Don't show again: %s\n", password);
        memset(password, 0, sizeof(password));
        ensure_memset(password);
}

int main(int argc, char* argv[])
{

        foo();
        int i;
        char foo3[] = "";
        char* bar = &foo3[0];
        for (i = -50; i < 50; i++)
                printf ("%c.", bar[i]);
        printf("\n");
        return 0;
}
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/