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

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Message-ID: <25e057c01002250833n1e13391drfcc806df369c5a55@mail.gmail.com>
Date:	Thu, 25 Feb 2010 17:33:03 +0100
From:	roel kluin <roel.kluin@...il.com>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Mikael Pettersson <mikpe@...uu.se>,
	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 5:16 PM, Pekka Enberg <penberg@...helsinki.fi> wrote:
> 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
>

        *(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;
}
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