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

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Date:	Thu, 25 Feb 2010 18:09:36 +0100
From:	Mikael Pettersson <mikpe@...uu.se>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Mikael Pettersson <mikpe@...uu.se>,
	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()

Brian Gerst wrote:
> Would barrier() (which is a simple memory clobber) after the memset work?

I don't know. It's implemented as an asm with a "memory" clobber,
but I wouldn't bet on that forcing previous writes to a dying object
to actally be performed (it would have to have a data-dependency on
the dying object, but I don't think there is one).

void secure_bzero(void *p, size_t n)
{
	memset(p, 0, n);
	asm("" : : "m"(*(char*)p));
}

seems to work, but as the object in general will be larger than a
single byte, I'd like to see some confirmation from the gcc folks
first that this will in fact work.
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