Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933475Ab0BYTry (ORCPT ); Thu, 25 Feb 2010 14:47:54 -0500 Received: from mail-wy0-f174.google.com ([74.125.82.174]:41792 "EHLO mail-wy0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932882Ab0BYTru (ORCPT ); Thu, 25 Feb 2010 14:47:50 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=message-id:date:from:user-agent:mime-version:to:cc:subject :references:in-reply-to:content-type:content-transfer-encoding; b=ISP5adX0bI9Pt20rBHXq0WMoeUhwKRj5ZHcjJJr/2EmK3p4zK9XAALchQuxgh5WpTq 8MmDsdAgMOGB6WJ9G2CWFX7qx5MAtGAXktMMAZutnaeSVrwnrn6pHlvCSxHAeq0zq/p1 NvFTNA3sME4AFpjz46/Re62ytNlGBPf2+I6G8= Message-ID: <4B86D3E4.4000205@gmail.com> Date: Thu, 25 Feb 2010 20:47:48 +0100 From: Roel Kluin User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.7) Gecko/20100120 Fedora/3.0.1-1.fc12 Thunderbird/3.0.1 MIME-Version: 1.0 To: Brian Gerst CC: Mikael Pettersson , Herbert Xu , "David S. Miller" , linux-crypto@vger.kernel.org, Andrew Morton , LKML Subject: Re: [PATCH] sha: prevent removal of memset as dead store in sha1_update() References: <4B8692E3.9030509@gmail.com> <19334.40337.651079.440912@pilspetsen.it.uu.se> <73c1f2161002250833n120cda05s9371e5ce13cc0aac@mail.gmail.com> <19334.44752.357207.382349@pilspetsen.it.uu.se> <73c1f2161002250932j5167e2fan51dc11970df00f7@mail.gmail.com> In-Reply-To: <73c1f2161002250932j5167e2fan51dc11970df00f7@mail.gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1964 Lines: 45 Op 25-02-10 18:32, Brian Gerst schreef: > On Thu, Feb 25, 2010 at 12:09 PM, Mikael Pettersson wrote: >> 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). > >>From the GCC manual, section 5.37: > If your assembler instructions access memory in an unpredictable > fashion, add `memory' to the list of clobbered registers. This will > cause GCC to not keep memory values cached in registers across the > assembler instruction and not optimize stores or loads to that memory. > You will also want to add the volatile keyword if the memory affected > is not listed in the inputs or outputs of the asm, as the `memory' > clobber does not count as a side-effect of the asm. > > -- > Brian Gerst > Also from that document: If you know how large the accessed memory is, you can add it as input or output but if this is not known, you should add memory. As an example, if you access ten bytes of a string, you can use a memory input like: {"m"( ({ struct { char x[10]; } *p = (void *)ptr ; *p; }) )}. I am new to assembly but does this mean we could use something like: #define SECURE_BZERO(x) do { \ memset(x, 0, sizeof(x)); \ asm("" : :"m"( ({ struct { char __y[ARRAY_SIZE(x)]; } *__z = \ (void *)x ; *__z; }) )); \ } while(0) Roel -- 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/