From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 50CADC2BB1D for ; Tue, 7 Apr 2020 09:07:02 +0000 (UTC) Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 278A6206F7 for ; Tue, 7 Apr 2020 09:07:02 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 278A6206F7 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=xen-devel-bounces@lists.xenproject.org Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.89) (envelope-from ) id 1jLkBv-0007Dv-KH; Tue, 07 Apr 2020 09:06:43 +0000 Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.89) (envelope-from ) id 1jLkBv-0007Dq-6t for xen-devel@lists.xenproject.org; Tue, 07 Apr 2020 09:06:43 +0000 X-Inumbo-ID: 18e4c1fa-78af-11ea-83d8-bc764e2007e4 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-rack-iad1.inumbo.com (Halon) with ESMTPS id 18e4c1fa-78af-11ea-83d8-bc764e2007e4; Tue, 07 Apr 2020 09:06:42 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id B5EDCAC26; Tue, 7 Apr 2020 09:06:40 +0000 (UTC) Subject: Re: [PATCH v2] xen/guest_access: Harden *copy_to_guest_offset() to prevent const dest operand To: Julien Grall References: <20200404130613.26428-1-julien@xen.org> <391ef401-b5d3-2f95-5fe3-c32f372dcc92@suse.com> From: Jan Beulich Message-ID: <814d1dba-980c-3436-842a-df504909e1f6@suse.com> Date: Tue, 7 Apr 2020 11:06:34 +0200 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-BeenThere: xen-devel@lists.xenproject.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Cc: Stefano Stabellini , Wei Liu , Andrew Cooper , Julien Grall , xen-devel@lists.xenproject.org, Volodymyr Babchuk , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" On 07.04.2020 11:01, Julien Grall wrote: > Hi Jan, > > On 06/04/2020 12:01, Jan Beulich wrote: >> On 04.04.2020 15:06, Julien Grall wrote: >>> From: Julien Grall >>> >>> At the moment, *copy_to_guest_offset() will allow the hypervisor to copy >>> data to guest handle marked const. >>> >>> Thankfully, no users of the helper will do that. Rather than hoping this >>> can be caught during review, harden copy_to_guest_offset() so the build >>> will fail if such users are introduced. >>> >>> There is no easy way to check whether a const is NULL in C99. The >>> approach used is to introduce an unused variable that is non-const and >>> assign the handle. If the handle were const, this would fail at build >>> because without an explicit cast, it is not possible to assign a const >>> variable to a non-const variable. >>> >>> Suggested-by: Jan Beulich >>> Signed-off-by: Julien Grall >> >> I'm not convinced it is a good idea to add (recurring) comments >> like you do - there are more aspects of these macros that would >> be worth commenting on, and commenting on some but not all may >> give the wrong impression of all subtleties being covered (also >> for others). > > I thought you would say that, but I don't think I am the best > person to describe all the other subtetly of the code. Yet I > didn't want to not comment the oddity of using a maybe_unused > variable. Well, to me the "__maybe_unused" is telling enough. >> In any event I'd like to ask that each header gain such a >> comment only once, with the other being a tiny reference to the >> one "complete" instance. > > I am not entirely sure how this would look like. We would need > to rely on _t having the same meaning across all the headers. > This is quite easy to miss during review, so my preference > still sticks to multiple comments. Well, I did say "each header" exactly because of this. > Although I can reduce the size of the comment to one on top > of the definition of _t. Something like: "Check if the handler > is not const". Ah yes, that would seem better (with s/handler/handle/ of course). Jan