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=-7.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 A055CC433E0 for ; Thu, 11 Mar 2021 08:33:30 +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 41BA664EF6 for ; Thu, 11 Mar 2021 08:33:30 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 41BA664EF6 Authentication-Results: mail.kernel.org; dmarc=fail (p=quarantine dis=none) header.from=suse.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=xen-devel-bounces@lists.xenproject.org Received: from list by lists.xenproject.org with outflank-mailman.96393.182294 (Exim 4.92) (envelope-from ) id 1lKGkt-0000FY-9a; Thu, 11 Mar 2021 08:33:15 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 96393.182294; Thu, 11 Mar 2021 08:33:15 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lKGkt-0000FR-6M; Thu, 11 Mar 2021 08:33:15 +0000 Received: by outflank-mailman (input) for mailman id 96393; Thu, 11 Mar 2021 08:33:13 +0000 Received: from all-amaz-eas1.inumbo.com ([34.197.232.57] helo=us1-amaz-eas2.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lKGkr-0000FM-Ir for xen-devel@lists.xenproject.org; Thu, 11 Mar 2021 08:33:13 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id 7dcdca86-8e75-472e-97bd-bf4c487d8215; Thu, 11 Mar 2021 08:33:12 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 0668FAC16; Thu, 11 Mar 2021 08:33:12 +0000 (UTC) X-BeenThere: xen-devel@lists.xenproject.org List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Precedence: list Sender: "Xen-devel" X-Inumbo-ID: 7dcdca86-8e75-472e-97bd-bf4c487d8215 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1615451592; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=v3TLjU1ZaXzkM5cq4AeizWMlcoE7mIvra9RmiAnL7qQ=; b=TGcDWXdqEEMN1g9FapfaMiT4WwAYEOYrwhHC8yVu9I3J4aspwYyjPv0VgM8GOMHlxdx+oP afvVdUAHolt2Jf8lZdzH7vlFnKjYujnHUgKc0T+OE+Ba5+I8OLNF52aYvOCUGKrwdMqQRg 5GoH58RJunnVc30HMTRByW12uGIt5Zs= Subject: Re: [PATCH][4.15] gnttab: work around "may be used uninitialized" warning To: =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= Cc: Julien Grall , Andrew Cooper , George Dunlap , Ian Jackson , Stefano Stabellini , Wei Liu , "xen-devel@lists.xenproject.org" References: <6bd14438-7dec-2176-eab5-5898f190c4d8@suse.com> <952ca444-2091-b7f1-3559-c728a63af37f@suse.com> From: Jan Beulich Message-ID: <957e6cab-657a-c72d-b5c7-f2e84e976a21@suse.com> Date: Thu, 11 Mar 2021 09:33:11 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit On 11.03.2021 09:25, Roger Pau Monné wrote: > On Thu, Mar 11, 2021 at 09:09:22AM +0100, Jan Beulich wrote: >> On 10.03.2021 18:52, Julien Grall wrote: >>> On 10/03/2021 16:21, Jan Beulich wrote: >>>> On 10.03.2021 15:58, Julien Grall wrote: >>>>> On 10/03/2021 10:13, Jan Beulich wrote: >>>>> 2) A compiler will not be able to help us if we are adding code >>>>> without initialized vaddrs. >>>>> >>>>> It also feels wrong to me to try to write Xen in a way that will make a >>>>> 10 years compiler happy... >>>> >>>> As said above - we've worked around limitations quite a few times >>>> in the past. This is just one more instance. >>> >>> I find amusing you wrote that when you complained multiple time when >>> someone was re-using existing bad pattern. :) >> >> Well, thing is - I don't view this as a bad pattern. The only question >> really is whether NULL is a good initializer here. As per above a non- >> canonical pointer may be better, but then we have quite a few places >> elsewhere to fix. > > Sorry for jumping in the middle but I think that would be a very > dangerous move for Xen to do. We have been using implicit conversions > of pointers to booleans all over the place, assuming that NULL == > false, hence NULL no longer mapping to false would break a lot of our > code. ie: > > if ( foo ) > free(foo); > > Would no longer work as expected. Funny you should give this example: Assuming you mean xfree(), it specifically tolerates both NULL and ZERO_BLOCK_PTR (the latter because xmalloc() may return it, and that's what it was invented for). But yes - other non-NULL checking guards would indeed break. I'm afraid every possible solution here has its downsides, and the suggested change simply follows the pattern we used so far in similar cases - without anyone objecting. Jan