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=-2.4 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, 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 7B365C4332B for ; Fri, 20 Mar 2020 14:58:00 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (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 41F3020714 for ; Fri, 20 Mar 2020 14:58:00 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="L9pHCVu3" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 41F3020714 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:53864 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jFJ5z-0002PH-F0 for qemu-devel@archiver.kernel.org; Fri, 20 Mar 2020 10:57:59 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:53933) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jFJ5K-0001zY-PO for qemu-devel@nongnu.org; Fri, 20 Mar 2020 10:57:19 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1jFJ5J-00042e-PX for qemu-devel@nongnu.org; Fri, 20 Mar 2020 10:57:18 -0400 Received: from us-smtp-delivery-74.mimecast.com ([216.205.24.74]:36275) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1jFJ5J-00042E-MF for qemu-devel@nongnu.org; Fri, 20 Mar 2020 10:57:17 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1584716237; h=from:from:reply-to:subject:subject: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=fHak6LAq0ZQ5K614OM/3hwbMpLmgWbl48+flHPHwe2Q=; b=L9pHCVu3zu+5xWG9vfOYJlmZYuOmuCUZ/XLnVD8IdnL+w96vVP/5UwqMd0u7tuDm1a+CCC y7N13beaDDKRBrwxW8q3r5eAZnCDoCQwZ1cXQmCAyLkl1A7citpfHmn8vzFooM1lHeuIGH uCRPMck9EvjmoMb9XKgJIP+dwsiehHQ= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-443-xgbIPjlJNkmfmr4DVGLKmw-1; Fri, 20 Mar 2020 10:57:12 -0400 X-MC-Unique: xgbIPjlJNkmfmr4DVGLKmw-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 6EDE48017CE; Fri, 20 Mar 2020 14:57:11 +0000 (UTC) Received: from [10.3.112.193] (ovpn-112-193.phx2.redhat.com [10.3.112.193]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 1ECEC5C1A5; Fri, 20 Mar 2020 14:57:11 +0000 (UTC) Subject: Re: [Qemu-devel] [PATCH v3] qga: Support enum names in guest-file-seek To: Peter Maydell References: <1455053236-22735-1-git-send-email-eblake@redhat.com> From: Eric Blake Organization: Red Hat, Inc. Message-ID: <55a50784-acfe-495c-f67a-9d5407c8c19b@redhat.com> Date: Fri, 20 Mar 2020 09:57:10 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 MIME-Version: 1.0 In-Reply-To: Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 216.205.24.74 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: QEMU Developers , Michael Roth Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 3/20/20 8:49 AM, Peter Maydell wrote: > On Tue, 9 Feb 2016 at 21:27, Eric Blake wrote: >> >> Magic constants are a pain to use, especially when we run the >> risk that our choice of '1' for QGA_SEEK_CUR might differ from >> the host or guest's choice of SEEK_CUR. Better is to use an >> enum value, via a qapi alternate type for back-compatibility. >> > Hi; dragging up this patch from 2016 to say that Coverity > has just noticed that there's some C undefined behaviour > in it (CID 1421990): Wow, took us a long time to find that! > >> +/* Convert GuestFileWhence (either a raw integer or an enum value) into >> + * the guest's SEEK_ constants. */ >> +int ga_parse_whence(GuestFileWhence *whence, Error **errp) >> +{ >> + /* Exploit the fact that we picked values to match QGA_SEEK_*. */ >> + if (whence->type == QTYPE_QSTRING) { >> + whence->type = QTYPE_QINT; >> + whence->u.value = whence->u.name; > > Here whence->u.value and whence->u.name are two different > fields in a union generated by QAPI: > > typedef enum QGASeek { > QGA_SEEK_SET, > QGA_SEEK_CUR, > QGA_SEEK_END, > QGA_SEEK__MAX, > } QGASeek; > > struct GuestFileWhence { > QType type; > union { /* union tag is @type */ > int64_t value; > QGASeek name; > } u; > }; > > So u.value and u.name overlap in storage. The C standard > says that this assignment is only valid if the overlap is > exact and the two objects have qualified or unqualified > versions of a compatible type. In this case the enum > type is likely not the same size as an int64_t, and so > we have undefined behaviour. You are (well, Coverity is) absolutely right! Patch coming up. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org