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 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 smtp.lore.kernel.org (Postfix) with ESMTPS id AD470C433EF for ; Fri, 25 Feb 2022 15:10:14 +0000 (UTC) Received: from localhost ([::1]:37450 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1nNcEX-0002mC-QM for qemu-devel@archiver.kernel.org; Fri, 25 Feb 2022 10:10:13 -0500 Received: from eggs.gnu.org ([209.51.188.92]:54478) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nNbhF-0003o7-HZ for qemu-devel@nongnu.org; Fri, 25 Feb 2022 09:35:49 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]:59413) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nNbhC-0001qs-Sa for qemu-devel@nongnu.org; Fri, 25 Feb 2022 09:35:48 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1645799745; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:in-reply-to:in-reply-to: references:references; bh=Nmdmzh1Dogp3by0aurcVM3Ow6vwPOYtiuXNm15bQhE4=; b=g868R1wm4/o4o98TsE/TevhvVQHQ0GCrMQN68CgkLHT5xq0MhwxNndS70gw1wS9VzQk5lx PiENDqA1ygJUHClUWVQISNxBEq7FLuqMF/JKm6YjD9g7lZ21t01xzVNlWCLmvFXeXiDQJf Npw8l2Zrgv17cNzG5ehii5yQK84Ab70= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-333-CnvNn60HOEahhiWZZq8oow-1; Fri, 25 Feb 2022 09:35:42 -0500 X-MC-Unique: CnvNn60HOEahhiWZZq8oow-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 156D0180FD71; Fri, 25 Feb 2022 14:35:41 +0000 (UTC) Received: from redhat.com (unknown [10.33.36.132]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 4331F6FB03; Fri, 25 Feb 2022 14:35:39 +0000 (UTC) Date: Fri, 25 Feb 2022 14:35:36 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: "Michael S. Tsirkin" Subject: Re: [PATCH] qom: assert integer does not overflow Message-ID: References: <20220225140955.63949-1-mst@redhat.com> MIME-Version: 1.0 In-Reply-To: <20220225140955.63949-1-mst@redhat.com> User-Agent: Mutt/2.1.5 (2021-12-30) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=berrange@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -28 X-Spam_score: -2.9 X-Spam_bar: -- X-Spam_report: (-2.9 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.082, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: Eduardo Habkost , Jason Wang , qemu-devel@nongnu.org, Victor Tom , Paolo Bonzini , Stefano Garzarella Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, Feb 25, 2022 at 09:10:44AM -0500, Michael S. Tsirkin wrote: > QOM reference counting is not designed with an infinite amount of > references in mind, trying to take a reference in a loop will overflow > the integer. We will then eventually assert when dereferencing, but the > real problem is in object_ref so let's assert there to make such issues > cleaner and easier to debug. What is the actual bug / scenario that led you to hit this problem ? I'm surprised you saw an assert in object_unref, as that would imply you had exactly UINT32_MAX calls to object_ref and then one to object_unref. > Some micro-benchmarking shows using fetch and add this is essentially > free on x86. > > Signed-off-by: Michael S. Tsirkin > --- > qom/object.c | 6 +++++- > 1 file changed, 5 insertions(+), 1 deletion(-) > > diff --git a/qom/object.c b/qom/object.c > index 4f0677cca9..5db3974f04 100644 > --- a/qom/object.c > +++ b/qom/object.c > @@ -1167,10 +1167,14 @@ GSList *object_class_get_list_sorted(const char *implements_type, > Object *object_ref(void *objptr) > { > Object *obj = OBJECT(objptr); > + uint32_t ref; > + > if (!obj) { > return NULL; > } > - qatomic_inc(&obj->ref); > + ref = qatomic_fetch_inc(&obj->ref); > + /* Assert waaay before the integer overflows */ > + g_assert(ref < INT_MAX); Not that I expect this to hit, but why choose this lower bound instead of g_assert(ref > 0) which is the actual failure scenario, matching the existing object_unref assert. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|