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 09B3ACAC59A for ; Wed, 17 Sep 2025 13:25:17 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1uys8u-0003bU-LF; Wed, 17 Sep 2025 09:24:16 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1uys8r-0003af-6e for qemu-devel@nongnu.org; Wed, 17 Sep 2025 09:24:14 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1uys8o-0001Sd-5f for qemu-devel@nongnu.org; Wed, 17 Sep 2025 09:24:12 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1758115448; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=hLRV015Povt+VNSvSRUbgvoZOhQwts5tuReFT3LIL+Q=; b=imINsUi1zaoak+U84XPEaMPtZMuQ08EdlbY0HlnOPKyR9BRjquPi6pSkK+YtjfaHGLJcFU 6awexKLxEfZlSyXdH7lRyXT4cB3Fgu/LTjg5nlugq0wlM5s/t2N8kYx4a3v+yMHA0CWCZU 6gNyx5g1lNES/maSblLlCUE8wBgtZvk= Received: from mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-658-PodyvTI3NAK437XH95VlJw-1; Wed, 17 Sep 2025 09:24:07 -0400 X-MC-Unique: PodyvTI3NAK437XH95VlJw-1 X-Mimecast-MFC-AGG-ID: PodyvTI3NAK437XH95VlJw_1758115441 Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.4]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 774A11800562; Wed, 17 Sep 2025 13:23:58 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.195]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 716B33002D26; Wed, 17 Sep 2025 13:23:39 +0000 (UTC) Date: Wed, 17 Sep 2025 14:23:35 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Akihiko Odaki , qemu-devel@nongnu.org, Alex Williamson , =?utf-8?Q?C=C3=A9dric?= Le Goater , Paolo Bonzini , Eduardo Habkost , Peter Xu , David Hildenbrand , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= , Richard Henderson , Helge Deller , =?utf-8?Q?Marc-Andr=C3=A9?= Lureau , "Michael S. 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Iglesias" , xen-devel@lists.xenproject.org Subject: Re: [PATCH v3 0/7] Do not unparent in instance_finalize() Message-ID: References: <20250917-use-v3-0-72c2a6887c6c@rsg.ci.i.u-tokyo.ac.jp> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/2.2.14 (2025-02-20) X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 Received-SPF: pass client-ip=170.10.133.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 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?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Wed, Sep 17, 2025 at 02:17:35PM +0100, Daniel P. Berrangé wrote: > On Wed, Sep 17, 2025 at 09:24:04PM +0900, Akihiko Odaki wrote: > > On 2025/09/17 20:57, Daniel P. Berrangé wrote: > > > On Wed, Sep 17, 2025 at 07:13:25PM +0900, Akihiko Odaki wrote: > > > > Based-on: > > > > ("[PATCH v2 00/14] hw/pci-host/raven clean ups") > > > > > > > > Supersedes: <20240829-memory-v1-1-ac07af2f4fa5@daynix.com> > > > > ("[PATCH] docs/devel: Prohibit calling object_unparent() for memory region") > > > > > > > > Children are automatically unparented so manually unparenting is > > > > unnecessary. > > > > > > Where is automatic unparenting you're referring to being done ? > > > > > > > Worse, automatic unparenting happens before the instance_finalize() > > > > callback of the parent gets called, so object_unparent() calls in > > > > the callback will refer to objects that are already unparented, which > > > > is semantically incorrect. > > > > > > IIUC, object_property_add_child will acquire a reference on > > > the child, and object_property_del_child (and thus > > > object_unparent) will release that reference. > > > > > > The 'object_finalize' method, and thus 'instance_finalize' > > > callback, won't be invoked until the last reference is > > > dropped on the object in question. > > > > > > IOW, it should be impossible for 'object_finalize' to ever > > > run, as long as the child has a parent set. > > > > > > So if we're in the 'finalize' then 'object_unparent' must > > > be a no-op as the child must already have no references > > > held and thus no parent. > > > > > > IOW, the reason to remove 'object_unparent' calls from > > > finalize is surely because they do nothing at all, > > > rather than this talk about callbacks being run at the > > > wrong time ? > > > > This patch series deals with the situation where the parent calls > > object_unparent() in its instance_finalize() callback. The process of > > finalization looks like as follows: > > > > 1. The parent's reference count reaches to zero. Please note that there can > > be remaining children that are referenced by the parent at this point. > > > > 2. object_finalize() is called. > > > > 2a. object_property_del_all() is called and the parent releases references > > to its children. This is what I referred as "automatic unparenting". The > > children without any other references will be finalized here. > > > > 2b. instance_finalize() is called. Past children may be already finalized, > > and calling object_unparent() here will cause dereferencing finalized > > objects in that case, which should be avoided. > > Oh, so these object_unparent calls run by the parent, against the child > in fact use-after-free flaws. Oh actually not a use-after-free since memory regions aren't directly freed by object_finalize. With 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 :|