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 46080D6B6B3 for ; Wed, 30 Oct 2024 17:59:01 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1t6Cxq-00084o-HD; Wed, 30 Oct 2024 13:58:38 -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 1t6Cxp-00084f-9g for qemu-devel@nongnu.org; Wed, 30 Oct 2024 13:58:37 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1t6Cxm-00074d-Oe for qemu-devel@nongnu.org; Wed, 30 Oct 2024 13:58:37 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1730311112; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=7yRY9Hy4yEeV2BpKTp3Zmo0SPoh783Z7PmWuiHqbKsw=; b=c/E9272wPNorhOfWO7Tr1Fh1qP85UWF39G345oqure6C+Gpj0uk5yQp9UNt8OYVMELZADL vZf8NM2LC+8vffWgzOk0T9wfGliOrIlchtxFFUDaqgYr1V1cUop8fOFr/Z8DyIfGZCq0tk nDOGI0tPbmBqfpO5x4iHosXcjskhb5c= Received: from mx-prod-mc-05.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-467-NXplRCQfMR-CcaV_KexWQA-1; Wed, 30 Oct 2024 13:58:27 -0400 X-MC-Unique: NXplRCQfMR-CcaV_KexWQA-1 Received: from mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.40]) (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-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 933D219560AB; Wed, 30 Oct 2024 17:58:25 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.92]) by mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 172931956054; Wed, 30 Oct 2024 17:58:19 +0000 (UTC) Date: Wed, 30 Oct 2024 17:58:15 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Peter Xu Cc: qemu-devel@nongnu.org, Markus Armbruster , Peter Maydell , Eduardo Habkost , Paolo Bonzini , Mark Cave-Ayland , Igor Mammedov , "Michael S . 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David Alan Gilbert" , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= , =?utf-8?Q?C=C3=A9dric?= Le Goater , Fabiano Rosas , Juraj Marcin Subject: Re: [PATCH RFC v2 0/7] QOM: Singleton interface Message-ID: References: <20241029211607.2114845-1-peterx@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/2.2.12 (2023-09-09) X-Scanned-By: MIMEDefang 3.0 on 10.30.177.40 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: -24 X-Spam_score: -2.5 X-Spam_bar: -- X-Spam_report: (-2.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.366, 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_H2=-0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_NONE=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, Oct 30, 2024 at 01:51:54PM -0400, Peter Xu wrote: > On Wed, Oct 30, 2024 at 04:13:57PM +0000, Daniel P. Berrangé wrote: > > On Wed, Oct 30, 2024 at 09:13:13AM -0400, Peter Xu wrote: > > > On Wed, Oct 30, 2024 at 09:48:07AM +0000, Daniel P. Berrangé wrote: > > > > On Tue, Oct 29, 2024 at 05:16:00PM -0400, Peter Xu wrote: > > > > > v1: https://lore.kernel.org/r/20241024165627.1372621-1-peterx@redhat.com > > > > > > > > > Meanwhile, migration has a long standing issue on current_migration > > > > > pointer, where it can point to freed data after the migration object is > > > > > finalized. It is debatable that the pointer can be cleared after the main > > > > > thread (1) join() the migration thread first, then (2) release the last > > > > > refcount for the migration object and clear the pointer. However there's > > > > > still major challenges [1]. With singleton, we could have a slightly but > > > > > hopefully working workaround to clear the pointer during finalize(). > > > > > > > > I'm still not entirely convinced that this singleton proposal is > > > > fixing the migration problem correctly. > > > > > > > > Based on discussions in v1, IIUC, the situation is that we have > > > > migration_shutdown() being called from qemu_cleanup(). The former > > > > will call object_unref(current_migration), but there may still > > > > be background migration threads running that access 'current_migration', > > > > and thus a potential use-after-free. > > > > > > migration thread is fine, it takes a refcount at the entry. > > > > > > And btw, taking it at the entry is racy, we've just fixed it, see (in my > > > next migration pull): > > > > > > https://lore.kernel.org/qemu-devel/20241024213056.1395400-2-peterx@redhat.com/ > > > > Yep, acquiring the refcount immediately before thread-create > > is what I meant. > > > > > The access reported was, IIUC, outside migration code, but after both > > > main/migration threads released the refcount, hence after finalize(). It > > > could be a random migration_is_running() call very late in device code, for > > > example. > > > > > > > > > > > > > > > > > Based on what the 7th patch here does, the key difference is that > > > > the finalize() method for MigrationState will set 'current_migration' > > > > to NULL after free'ing it. > > > > > > Yes. But this show case series isn't complete. We need a migration-side > > > lock finally to make it safe to access. For that, see: > > > > > > https://lore.kernel.org/qemu-devel/20241024213056.1395400-9-peterx@redhat.com/ > > > > > > > > > > > I don't believe that is safe. > > > > > > I hope after the other series applied it will be 100% safe, even though I > > > agree it's tricky. But hopefully QOM is very clean, the trickly part is > > > still within migration, and it should be less tricky than migration > > > implement a refcount on top of Object.. > > > > Ok, so with the other series applied, this does look safe, but > > it also doesn't seem to really have any dependancy on the > > single interface code. Patch 7 here looks sufficient, in combo > > with the other 2 series to avoid the use-after-free flaws. > > Patch 7, when applied without patch 6 and prior, will crash in > device-introspect-test, trying to create yet another migration object when > processing the "device-list-properties" QMP command. And it turns out > that's also not the only way QEMU can crash by that. > > Fundamentally it's because patch 7 has global operations within > init()/finalize() to fix the migration dangling pointer, hence it must not > be instanciated more than once. That's a result from moving the "assert()" into the constructor. The assert(!current_migration) can be kept in migration_object_init, the constructor could conditionally set current_migration only if it is NULL, and the finalizer could conditionally clear current_migration only if it matches the current object. There's no conceptual dependancy on having a singleton interface in the patch. 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 :|