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 4D1E6D0EE14 for ; Tue, 25 Nov 2025 19:03:31 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1vNyJg-0002v7-0n; Tue, 25 Nov 2025 14:03:08 -0500 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 1vNyJe-0002sm-Cv for qemu-devel@nongnu.org; Tue, 25 Nov 2025 14:03:06 -0500 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 1vNyJb-0006yJ-As for qemu-devel@nongnu.org; Tue, 25 Nov 2025 14:03:06 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1764097381; 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=DWnyHATjJsphW/0iYKwSv64tZark0wKKl/qXRN9veNw=; b=LHnUI0c+w3KWS30AJfcJC97ae34G92SnLW/BLIm5R73jsmnA/2NzevQcSBX0/aPDq0MP0T lP0jkO55Z4jOF+XqXWVnxPv0iD+uKU0rffqEl7i6qgBVp5tCz5X2OiM49vQNFKt147mWrG ov0B+epbUEWZJ/INNrTFXFODAne+q5E= Received: from mx-prod-mc-08.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-653-XOhloA8SN7aR-LkksWsnJg-1; Tue, 25 Nov 2025 14:02:59 -0500 X-MC-Unique: XOhloA8SN7aR-LkksWsnJg-1 X-Mimecast-MFC-AGG-ID: XOhloA8SN7aR-LkksWsnJg_1764097379 Received: from mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.93]) (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-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id CAA9E1800342; Tue, 25 Nov 2025 19:02:58 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.45.242.3]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 5D9531800451; Tue, 25 Nov 2025 19:02:58 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id F229D21E6A27; Tue, 25 Nov 2025 20:02:55 +0100 (CET) From: Markus Armbruster To: Peter Xu Cc: Daniel P. =?utf-8?Q?Berrang=C3=A9?= , qemu-devel@nongnu.org, farosas@suse.de, peter.maydell@linaro.org Subject: Re: g_autoptr(Error) In-Reply-To: (Peter Xu's message of "Tue, 25 Nov 2025 11:15:37 -0500") References: <20251125070554.2256181-1-armbru@redhat.com> <871plmk1bc.fsf@pond.sub.org> <87jyzexrly.fsf@pond.sub.org> Date: Tue, 25 Nov 2025 20:02:55 +0100 Message-ID: <87cy56gckg.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 Received-SPF: pass client-ip=170.10.129.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -22 X-Spam_score: -2.3 X-Spam_bar: -- X-Spam_report: (-2.3 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.152, 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_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: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Peter Xu writes: > On Tue, Nov 25, 2025 at 12:46:01PM +0100, Markus Armbruster wrote: >> Daniel P. Berrang=C3=A9 writes: >>=20 >> > On Tue, Nov 25, 2025 at 08:40:07AM +0100, Markus Armbruster wrote: >> >> g_autoptr(T) is quite useful when the object's extent matches the >> >> function's. >> >>=20 >> >> This isn't the case for an Error object the function propagates to its >> >> caller. It is the case for an Error object the function reports or >> >> handles itself. However, the functions to report Error also free it. > > I'd confess I didn't pay enough attention on how the error API was design= ed > deliberately to always free the Error objects before almost whenever > possible. But I see now, thanks for the write up. You're welcome! >> >>=20 >> >> Thus, g_autoptr(Error) is rarely applicable. We have just three >> >> instances out of >1100 local Error variables, all in migration code. >> >>=20 >> >> Two want to move the error to the MigrationState for later handling / >> >> reporting. Since migrate_set_error() doesn't move, but stores a copy, >> >> the original needs to be freed, and g_autoptr() is correct there. We >> >> have 17 more that instead manually free with error_free() or >> >> error_report_err() right after migrate_set_error(). >> >>=20 >> >> We recently discussed storing a copy vs. move the original: >> >>=20 >> >> From: Peter Xu >> >> Subject: Re: [PATCH 0/3] migration: Error fixes and improvements >> >> Date: Mon, 17 Nov 2025 11:03:37 -0500 >> >> Message-ID: >> >>=20 >> >> The two g_autoptr() gave me pause when I investigated this topic, sim= ply >> >> because they deviate from the common pattern migrate_set_error(s, err) >> >> followed by error_free() or error_report_err(). >> >>=20 >> >> The third one became wrong when I cleaned up the reporting (missed in >> >> the cleanup patch, fixed in the patch I'm replying to). I suspect my >> >> mistake escaped review for the same reason I made it: g_autoptr(Error) >> >> is unusual and not visible in the patch hunk. >> >>=20 >> >> Would you like me to replace the two correct uses of g_autoptr(Error)= by >> >> more common usage? > > Works for me. > > Now I also think it should be good migrate_set_error() follow QEMU's Error > API design if we decide to stick with it freeing errors in such APIs. > > Said that, I wonder if you think we could still consider passing Error** > into migrate_set_error(), though, which will be a merged solution of > current Error API and what Marc-Andre proposed on resetting pointers to > avoid any possible UAF, which I would still slightly prefer personally. > > If we rework migrate_set_error() to take ownership first, then we can > naturally drop the two use cases, and remove the cleanup function. > > Markus, please also let me know if you want me to do it. I think the first step should replace the two g_autoptr() by error_free(), then delete g_autoptr() support. A possible second step is to replace migrate_set_error() by a function that takes ownership. "Replace" because I think migrate_set_error() would be a bad name for such a function. What's a better name? Naming is hard... migrate_error_propagate_to_state()? Because there's similarity: error_propagate(errp, err); stores @err in @errp, or else frees it, and migrate_error_propagate_to_state(s, err) stores @err in @s, or else frees it. We could also forgo encapsulation and simply use error_propagate(&s->error, err); Matter of taste, which means migration maintainers decide. I can do just the first step, or both. Up to you. [...]