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Tue, 25 Nov 2025 18:48:47 +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 9F4C918004D8; Tue, 25 Nov 2025 18:48:46 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 3E36221E6A27; Tue, 25 Nov 2025 19:48:44 +0100 (CET) From: Markus Armbruster To: Peter Xu Cc: =?utf-8?Q?Marc-Andr=C3=A9?= Lureau , qemu-devel@nongnu.org, farosas@suse.de, peter.maydell@linaro.org Subject: Re: [PATCH] migration: Fix double-free on error path In-Reply-To: (Peter Xu's message of "Tue, 25 Nov 2025 10:47:16 -0500") References: <20251125070554.2256181-1-armbru@redhat.com> <875xayxo6t.fsf@pond.sub.org> Date: Tue, 25 Nov 2025 19:48:44 +0100 Message-ID: <87ikeygd83.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=170.10.133.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 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_H3=0.001, RCVD_IN_MSPIKE_WL=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 01:59:54PM +0100, Markus Armbruster wrote: >> Marc-Andr=C3=A9 Lureau writes: >>=20 >> > Hi >> > >> > On Tue, Nov 25, 2025 at 11:06=E2=80=AFAM Markus Armbruster wrote: >> >> >> >> Fixes: ffaa1b50a879 (migration: Use warn_reportf_err() where appropri= ate) >> >> Resolves: Coverity CID 1643463 >> >> Signed-off-by: Markus Armbruster >> > >> > Reviewed-by: Marc-Andr=C3=A9 Lureau >> > >> >> --- >> >> migration/multifd.c | 2 +- >> >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> >> >> diff --git a/migration/multifd.c b/migration/multifd.c >> >> index 6210454838..3203dc98e1 100644 >> >> --- a/migration/multifd.c >> >> +++ b/migration/multifd.c >> >> @@ -450,7 +450,7 @@ static void multifd_send_set_error(Error *err) >> >> */ >> >> static void migration_ioc_shutdown_gracefully(QIOChannel *ioc) >> >> { >> >> - g_autoptr(Error) local_err =3D NULL; >> >> + Error *local_err =3D NULL; >> >> >> >> if (!migration_has_failed(migrate_get_current()) && >> >> object_dynamic_cast((Object *)ioc, TYPE_QIO_CHANNEL_TLS)) { >> >> -- >> >> 2.49.0 >> >> >> > >> > Maybe warn_reportf_err() should take a Error **err instead, and clear >> > it (and accept NULL values) >>=20 >> Our deallocating functions don't work that way. g_free(), g_realloc(), freeaddrinfo(), qapi_free_T(), visit_free(), qcrypto_FOO_free(), aio_task_pool_free(), qemu_opts_free(), timer_free(), ... >> Having them take a pointer by reference and clear it gets rid of *one* >> dangling reference. There may be more. > > True. However I need to confess I like Marc-Andr=C3=A9's proposal.. Norm= ally we > only have one Error object, or >1 objects. > > The only thing I'm not sure is such design doesn't match with the error A= PI > (e.g. current form matches the more famous error_report_err(), and likely > others that I'm not familiar). So at least this will need some more > thoughts before all the code churns. The error.h functions and macros that free an Error object are: * error_free(), error_free_or_abort() These take a single Error * argument. * error_report_err(), warn_report_err(), error_reportf_err(), warn_reportf_err(). warn_report_err_once_cond(), warn_report_err_once() These take also a single Error * argument. * error_propagate(), error_propagate_prepend() These take an Error ** destination, and an Error * object to be propagated. They take ownership of the latter. They either store it in the destination, or free it. Changing the latter to Error ** makes these functions vulnerable to swapped arguments. See "Error ** parameters are almost always for returning errors" below. * ERRP_GUARD() Includes automatic error_propagate() on return, immune to use after free. > > Thanks, > >>=20 >> Coverity is fairly good at finding the kind of use after free this could >> avoid. >>=20 >> Error ** parameters are almost always for returning errors. Not having >> to wonder what such a parameter is for makes code easier to read. My main argument remains this one: our deallocating functions don't work that way. For better or worse. I don't want the Error API free differently.