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Tue, 25 Nov 2025 08:15:41 -0800 (PST) X-Google-Smtp-Source: AGHT+IF15aZGlvoT+7lmnZg1qcS30NcleLngXiJvk3/yDQGfgc6SOZAAVCROZnhckj4DsCNerhcW+g== X-Received: by 2002:a05:622a:50:b0:4ee:2a31:6795 with SMTP id d75a77b69052e-4ee5883f9a9mr236309801cf.12.1764087339437; Tue, 25 Nov 2025 08:15:39 -0800 (PST) Received: from x1.local ([142.188.210.156]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-4ee48d3a465sm110227231cf.4.2025.11.25.08.15.38 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 25 Nov 2025 08:15:39 -0800 (PST) Date: Tue, 25 Nov 2025 11:15:37 -0500 From: Peter Xu To: Markus Armbruster Cc: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= , qemu-devel@nongnu.org, farosas@suse.de, peter.maydell@linaro.org Subject: Re: g_autoptr(Error) Message-ID: References: <20251125070554.2256181-1-armbru@redhat.com> <871plmk1bc.fsf@pond.sub.org> <87jyzexrly.fsf@pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <87jyzexrly.fsf@pond.sub.org> Received-SPF: pass client-ip=170.10.133.124; envelope-from=peterx@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 On Tue, Nov 25, 2025 at 12:46:01PM +0100, Markus Armbruster wrote: > Daniel P. Berrangé writes: > > > 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. > >> > >> 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 designed deliberately to always free the Error objects before almost whenever possible. But I see now, thanks for the write up. > >> > >> Thus, g_autoptr(Error) is rarely applicable. We have just three > >> instances out of >1100 local Error variables, all in migration code. > >> > >> 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(). > >> > >> We recently discussed storing a copy vs. move the original: > >> > >> From: Peter Xu > >> Subject: Re: [PATCH 0/3] migration: Error fixes and improvements > >> Date: Mon, 17 Nov 2025 11:03:37 -0500 > >> Message-ID: > >> > >> The two g_autoptr() gave me pause when I investigated this topic, simply > >> because they deviate from the common pattern migrate_set_error(s, err) > >> followed by error_free() or error_report_err(). > >> > >> 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. > >> > >> 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 had previously proposed g_autoptr(Error) a year or two back and you > > rejected it then, so I'm surprised to see that it got into the code, > > because it requires explicit opt-in via a G_DEFINE_AUTOPTR_CLEANUP_FUNC. > > > > Unfortunately it appears exactly that was added earlier this year in > > > > commit 18eb55546a54e443d94a4c49286348176ad4b00a > > Author: Maciej S. Szmigiero > > Date: Tue Mar 4 23:03:35 2025 +0100 > > > > error: define g_autoptr() cleanup function for the Error type > > > > Automatic memory management helps avoid memory safety issues. > > > > Reviewed-by: Peter Xu > > Signed-off-by: Maciej S. Szmigiero > > Link: https://lore.kernel.org/qemu-devel/a5843c5fa64d7e5239a4316092ec0ef0d10c2320.1741124640.git.maciej.szmigiero@oracle.com > > Signed-off-by: Cédric Le Goater > > I missed it. Not he submitter's fault; it was cc'ed to me. If someone to blame, it's the reviewer. Thanks! -- Peter Xu