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Mon, 22 Nov 2021 10:03:32 +0000 Message-ID: <7fd116a3-90e9-5d4f-3a9f-bcc15338c14c@virtuozzo.com> Date: Mon, 22 Nov 2021 13:03:29 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.1.0 Subject: Re: [PATCH] ui/vnc-clipboard: fix adding notifier twice Content-Language: en-US To: Nikta Lapshin , qemu-devel@nongnu.org Cc: kraxel@redhat.com, den@openvz.org References: <20211110103800.2266729-1-vsementsov@virtuozzo.com> <5de2287b-5c1d-7350-b4f6-eaef67095315@virtuozzo.com> From: Vladimir Sementsov-Ogievskiy In-Reply-To: <5de2287b-5c1d-7350-b4f6-eaef67095315@virtuozzo.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: AM6PR01CA0068.eurprd01.prod.exchangelabs.com (2603:10a6:20b:e0::45) To AM9PR08MB6737.eurprd08.prod.outlook.com (2603:10a6:20b:304::18) MIME-Version: 1.0 Received: from [192.168.100.10] (185.215.60.233) by AM6PR01CA0068.eurprd01.prod.exchangelabs.com (2603:10a6:20b:e0::45) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4713.19 via Frontend Transport; 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envelope-from=vsementsov@virtuozzo.com; helo=EUR03-AM5-obe.outbound.protection.outlook.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, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, MSGID_FROM_MTA_HEADER=0.001, NICE_REPLY_A=-0.097, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-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" 21.11.2021 22:12, Nikta Lapshin wrote: > > On 11/10/21 13:38, Vladimir Sementsov-Ogievskiy wrote: >> vnc_server_cut_text_caps() is not guaranteed to be called only once. >> >> If it called twice, we finally call notifier_list_add() twice with same >> element. Which leads to loopback QLIST. So, on next >> notifier_list_notify() we'll loop forever and QEMU stuck. >> >> So, let's only register new notifier if it's not yet registered. >> >> Note, that similar check is used in vdagent_chr_recv_caps() (before >> call qemu_clipboard_peer_register()), and also before >> qemu_clipboard_peer_unregister() call in vdagent_disconnect() and in >> vnc_disconnect_finish(). >> >> Signed-off-by: Vladimir Sementsov-Ogievskiy >> --- >> >> Hi all! >> >> After backporting clipboard patches to our Rhel7-based downstream, we >> faced Qemu stuck in notifier_list_notify(): >> >>      (gdb) bt >>    #0  vnc_clipboard_notify (notifier=0x564427f283f8, data=0x564426c59a70) at ui/vnc-clipboard.c:193 >>    #1  0x0000564423455887 in notifier_list_notify (list=list@entry=0x564423d2b258 , data=data@entry=0x564426c59a70) at util/notify.c:40 >>    #2  0x00005644233273bf in qemu_clipboard_update (info=info@entry=0x564426c59a70) at ui/clipboard.c:19 >>    #3  0x000056442334efd2 in vnc_client_cut_text_ext (vs=vs@entry=0x564427f18000, len=len@entry=4, flags=, >>              data=data@entry=0x5644263cc00c "\002\f\001\251\020\377\377\377!\377\377\377\314\376\377\377\315\376\377\377 \377\377\377\316\345\241\300\307\376\377\377\310\376\377\377\376\376\377\377\a") >>              at ui/vnc-clipboard.c:256 >>    #4  0x000056442333b172 in protocol_client_msg (vs=0x564427f18000, data=0x5644263cc000 "\006", len=) at ui/vnc.c:2396 >>    #5  0x0000564423338af6 in vnc_client_read (vs=0x564427f18000) at ui/vnc.c:1537 >>    #6  vnc_client_io (ioc=, condition=G_IO_IN, opaque=0x564427f18000) at ui/vnc.c:1559 >>    #7  0x00007f07b02cf147 in g_main_dispatch (context=0x564425546bb0) at gmain.c:3192 >>    #8  g_main_context_dispatch (context=context@entry=0x564425546bb0) at gmain.c:3845 >>    #9  0x00005644234468f7 in glib_pollfds_poll () at util/main-loop.c:215 >>    #10 os_host_main_loop_wait (timeout=) at util/main-loop.c:238 >>    #11 main_loop_wait (nonblocking=nonblocking@entry=0) at util/main-loop.c:497 >> >> >> investigations shows, that notifier list has only one element which points to itself as next. So, we are in the endless loop. >> >> Seems that it's possible, if vnc_server_cut_text_caps() called twice. Then it registers notifier twice and it added to QLIST twice, which leads to the situation. >> >> >> I don't have any reproducer and not sure that bug may be reproduced on >> master. >> >> I'm not familiar with ui code - may be vnc_server_cut_text_caps() should >> never be called twice? Or notifier should be removed somehow before the >> second call? Maybe this patch just shadows another bug? >> >> But what I do know, is that we should not put same element into QLIST >> twice. And if the check I propose is not needed we should add an >> assertion instead: >> >>    assert(!vs->cbpeer.update.notify); >> >> >>   ui/vnc-clipboard.c | 10 ++++++---- >>   1 file changed, 6 insertions(+), 4 deletions(-) >> >> diff --git a/ui/vnc-clipboard.c b/ui/vnc-clipboard.c >> index 9f077965d0..67284b556c 100644 >> --- a/ui/vnc-clipboard.c >> +++ b/ui/vnc-clipboard.c >> @@ -316,8 +316,10 @@ void vnc_server_cut_text_caps(VncState *vs) >>       caps[1] = 0; >>       vnc_clipboard_send(vs, 2, caps); >> -    vs->cbpeer.name = "vnc"; >> -    vs->cbpeer.update.notify = vnc_clipboard_notify; >> -    vs->cbpeer.request = vnc_clipboard_request; >> -    qemu_clipboard_peer_register(&vs->cbpeer); >> +    if (!vs->cbpeer.update.notify) { >> +        vs->cbpeer.name = "vnc"; >> +        vs->cbpeer.update.notify = vnc_clipboard_notify; >> +        vs->cbpeer.request = vnc_clipboard_request; >> +        qemu_clipboard_peer_register(&vs->cbpeer); >> +    } >>   } > > > Perhaps QLIST_IS_INSERTED will be suitable for such checks because I couldn't find any initialize of .notify pointer so it can potentially be UB. > I think, vs structure should be initialized to zero at start. For example at start of vnc_connect(): "VncState *vs = g_new0(VncState, 1);", and I didn't find another place where it is allocated. Also similar checks are already used in the code, so I think better to behave similarly here. -- Best regards, Vladimir