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 X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id AAAE3C7618A for ; Mon, 15 Jul 2019 05:38:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 88B8C20838 for ; Mon, 15 Jul 2019 05:38:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729098AbfGOFij (ORCPT ); Mon, 15 Jul 2019 01:38:39 -0400 Received: from relay.sw.ru ([185.231.240.75]:40858 "EHLO relay.sw.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726052AbfGOFii (ORCPT ); Mon, 15 Jul 2019 01:38:38 -0400 Received: from [172.16.24.21] by relay.sw.ru with esmtp (Exim 4.92) (envelope-from ) id 1hmtgp-00084E-4u; Mon, 15 Jul 2019 08:38:19 +0300 Subject: Re: [PATCH v3 0/3] kernel/notifier.c: avoid duplicate registration To: Xiaoming Ni , "gregkh@linuxfoundation.org" Cc: "adobriyan@gmail.com" , "akpm@linux-foundation.org" , "anna.schumaker@netapp.com" , "arjan@linux.intel.com" , "bfields@fieldses.org" , "chuck.lever@oracle.com" , "davem@davemloft.net" , "jlayton@kernel.org" , "luto@kernel.org" , "mingo@kernel.org" , "Nadia.Derbey@bull.net" , "paulmck@linux.vnet.ibm.com" , "semen.protsenko@linaro.org" , "stable@kernel.org" , "stern@rowland.harvard.edu" , "tglx@linutronix.de" , "torvalds@linux-foundation.org" , "trond.myklebust@hammerspace.com" , "viresh.kumar@linaro.org" , "Huangjianhui (Alex)" , Dailei , "linux-kernel@vger.kernel.org" , "linux-nfs@vger.kernel.org" , "netdev@vger.kernel.org" References: <1562728147-30251-1-git-send-email-nixiaoming@huawei.com> <8ee6f763-ccce-ab58-3d96-21f5e1622916@huawei.com> <20190712140729.GA11583@kroah.com> <65f50cf2-3051-ab55-078f-30930fe0c9bc@huawei.com> From: Vasily Averin Message-ID: <5521e5a4-66d9-aaf8-3a12-3999bfc6be8b@virtuozzo.com> Date: Mon, 15 Jul 2019 08:38:07 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: <65f50cf2-3051-ab55-078f-30930fe0c9bc@huawei.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 7/14/19 5:45 AM, Xiaoming Ni wrote: > On 2019/7/12 22:07, gregkh@linuxfoundation.org wrote: >> On Fri, Jul 12, 2019 at 09:11:57PM +0800, Xiaoming Ni wrote: >>> On 2019/7/11 21:57, Vasily Averin wrote: >>>> On 7/11/19 4:55 AM, Nixiaoming wrote: >>>>> On Wed, July 10, 2019 1:49 PM Vasily Averin wrote: >>>>>> On 7/10/19 6:09 AM, Xiaoming Ni wrote: >>>>>>> Registering the same notifier to a hook repeatedly can cause the hook >>>>>>> list to form a ring or lose other members of the list. >>>>>> >>>>>> I think is not enough to _prevent_ 2nd register attempt, >>>>>> it's enough to detect just attempt and generate warning to mark host in bad state. >>>>>> >>>>> >>>>> Duplicate registration is prevented in my patch, not just "mark host in bad state" >>>>> >>>>> Duplicate registration is checked and exited in notifier_chain_cond_register() >>>>> >>>>> Duplicate registration was checked in notifier_chain_register() but only >>>>> the alarm was triggered without exiting. added by commit 831246570d34692e >>>>> ("kernel/notifier.c: double register detection") >>>>> >>>>> My patch is like a combination of 831246570d34692e and notifier_chain_cond_register(), >>>>> which triggers an alarm and exits when a duplicate registration is detected. >>>>> >>>>>> Unexpected 2nd register of the same hook most likely will lead to 2nd unregister, >>>>>> and it can lead to host crash in any time: >>>>>> you can unregister notifier on first attempt it can be too early, it can be still in use. >>>>>> on the other hand you can never call 2nd unregister at all. >>>>> >>>>> Since the member was not added to the linked list at the time of the second registration, >>>>> no linked list ring was formed. >>>>> The member is released on the first unregistration and -ENOENT on the second unregistration. >>>>> After patching, the fault has been alleviated >>>> >>>> You are wrong here. >>>> 2nd notifier's registration is a pure bug, this should never happen. >>>> If you know the way to reproduce this situation -- you need to fix it. >>>> >>>> 2nd registration can happen in 2 cases: >>>> 1) missed rollback, when someone forget to call unregister after successfull registration, >>>> and then tried to call register again. It can lead to crash for example when according module will be unloaded. >>>> 2) some subsystem is registered twice, for example from different namespaces. >>>> in this case unregister called during sybsystem cleanup in first namespace will incorrectly remove notifier used >>>> in second namespace, it also can lead to unexpacted behaviour. >>>> >>> So in these two cases, is it more reasonable to trigger BUG() directly when checking for duplicate registration ? >>> But why does current notifier_chain_register() just trigger WARN() without exiting ? >>> notifier_chain_cond_register() direct exit without triggering WARN() ? >> >> It should recover from this, if it can be detected. The main point is >> that not all apis have to be this "robust" when used within the kernel >> as we do allow for the callers to know what they are doing :) >> > In the notifier_chain_register(), the condition ( (*nl) == n) is the same registration of the same hook. > We can intercept this situation and avoid forming a linked list ring to make the API more rob Once again -- yes, you CAN prevent list corruption, but you CANNOT recover the host and return it back to safe state. If double register event was detected -- it means you have bug in kernel. Yes, you can add BUG here and crash the host immediately, but I prefer to use warning in such situations. >> If this does not cause any additional problems or slow downs, it's >> probably fine to add. >> > Notifier_chain_register() is not a system hotspot function. > At the same time, there is already a WARN_ONCE judgment. There is no new judgment in the new patch. > It only changes the processing under the condition of (*nl) == n, which will not cause performance problems. > At the same time, avoiding the formation of a link ring can make the system more robust. I disagree, yes, node will have correct list, but anyway node will work wrong and can crash the host in any time. >> thanks, >> >> greg k-h >> >> . >> > Thanks > > Xiaoming Ni > > >