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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,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 DE71BC73C66 for ; Sun, 14 Jul 2019 02:45:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C1E7320848 for ; Sun, 14 Jul 2019 02:45:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728193AbfGNCpu (ORCPT ); Sat, 13 Jul 2019 22:45:50 -0400 Received: from szxga06-in.huawei.com ([45.249.212.32]:56550 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727918AbfGNCpt (ORCPT ); Sat, 13 Jul 2019 22:45:49 -0400 Received: from DGGEMS414-HUB.china.huawei.com (unknown [172.30.72.58]) by Forcepoint Email with ESMTP id 12EC11D3CA4F278F5E32; Sun, 14 Jul 2019 10:45:46 +0800 (CST) Received: from [127.0.0.1] (10.57.88.168) by DGGEMS414-HUB.china.huawei.com (10.3.19.214) with Microsoft SMTP Server id 14.3.439.0; Sun, 14 Jul 2019 10:45:40 +0800 Subject: Re: [PATCH v3 0/3] kernel/notifier.c: avoid duplicate registration To: "gregkh@linuxfoundation.org" CC: Vasily Averin , "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> From: Xiaoming Ni Message-ID: <65f50cf2-3051-ab55-078f-30930fe0c9bc@huawei.com> Date: Sun, 14 Jul 2019 10:45:39 +0800 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <20190712140729.GA11583@kroah.com> Content-Type: text/plain; charset="gbk" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.57.88.168] X-CFilter-Loop: Reflected Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > 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. > thanks, > > greg k-h > > . > Thanks Xiaoming Ni