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=-13.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham 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 926DFC64E7B for ; Mon, 30 Nov 2020 18:43:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3F7C420705 for ; Mon, 30 Nov 2020 18:43:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388181AbgK3Smp (ORCPT ); Mon, 30 Nov 2020 13:42:45 -0500 Received: from out03.mta.xmission.com ([166.70.13.233]:53894 "EHLO out03.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727626AbgK3Smp (ORCPT ); Mon, 30 Nov 2020 13:42:45 -0500 Received: from in01.mta.xmission.com ([166.70.13.51]) by out03.mta.xmission.com with esmtps (TLS1.2) tls TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 (Exim 4.93) (envelope-from ) id 1kjo7g-004lQG-08; Mon, 30 Nov 2020 11:42:04 -0700 Received: from ip68-227-160-95.om.om.cox.net ([68.227.160.95] helo=x220.xmission.com) by in01.mta.xmission.com with esmtpsa (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.87) (envelope-from ) id 1kjo7e-0001dC-QW; Mon, 30 Nov 2020 11:42:03 -0700 From: ebiederm@xmission.com (Eric W. Biederman) To: Wen Yang Cc: Alexey Dobriyan , Christian Brauner , Oleg Nesterov , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org References: <20201128175850.19484-1-wenyang@linux.alibaba.com> Date: Mon, 30 Nov 2020 12:41:33 -0600 In-Reply-To: <20201128175850.19484-1-wenyang@linux.alibaba.com> (Wen Yang's message of "Sun, 29 Nov 2020 01:58:50 +0800") Message-ID: <87zh2yit5u.fsf@x220.int.ebiederm.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-SPF: eid=1kjo7e-0001dC-QW;;;mid=<87zh2yit5u.fsf@x220.int.ebiederm.org>;;;hst=in01.mta.xmission.com;;;ip=68.227.160.95;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX1+D1VhZMNzyY+vufufIEE54onRJFTivgMQ= X-SA-Exim-Connect-IP: 68.227.160.95 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [PATCH] proc: add locking checks in proc_inode_is_dead X-SA-Exim-Version: 4.2.1 (built Thu, 05 May 2016 13:38:54 -0600) X-SA-Exim-Scanned: Yes (on in01.mta.xmission.com) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Wen Yang writes: > The proc_inode_is_dead function might race with __unhash_process. > This will result in a whole bunch of stale proc entries being cached. > To prevent that, add the required locking. I assume you are talking about during proc_task_readdir? It is completely possible for the proc_inode_is_dead to report the inode is still alive and then for unhash_process to happen when afterwards. Have you been able to trigger this race in practice? Ouch!!!! Oleg I just looked the introduction of proc_inode_is_dead in d855a4b79f49 ("proc: don't (ab)use ->group_leader in proc_task_readdir() paths") introduced a ``regression''. Breaking the logic introduced in 7d8952440f40 ("[PATCH] procfs: Fix listing of /proc/NOT_A_TGID/task") to keep those directory listings not showing up. Given that it has been 6 years and no one has cared it doesn't look like an actual regression but it does suggest the proc_inode_is_dead can be removed entirely as it does not achieve anything in proc_task_readdir. As for removing the race. I expect the thing to do is to modify proc_pid_is_alive to verify the that the pid is still alive. Oh but look get_task_pid verifies that thread_pid is not NULL and unhash_process sets thread_pid to NULL. My brain is too fuzzy right now to tell if it is possible for get_task_pid to return a pid and then for that pid to pass through unhash_process, while the code is still in proc_pid_make_inode. proc_inode_is_dead is definitely not the place to look to close races. Eric > Signed-off-by: Wen Yang > Cc: Oleg Nesterov > Cc: "Eric W. Biederman" > Cc: Alexey Dobriyan > Cc: Christian Brauner > Cc: linux-kernel@vger.kernel.org > Cc: linux-fsdevel@vger.kernel.org > --- > fs/proc/base.c | 8 +++++++- > 1 file changed, 7 insertions(+), 1 deletion(-) > > diff --git a/fs/proc/base.c b/fs/proc/base.c > index 1bc9bcd..59720bc 100644 > --- a/fs/proc/base.c > +++ b/fs/proc/base.c > @@ -1994,7 +1994,13 @@ static int pid_revalidate(struct dentry *dentry, unsigned int flags) > > static inline bool proc_inode_is_dead(struct inode *inode) > { > - return !proc_pid(inode)->tasks[PIDTYPE_PID].first; > + bool has_task; > + > + read_lock(&tasklist_lock); > + has_task = pid_has_task(proc_pid(inode), PIDTYPE_PID); > + read_unlock(&tasklist_lock); > + > + return !has_task; > } > > int pid_delete_dentry(const struct dentry *dentry)