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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS 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 D307FC433DB for ; Thu, 7 Jan 2021 12:17:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7F37823359 for ; Thu, 7 Jan 2021 12:17:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728034AbhAGMRR (ORCPT ); Thu, 7 Jan 2021 07:17:17 -0500 Received: from mail.kernel.org ([198.145.29.99]:42586 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726326AbhAGMRQ (ORCPT ); Thu, 7 Jan 2021 07:17:16 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id AAB2D23359; Thu, 7 Jan 2021 12:16:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1610021796; bh=MrM1/LVzBvycUoSWnH8Oj7Yg2SWMfo0FFP8owU5Stbs=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=UBYGHp4pt329Qv7BLbuDDktSU2MhFS5vXSqQRqd7nZzMz4I7cahCiCWdSmOHjE56R Xuy0ffJGQQIBMQwrYkM9MW0UHc3xyWQwiDFjVoyIotCXEyaQeSjfxD4T49cXMlt/y8 dAUgv57BIQ/c9OW4wykiV3lwpwv/b4bS8LGkOLRM= Date: Thu, 7 Jan 2021 13:17:55 +0100 From: Greg Kroah-Hartman To: Wen Yang Cc: Sasha Levin , Xunlei Pang , linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 4.9 00/10] fix a race in release_task when flushing the dentry Message-ID: References: <20210107075222.62623-1-wenyang@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20210107075222.62623-1-wenyang@linux.alibaba.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 07, 2021 at 03:52:12PM +0800, Wen Yang wrote: > The dentries such as /proc//ns/ have the DCACHE_OP_DELETE flag, they > should be deleted when the process exits. > > Suppose the following race appears： > > release_task dput > -> proc_flush_task > -> dentry->d_op->d_delete(dentry) > -> __exit_signal > -> dentry->d_lockref.count-- and return. > > In the proc_flush_task(), if another process is using this dentry, it will > not be deleted. At the same time, in dput(), d_op->d_delete() can be executed > before __exit_signal(pid has not been hashed), d_delete returns false, so > this dentry still cannot be deleted. > > This dentry will always be cached (although its count is 0 and the > DCACHE_OP_DELETE flag is set), its parent denry will also be cached too, and > these dentries can only be deleted when drop_caches is manually triggered. > > This will result in wasted memory. What's more troublesome is that these > dentries reference pid, according to the commit f333c700c610 ("pidns: Add a > limit on the number of pid namespaces"), if the pid cannot be released, it > may result in the inability to create a new pid_ns. > > This issue was introduced by 60347f6716aa ("pid namespaces: prepare > proc_flust_task() to flush entries from multiple proc trees"), exposed by > f333c700c610 ("pidns: Add a limit on the number of pid namespaces"), and then > fixed by 7bc3e6e55acf ("proc: Use a list of inodes to flush from proc"). Why are you just submitting a series for 4.9 and 4.19, what about 4.14? We can't have users move to a newer kernel and then experience old bugs, right? But the larger question is why are you backporting a whole new feature here? Why is CLONE_PIDFD needed? That feels really wrong... thanks, greg k-h