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=-2.1 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT 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 59472ECDE32 for ; Wed, 17 Oct 2018 12:08:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CAB4A21527 for ; Wed, 17 Oct 2018 12:08:31 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="EU3G2Elc" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org CAB4A21527 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727049AbeJQUDz (ORCPT ); Wed, 17 Oct 2018 16:03:55 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:44250 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726936AbeJQUDz (ORCPT ); Wed, 17 Oct 2018 16:03:55 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=INthpbZcLMtOLXqHlawkIhN/DbRyhVWYqCFt/3e1ceY=; b=EU3G2ElcsM8cnxl+IeHs5JSOB ZJNg5lndXqe1xKDAfksa6ghs01HzoIiRe0th8sO8GXlFsfqB6nIy+JoK4vI1dNj6v8X6SU8jAO4uD 5fePp3r/ximdVrTjtyhpY20srB9iC6rLrGfStpPEYPSmhLev0oGfRKjxA54GHYZvs/luJDnn8Y4wJ 84EvL3wV/TxX2SuXlJ2mdI43IpyuDzk1BkFr39ZADtGEa13KSIEhsuDHRqDQQ39sAnUzpxDZ8T97m af2yOZ0tNf0zDy50r/3nN72mBD1XZFNjdIcGRLry0d95/DJ7L1v5UbjH9tdAMQCqHOmuTcvQdbUsr CuK+G/AcQ==; Received: from hch by bombadil.infradead.org with local (Exim 4.90_1 #2 (Red Hat Linux)) id 1gCkco-00008F-3d; Wed, 17 Oct 2018 12:08:30 +0000 Date: Wed, 17 Oct 2018 05:08:29 -0700 From: Christoph Hellwig To: Joel Fernandes Cc: Christoph Hellwig , linux-kernel@vger.kernel.org, kernel-team@android.com, jreck@google.com, john.stultz@linaro.org, tkjos@google.com, gregkh@linuxfoundation.org, Andrew Morton , dancol@google.com, "J. Bruce Fields" , Jeff Layton , Khalid Aziz , linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mm@kvack.org, Mike Kravetz , minchan@google.com, Shuah Khan Subject: Re: [PATCH v2 1/2] mm: Add an F_SEAL_FS_WRITE seal to memfd Message-ID: <20181017120829.GA19731@infradead.org> References: <20181009222042.9781-1-joel@joelfernandes.org> <20181017095155.GA354@infradead.org> <20181017103958.GB230639@joelaf.mtv.corp.google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181017103958.GB230639@joelaf.mtv.corp.google.com> User-Agent: Mutt/1.9.2 (2017-12-15) X-SRS-Rewrite: SMTP reverse-path rewritten from by bombadil.infradead.org. See http://www.infradead.org/rpr.html Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 17, 2018 at 03:39:58AM -0700, Joel Fernandes wrote: > > > This usecase cannot be implemented with the existing F_SEAL_WRITE seal. > > > To support the usecase, this patch adds a new F_SEAL_FS_WRITE seal which > > > prevents any future mmap and write syscalls from succeeding while > > > keeping the existing mmap active. The following program shows the seal > > > working in action: > > > > Where does the FS come from? I'd rather expect this to be implemented > > as a 'force' style flag that applies the seal even if the otherwise > > required precondition is not met. > > The "FS" was meant to convey that the seal is preventing writes at the VFS > layer itself, for example vfs_write checks FMODE_WRITE and does not proceed, > it instead returns an error if the flag is not set. I could not find a better > name for it, I could call it F_SEAL_VFS_WRITE if you prefer? I don't think there is anything VFS or FS about that - at best that is an implementation detail. Either do something like the force flag I suggested in the last mail, or give it a name that matches the intention, e.g F_SEAL_FUTURE_WRITE. > I could make it such that this seal would not be allowed unless F_SEAL_SHRINK > and F_SEAL_GROW are either previously set, or they are passed along with this > seal. Would that make more sense to you? Yes. > > > static int memfd_add_seals(struct file *file, unsigned int seals) > > > { > > > @@ -219,6 +220,9 @@ static int memfd_add_seals(struct file *file, unsigned int seals) > > > } > > > } > > > > > > + if ((seals & F_SEAL_FS_WRITE) && !(*file_seals & F_SEAL_FS_WRITE)) > > > + file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE); > > > + > > > > This seems to lack any synchronization for f_mode. > > The f_mode is set when the struct file is first created and then memfd sets > additional flags in memfd_create. Then later we are changing it here at the > time of setting the seal. I donot see any possiblity of a race since it is > impossible to set the seal before memfd_create returns. Could you provide > more details about what kind of synchronization is needed and what is the > race condition scenario you were thinking off? Even if no one changes these specific flags we still need a lock due to rmw cycles on the field. For example fadvise can set or clear FMODE_RANDOM. It seems to use file->f_lock for synchronization.