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 14489C352A4 for ; Wed, 12 Feb 2020 20:38:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DFB652467B for ; Wed, 12 Feb 2020 20:38:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728098AbgBLUi0 (ORCPT ); Wed, 12 Feb 2020 15:38:26 -0500 Received: from 216-12-86-13.cv.mvl.ntelos.net ([216.12.86.13]:50594 "EHLO brightrain.aerifal.cx" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727692AbgBLUi0 (ORCPT ); Wed, 12 Feb 2020 15:38:26 -0500 Received: from dalias by brightrain.aerifal.cx with local (Exim 3.15 #2) id 1j1ylw-0004xz-00; Wed, 12 Feb 2020 20:38:12 +0000 Date: Wed, 12 Feb 2020 15:38:12 -0500 From: Rich Felker To: Florian Weimer Cc: Andreas Schwab , Al Viro , "Darrick J. Wong" , Christoph Hellwig , linux-xfs@vger.kernel.org, libc-alpha@sourceware.org, linux-fsdevel@vger.kernel.org Subject: Re: XFS reports lchmod failure, but changes file system contents Message-ID: <20200212203812.GE1663@brightrain.aerifal.cx> References: <874kvwowke.fsf@mid.deneb.enyo.de> <20200212161604.GP6870@magnolia> <20200212181128.GA31394@infradead.org> <20200212183718.GQ6870@magnolia> <87d0ajmxc3.fsf@mid.deneb.enyo.de> <20200212195118.GN23230@ZenIV.linux.org.uk> <87wo8rlgml.fsf@mid.deneb.enyo.de> <87wo8r1rx6.fsf@igel.home> <20200212201951.GC1663@brightrain.aerifal.cx> <87sgjflfh8.fsf@mid.deneb.enyo.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87sgjflfh8.fsf@mid.deneb.enyo.de> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-xfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org On Wed, Feb 12, 2020 at 09:26:11PM +0100, Florian Weimer wrote: > * Rich Felker: > > > On Wed, Feb 12, 2020 at 09:17:41PM +0100, Andreas Schwab wrote: > >> On Feb 12 2020, Florian Weimer wrote: > >> > >> > * Al Viro: > >> > > >> >> On Wed, Feb 12, 2020 at 08:15:08PM +0100, Florian Weimer wrote: > >> >> > >> >>> | Further, I've found some inconsistent behavior with ext4: chmod on the > >> >>> | magic symlink fails with EOPNOTSUPP as in Florian's test, but fchmod > >> >>> | on the O_PATH fd succeeds and changes the symlink mode. This is with > >> >>> | 5.4. Cany anyone else confirm this? Is it a problem? > >> >>> > >> >>> It looks broken to me because fchmod (as an inode-changing operation) > >> >>> is not supposed to work on O_PATH descriptors. > >> >> > >> >> Why? O_PATH does have an associated inode just fine; where does > >> >> that "not supposed to" come from? > >> > > >> > It fails on most file systems right now. I thought that was expected. > >> > Other system calls (fsetxattr IIRC) do not work on O_PATH descriptors, > >> > either. I assumed that an O_PATH descriptor was not intending to > >> > confer that capability. Even openat fails. > >> > >> According to open(2), this is expected: > >> > >> O_PATH (since Linux 2.6.39) > >> Obtain a file descriptor that can be used for two purposes: to > >> indicate a location in the filesystem tree and to perform opera- > >> tions that act purely at the file descriptor level. The file > >> itself is not opened, and other file operations (e.g., read(2), > >> write(2), fchmod(2), fchown(2), fgetxattr(2), ioctl(2), mmap(2)) > >> fail with the error EBADF. > > > > That text is outdated and should be corrected. Fixing fchmod fchown, > > fstat, etc. to operate on O_PATH file descriptors was a very > > intentional change in the kernel. > > I suppose we could do the S_ISLNK check, try fchmod, and if that > fails, go via /proc. Is this the direction you want to go in? It was, but Al Viro just pointed out to me that I was wrong. I think we could use fstat (which AIUI now works) to do the S_ISLNK check, so that it doesn't depend on /proc, but I don't see a way to do the chmod operation without /proc at this time. Rich