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=-0.8 required=3.0 tests=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 82E33C352A4 for ; Wed, 12 Feb 2020 20:27:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 550AF246C9 for ; Wed, 12 Feb 2020 20:27:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728674AbgBLU1q (ORCPT ); Wed, 12 Feb 2020 15:27:46 -0500 Received: from albireo.enyo.de ([37.24.231.21]:38502 "EHLO albireo.enyo.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727548AbgBLU1p (ORCPT ); Wed, 12 Feb 2020 15:27:45 -0500 Received: from [172.17.203.2] (helo=deneb.enyo.de) by albireo.enyo.de with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) id 1j1ybe-0006mm-7t; Wed, 12 Feb 2020 20:27:34 +0000 Received: from fw by deneb.enyo.de with local (Exim 4.92) (envelope-from ) id 1j1yaJ-0006Eo-S9; Wed, 12 Feb 2020 21:26:11 +0100 From: Florian Weimer To: Rich Felker 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 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> Date: Wed, 12 Feb 2020 21:26:11 +0100 In-Reply-To: <20200212201951.GC1663@brightrain.aerifal.cx> (Rich Felker's message of "Wed, 12 Feb 2020 15:19:51 -0500") Message-ID: <87sgjflfh8.fsf@mid.deneb.enyo.de> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-xfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org * 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?