From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on inbox.vuxu.org X-Spam-Level: X-Spam-Status: No, score=-3.0 required=5.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,RCVD_IN_DNSWL_MED,RCVD_IN_MSPIKE_H3, RCVD_IN_MSPIKE_WL autolearn=ham autolearn_force=no version=3.4.2 Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by inbox.vuxu.org (OpenSMTPD) with SMTP id b9fdcf8b for ; Wed, 22 Jan 2020 15:15:29 +0000 (UTC) Received: (qmail 3177 invoked by uid 550); 22 Jan 2020 15:15:28 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Reply-To: musl@lists.openwall.com Received: (qmail 3159 invoked from network); 22 Jan 2020 15:15:28 -0000 Date: Wed, 22 Jan 2020 10:15:07 -0500 From: Rich Felker To: Florian Weimer Cc: 39236@debbugs.gnu.org, musl@lists.openwall.com Message-ID: <20200122151507.GB30412@brightrain.aerifal.cx> References: <20200122141557.GA8157@brightrain.aerifal.cx> <87ftg7k1at.fsf@oldenburg2.str.redhat.com> <20200122144243.GZ30412@brightrain.aerifal.cx> <87a76fjzpx.fsf@oldenburg2.str.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87a76fjzpx.fsf@oldenburg2.str.redhat.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: Rich Felker Subject: Re: [musl] coreutils cp mishandles error return from lchmod On Wed, Jan 22, 2020 at 04:08:26PM +0100, Florian Weimer wrote: > * Rich Felker: > > > On Wed, Jan 22, 2020 at 03:34:18PM +0100, Florian Weimer wrote: > >> * Rich Felker: > >> > >> > coreutils should be opting to use the system-provided lchmod, which is > >> > safe, and correctly handling error returns (silently treating > >> > EOPNOTSUPP as success) rather than as hard errors. > >> > >> glibc's lchmod always returns ENOSYS (except on Hurd). I don't know how > >> lchmod is used in coreutils, but I suspect it is not particularly > >> useful. > > > > When preserving permissions (cp -p, archive extraction, etc.), you > > want lchmod to work correctly just for the purpose of *not* following > > the link and thereby unwantedly changing the permissions of the link > > target. But, fchmodat with AT_SYMLINK_NOFOLLOW works just as well and > > is standard, and that's really what coreutils should be using. > > I think you misread what I wrote: lchmod *always* returns ENOSYS. Even > if the file is not a symbolic link. Likewise, fchmodat with > AT_SYMLINK_NOFOLLOW *always* returns ENOTSUP. Yes, I understood that. I was going into why there should be a real implementation, but didn't make it clear that that was what I was doing. > The reason for this is that the kernel does not provide a suitable > system call to implement this, even though some file systems allow a > mode change for symbolic links. I think we can do better, although I > should note that each time we implement such emulation in userspace, it > comes back to bite us eventually. Emulations in userspace that are approximate, have race conditions, etc. are bad. Ones that are rigorous are good, though. Rich