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 f855168b for ; Wed, 22 Jan 2020 16:08:13 +0000 (UTC) Received: (qmail 21636 invoked by uid 550); 22 Jan 2020 16:08:11 -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 21618 invoked from network); 22 Jan 2020 16:08:11 -0000 Date: Wed, 22 Jan 2020 11:07:43 -0500 From: Rich Felker To: Florian Weimer Cc: 39236@debbugs.gnu.org, musl@lists.openwall.com Message-ID: <20200122160743.GC30412@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> <20200122151507.GB30412@brightrain.aerifal.cx> <87zhefik0y.fsf@oldenburg2.str.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87zhefik0y.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 04:32:45PM +0100, Florian Weimer wrote: > * Rich Felker: > > > 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. > > Ah, yes, there should be a real implementation if we can get full > lchmod/AT_SYMLINK_NOFOLLOW behavior on file systems that support it. If > we can't, I'm not sure if there is a point to it. The point is to fail when the target is a symlink, rather than (erroneously and possibly dangerously) applying the chmod to the link target. Actually supporting link modes is useless. It's the "not modifying the target" that's important. > >> 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. > > Is there a reason for the S_ISLNK check in the musl implementation? > With current kernels, chmod on the proc pseudo-file will not traverse > the symbolic link, but I have yet to check if this has always been the > case. It's explained in the bz you just replied on, https://sourceware.org/bugzilla/show_bug.cgi?id=14578 The point of the S_ISLNK check is to fail out early with the ENOTSUPP, which the caller should treat as "success-like", in the non-racing condition, without the need to open a fd (which may fail with ENFILE/EMFILE) and without the need for /proc to be mounted. Otherwise, a different error will be produced when one of those cases is hit, and the caller will treat it as a real error. Rich