Date: Wed, 22 Jan 2020 10:15:07 -0500 From: Rich Felker <dalias@...c.org> To: Florian Weimer <fweimer@...hat.com> Cc: 39236@...bugs.gnu.org, musl@...ts.openwall.com Subject: Re: 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
Powered by blists - more mailing lists
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.