Date: Wed, 22 Jan 2020 16:32:45 +0100 From: Florian Weimer <fweimer@...hat.com> To: Rich Felker <dalias@...c.org> Cc: 39236@...bugs.gnu.org, musl@...ts.openwall.com Subject: Re: coreutils cp mishandles error return from lchmod * 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 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. Thanks, Florian
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