bug#39236: [musl] 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.

> 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