bug#39236: [musl] coreutils cp mishandles error return from lchmod

[Florian Weimer]

* 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.

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.

Thanks,
Florian