bug#9446: Bug in cp (or strange behaviour and unclear documentation)

[Jim Meyering]

Torbjorn Granlund wrote:
> On FreeBSD (tested on 8.1, but exact version is unlikely relevant)
> copying a file from a ZFS file system to a tmpfs or tmpmfs filesystem,
> the -p option causes cp to return with a non-zero exit status.
>
> Sample run:
>
> $ cp -p /etc/profile /tmp/foo || echo non-zero exit
> cp: preserving permissions for `/tmp/foo': Invalid argument
> non-zero exit
>
> Some debugging shows that the function acl_access_nontrivial returns 1,
> caused by the tag ACL_EVERYONE.  (It is present in every file by
> default.)
>
> ACLs are not supported on FreeBSD's tmpfs or tmpmfs file system types.
>
> Perhaps this failure is intentional?  Then it should be clearly
> documented for the -p/--preserve option.  I have read it carefully, and
> I cannot see support for the observed behaviour.
>
> Consider what happens under -p when copying a file owned by somebody
> else.  The used and group owners can then not (typically) be preserved.
> This is not considered an error in cp.
>
> Similarly, when copying a file with the setuid bit set, and the user is
> changed (per above) the setuid bit is not preserved, and this is also
> not considered an error by cp.
>
> The failure to preserve (partial) ACL data should IMHO not be considered
> an error, except perhaps under some --preserve=blah options, where blah
> denoted preservation of exactly this.

Hi!
Long time ;-)

ACL-preservation has always been lumped in with the
permission-preservation that you get with cp's "-p".
However, that is not POSIX-mandated.

This is the responsible code in copy.c:

  if (x->preserve_mode || x->move_mode)
    {
      if (copy_acl (src_name, -1, dst_name, -1, src_mode) != 0
          && x->require_preserve)
        return false;
    }

However, upon copy_acl failure, rather than simply returning false, it
might be ok to return true for EINVAL, or even to perform a relatively
expensive test (the first time for each FS) to see if the failure is
due to lack of ACL support in the destination file system.

I'm less inclined to add a separate --preserve=acl option, because
that would involve a behavior change (of not copying ACLs by default).