Hmm, interesting.
First of all, I do not quite understand where this:
|"GNU coreutils upstream says that NFS-related entries should be removed|
|from the default /etc/xattr.conf to make copying of NFSv4 ACLs work:"|
...comes from. Do we know who told Kamil to remove "system.nfs4acl" entries
from /etc/xattr.conf?
Because when removed, then "cp -p" is calmed down indeed - simply because it
will no longer be able to copy NFSv4 ACLs
The example Ross mentioned is quite clear as you are trying to copy NFSv4
filesystem with a non-trivial ACLs to ext4 mount.
Since ext4 filesystem is unable to store these ACLs, then "cp -p" fails with message
"Operation not supported"
As Padraig says, on Linux, this operation will currently (sadly enough) only
succeed when copying NFSv4->NFSv4.
In my view, the commit to xattr.conf should be reverted - yes people might
scream about that message, but that's a not a bug, it is a new feature!