The two sides of reflink()

You raise an excellent point. The useful implementation of "reflink" would
have semantics as a file copy. Since an unprivileged user cannot change the
ownership of an existing file, a general purpose implementation *must* be
able to change the ownership of the new file to that of the current user in
the process.

Otherwise you would get a highly restricted operation that would only be
useful to unprivileged users for making efficient copies of files they
already own.