part of original file [ copy.c ]

/* copy.c -- core functions for copying files and directories
   Copyright (C) 89, 90, 91, 1995-2009 Free Software Foundation, Inc.

   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation, either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see .  */

/* Extracted from cp.c and librarified by Jim Meyering.  */

 /* To allow copying xattrs on read-only files, temporarily chmod u+rw.
     This workaround is required as an inode permission check is done
     by xattr_permission() in fs/xattr.c of the GNU/Linux kernel tree.  */
  if (x->preserve_xattr)
    {
      bool access_changed = false;

      if (!(sb.st_mode & S_IWUSR) && geteuid() != 0)
        access_changed = fchmod_or_lchmod (dest_desc, dst_name, 0600) == 0;

      if (!copy_attr_by_fd (src_name, source_desc, dst_name, dest_desc, x)
          && x->require_preserve_xattr)
        return_val = false;

      if (access_changed)
        fchmod_or_lchmod (dest_desc, dst_name, dst_mode & ~omitted_permissions);
    }

http://en.wikipedia.org/wiki/Extended_file_attributes
http://sourceforge.net/projects/xattr/

http://www.opengroup.org/onlinepubs/007908799/xsh/fchmod.html