From the Wikipedia article on Linux:
The Linux kernel is licensed explicitly only under version 2 of the GPL, without offering the licensee the option to choose "any later version", which is a common GPL extension. There was a debate over how easily it could be changed to use later GPL versions such as version 3 and whether this is even desirable.
But I've visited the current COPYING file and that doesn't mention it. So, considering the following text in the file:
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 2 of the License, or (at your option) any later version.
So, can Linux be released under GPLv3?
(or say I want to know can GNU release linux-libre under GPLv3?)