True, but Linux originally used the GPLv2, but IIRC never said anywhere whether it was GPLv2-only or GPLv2 or any later version, or just "GPL". According to the GPL, that means:
> If the Program does not specify a version number of this License, you may choose any version ever published by the Free Software Foundation.
So Linux could've theoretically upgraded to the GPLv3 (or GPLv2+). However, Linus didn't like that idea, and in 2006 edited the COPYING file to say that it was GPLv2-only. If you really wanted to, you could take the 2006 source code and make a GPLv3 fork of Linux.
I read through the entire post without realizing who wrote it. I just want to let you know your work is fantastic. Thank you for everything you've contributed to emulation.
> If the Program does not specify a version number of this License, you may choose any version ever published by the Free Software Foundation.
So Linux could've theoretically upgraded to the GPLv3 (or GPLv2+). However, Linus didn't like that idea, and in 2006 edited the COPYING file to say that it was GPLv2-only. If you really wanted to, you could take the 2006 source code and make a GPLv3 fork of Linux.