Interesting
Obviously the next version of the GPL will have to be rewritten so that Linus Torvalds or perhaps even Richard Stallman could, at the stroke of a pen, take away Grsecurity's right to use, modify, or redistribute Linux in return for having the temerity to file such a lawsuit.
Of course, as long as the controversial clause in Grsecurity's agreement has not been tested in court, such a lawsuit is possible. Since the penalty for redistributing Grsecurity's code is simply termination of a relationship with Grsecurity, it could indeed be argued that they can get away with what they're doing, even if it's contrary to the spirit of the GPL. It might not contradict the letter of the GPL.
Biting the hand that feeds IT
