Another push for sched_ext

1. Is it legal to make a modified version of the BPF verifier which does not reject non-GPL-licensed programs? It might violate 17 USC 1201 (and similar laws in other countries). Version 3 of the GPL explicitly repudiates that provision of copyright law, but the kernel is licensed under GPLv2. OTOH, the GPL is much more permissive than a "typical" (proprietary) copyright license, so I don't know what courts would say in that situation. Certainly the spirit of the license is that you can change whatever you want, so long as you distribute those changes (if at all) under the GPL. I also believe this is the intent of most if not all of the people who have actually contributed source code to the kernel.
2. Is it legal to make a BPF sched_ext program that is not GPL? In most countries, this will turn on some kind of substantial similarity analysis. In the US, some courts have adopted a software-specific test called the "abstraction-filtration-comparison test," which complicates things, but the short version is that there must be portions of the allegedly infringing work (the BPF program) which are both eligible for copyright protection and also present in the original work (the kernel), and those portions must be substantial enough to infer that some kind of "copying" happened. This is not necessarily limited to literal copying of source code, but it can't be something as abstract as an entire algorithm, either (nobody owns quicksort, for example). The AFC test is basically just a more methodical and specific way of doing that analysis for software. Probably other countries won't do the exact same analysis, but it is hard to imagine them doing something wildly different.