Another push for sched_ext

>Is it legal to make a modified version of the BPF verifier which does not reject non-GPL-licensed programs?

Yes, of course. That does not change the licensing situation at all.

You can:
- Remove this check from the verifier.
- Use the modified verifier.
- Distribute the resulting verifier code.
- Write a non-GPL BPF program and use it locally without distributing.

It may be a license violation (courts have to decide), if:
- You distribute your non-GPL BPF program.

It might also be totally fine to distribute the non-GPL BPF program along with the modified verifier, if a court rules that the BPF program is not a derived work.