They didn't rewrite ZFS for Linux support, they did the syscall stuff in such a way that it's reasonably compatible license-wise. I don't know the specifics, I

They didn't rewrite ZFS for Linux support, they did the syscall stuff in such a way that it's reasonably compatible license-wise. I don't know the specifics, I just know it's a forked codebase that improves compatibility with Linux. FreeBSD moved to this new codebase because it is getting a lot of attention and better support.