If combining gpl and cddl in a redistributed file is a violation of gpl but not cddl, then what would the holder of the copyright of the cddl code be able to sue about? I think the concern is that a contributor to Linux might sue, just like in the Nvidia/and case.
Oracle is a contributor to Linux, so they could sue from the GPL side. While Nvidia and AMD also are Linux contributors, the fact they ship proprietary modules would make it hard to argue that they aren't implicitly permitting users to redistribute it (and thus they would be forced to license their drivers under GPLv2).
Not to mention that ZFS is covered by Oracle patents. CDDL provides a patent license, but it might be possible for Oracle to sue you for patent infringement if you're distributing code in a way that complicates the licensing. Not that I'm saying that's likely, but Oracle has enough money to ruin you if they want to.
