I keep hearing that the GPL and CDDL are incompatible. But repeating something doesn't make it true. And I have read both licenses, and have come to the conclusion that they're unlikely to be incompatible in the first place. Unfortunately, every time this line of argument come up, it tends to get buried in discussion.

The basis of the incompatibility claim seems to be from 3 arguments:

1) Statements from the FSF website, but those have no legal bearing against the actual text of the licenses

2) Claims that the CDDL was engineered to be incompatible with the GPL, again, it's an interesting hypothesis but has little bearing against the actual text of the licenses

3) Derivative Work argument from the GPL. This seems to be the only one that could hold water; however, I doubt that a court would find ZFS to be derivative work of the Linux kernel.

Furthermore, even if you argue that the "derviative work" clause makes them incompatible, there’s no way actually to prosecute such a violation. Copyright law is something called a tort in law, which means you have to show that someone violated the rules AND also caused quantifiable harm. What would our theory of harm be? (https://blog.hansenpartnership.com/are-gplv2-and-cddl-incomp...)