Specifically, the issue is with GPL, which disallows licenses that have more requirements than itself - in case of CDDL, it's IIRC patent sharing requirements a

Specifically, the issue is with GPL, which disallows licenses that have more requirements than itself - in case of CDDL, it's IIRC patent sharing requirements and language around file-specific applicability. And of course then there's the part that GPL doesn't apply to linking at all - it applies to derivative code, which OpenZFS is not and thus it does not violate GPL to ship OpenZFS code linked with Linux kernel.