I always thought of MariaDB as a fork of GPLed MySQL code that is now owned by Oracle. However, I see that MariaDB uses a contributor agreement, and reserves s

I always thought of MariaDB as a fork of GPLed MySQL code that is now owned by Oracle. However, I see that MariaDB uses a contributor agreement, and reserves some features for a proprietary(?) enterprise edition. Do I misunderstand the copyright status of MySQL/MariaDB, or has MariaDB been clever about how it adds features to the GPL core, such that they're not legally derivative works?