The definition of a derivative work comes into play here. An argument has been made by Illumos who ported the KVM module from Linux that since they did not have

The definition of a derivative work comes into play here. An argument has been made by Illumos who ported the KVM module from Linux that since they did not have to make any changes to Illumos to support KVM, and they only made minor modifications to allow KVM to be loaded into the Illumos kernel that it does not constitute a derived work since it in no way depends on Illumos to function and that there was no changes made to Illumos to support it. Illumos gets to stay as CDDL and KVM remains GPL. A program that functionally requires libreadline to function is very obviously a derived work in copyright law, calling dlopen() isn't going to save you from that.