As far as I understand this would be a "mere aggregation" since the application and the kernel module communicate at arms length through the kernel syscall inte

As far as I understand this would be a "mere aggregation" since the application and the kernel module communicate at arms length through the kernel syscall interface https://www.gnu.org/licenses/gpl-faq.en.html#MereAggregation https://www.gnu.org/licenses/gpl-faq.en.html#MereAggregation . So they'd have to make the source of the kernel extension available (as the GPL requires) but it would not be viral to the application using the kernel extension. As the FAQ entry says this is a legal question "which ultimately judges will decide" and as far as I know that hasn't happend yet. Which is probably one reason many don't want to do it and err on the safe side and choose another project with non-viral license.