[cwzwarich]

You could test data structures (which tend to be implemented using macros), but any nontrivial subsystem of a monolithic kernel lives in a web of dependencies with other subsystems. This is especially true of Linux.

To solve this, you would need to either use a hierarchical decomposition of subsystems or do some crazy mocking to run subsystems outside of the full kernel.

There is a recent project (KUnit) to add a unit testing framework to the Linux kernel, but it remains to be seen how much adoption it will get.

[yjftsjthsd-h]

NetBSD has their rump kernel tech, which specifically exists to run chunks of kernel code independently. It can do a lot of neat things (I liked the "run netbsd drivers on other OSs" trick, personally), but one of the big uses they've mentioned is that it helps testing and development.

[syrrim]

A lot of the linux kernel is implemented as modules, which does allow said modules to be run under different OSes. You could probably use the module interface to implement unit testing.