They do keep adding stuff to the open source portion, but you're right that there's plenty of closed source code running on a stock OS X kernel. This is loaded

They do keep adding stuff to the open source portion, but you're right that there's plenty of closed source code running on a stock OS X kernel. This is loaded in as kexts, so the mach_kernel binary itself is open and reproducible. Drivers for anything vaguely recent are all closed, as are any security related things (crypto, firewall, sandbox, quarantine, etc.) and a few other odds and ends. Recompiling the kernel is entirely doable [1] (and occasionally useful for kext development), though running a kernel with literally no proprietary code will be quite a lot of effort. (and various important bits of OSX userspace will cease functioning) [1] I've personally had success with the instructions at http://shantonu.blogspot.com/ http://shantonu.blogspot.com/ - not that Apple has accepted any of my patches...