kdbus is a somewhat contentious kernel patch that is intended to provide the dbus api in kernel space. It is slated to be a drop in replacement for dbus (user space), with the initial beneficiary of the merge the systemd software that is present on most recent distributions. With linux 4.3rc1 out (which does not include kdbus), linux-next (proposals for inclusion of patches into kernel 4.4) has been made available, and it does indeed include kdbus.
https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git/log/?id=refs/tags/next-20150912
Merge remote-tracking branch ‘kdbus/kdbus’
Does this mean that kdbus will make it into the mainline kernel? I have no idea. My understanding is that the merge window for 4.4 will close, and Linus will decide what to pull into 4.4rc1. At that time, kdbus may or may not be pulled into the Linus-tree, aka the mainline kernel.
Is kdbus a bad thing? Is it bad software, or is it in conflict with the “unix philosophy?” I don’t really have an opinion on that. I don’t think including more software in the kernel is a bad thing (other than bloat), but what I do feel is somewhat less than desirable is an ever-increasing number of software projects relying on said software.