This really seems like something that shouldn't belong in the kernel. The D-Bus protocol is a bit more complex than other IPC, and it makes sense to me to keep

This really seems like something that shouldn't belong in the kernel. The D-Bus protocol is a bit more complex than other IPC, and it makes sense to me to keep that level of complexity in userspace. (Of course if it were a simpler model -- just a bunch of multicast channels, rather than all this filtering, it might make sense to put bits in the kernel).