There is not really a permission system in bus1 any more than fd passing is a permission system. What bus1 gives you is the primitives to build a permission system though.

There is no sealing in bus1.

The payload of a bus1 message is exactly the same as the payload of a UDS message: unstructured binary data. bus1 is (like UDS) just a transport, it would be up to the consumer to add structured payload if they wish.

kdbus is higher level than bus1, don't confuse the two. kdbus is still not at the application level though, there would have to be a userspace component inbetween.

High-level abstractions for applications don't belong in the kernel, I agree. But that is a strawman, bus1 is not doing that at all.