Sorry, I was imprecise. If a principal starts a new process that indiscriminately inherits the same capabilities of its parent, the security posture is no different from what we have today.
I played with augmenting factotum with a form of mandatory access control, whereby I could ask the hostowner's factotum to mount and bind things into a freshly rfork()'d namespace on an as-needed basis, but the feeling was that it could build up access delegation fatigue pretty quickly.
plash did get a reasonably ergonomic pola shell working under linux, but unfortunately it has bitrotted. fundamentally the insight is that most of the time when you want a process to open a file you have to tell it which file to open. if you give it an open file descriptor instead of a string, it doesn't need the authority to open existing files itself. just one step further down the path where the shell does the globbing, not the program launched
in theory you could do the same thing with per-process filesystem namespaces as you are describing, but it seems like it would take a lot more work to make it reliable and usable
My mandatory access control experiments abused the fact that I could have open file descriptors and barely anything bound in the namespace by the time I exec()ed what I wanted to run. What files did those file descriptors reference? You didn't need to know, and you couldn't see them.
the #s mechanism sounds considerably less horrific than scm_rights
The Plan 9 primitives were refreshingly simple and consistent.
i feel like plan9 still tends to pass a lot of ambient authority to new processes by default
Plan 9 is, at its core, still an ambient authority system, and I will never try to misrepresent it otherwise. That said, I've found it does an admirable job emulating what a "familiar" (in the sense of not looking completely alien to someone who only knows ambient authority) object-capability system could look like, kind of like how you can emulate the free monad in Ruby despite it not being a functional programming language.
This is possible principally because the mantra of Plan 9 is that everything is a filesystem: if you can bind it into your namespace, you can access it. As a corollary, if you can't bind it into your namespace, you can't access it. It makes shaping one's ambient authority a remarkably productive endeavor.