[YZF]

I agree. Bestowing certain permissions/properties on a thread doesn't make sense. The thread shouldn't "enter the namespace", an API should retrieve a handle to that namespace that should then be usable from any thread in the process via some appropriate calls.

[the8472]

On the other hand having individual threads entering namespaces means your broker process does not have to constantly switch namespaces, it can use shared memory between differently privileged threads to do the brokering.

[YZF]

I know nothing of the implementation here but presumably these namespaces can exist side by side in a way that doesn't require any "switching". If switching is expensive that would make context switching between a thread in one namespace and a thread in another namespace just as expensive?

If you have one thread in one namespace and another in another you now have to worry about what you can do in the context of a callback. This asymmetry just makes any multi-threaded program more complicated than it needs to be (and already is).

[the8472]

Switching is a system call (setns), in principle shared-memory IPC does not involve context switches, just lock-free data structures. I'm not sure how common this is in practice since shared memory also has some downsides if you're doing this for security.

But there also are non-security applications of namespaces.

And it's not like namespaces are the only per-thread thing in linux. Capabilities, uid and signal handlers come to mind.