[jacquesm]

> That your code is running in kernel mode, that is the entire concept of ring 0 vs ring 3.

That need not be the case though. You could have a kernel side allocator that sets up the MMU to map that memory to a pointer that you return which lives in the space of the process. The MMU would take care of the required arithmetic to access the memory at its actual location using an offset.

That way you can map resources from real addresses into arbitrary addresses on the user side.

I think the correct term for this mechanism is 'system address translation'.

[steveklabnik]

The language would still have to understand all of that in order to write that kernel side allocator in safe code.

[jacquesm]

I don't see how that follows. The language can't possibly understand the intricacies of what the MMU is capable of (besides, every MMU is different), and as far as the language is concerned what is returned is simply a valid offset and a length to go with it to indicate where the allocated segment ends.

[steveklabnik]

I think you're strongly agreeing with me. It's not feasible to have in the language.