MPU Backed Userspace
The MPU backed userspace implementation requires the creation of a secondary set of stacks. These stacks exist in a 1:1 relationship with each thread stack defined in the system. The privileged stacks are created as a part of the build process.
A post-build script scripts/build/gen_kobject_list.py scans the generated ELF file and finds all of the thread stack objects. A set of privileged stacks, a lookup table, and a set of helper functions are created and added to the image.
During the process of dropping a thread to user mode, the privileged stack information is filled in and later used by the swap and system call infrastructure to configure the MPU regions properly for the thread stack and guard (if applicable).
During system calls, the user mode thread’s access to the system call and the passed-in parameters are all validated. The user mode thread is then elevated to privileged mode, the stack is switched to use the privileged stack, and the call is made to the specified kernel API. On return from the kernel API, the thread is set back to user mode and the stack is restored to the user stack.