How does the Linux kernel launch user space?

The process of launching a user space in Linux kernel starts after the kernel initialization and all necessary hardware checks and configurations. The main process that starts user space is init (or its modern equivalents like systemd).

The process is as follows:

1. After loading the kernel and executing start_kernel(), the init() function is called.
2. The kernel mounts the root filesystem and searches for the executable init in standard paths, such as /sbin/init, /etc/init, /bin/init.
3. The kernel creates the first process (PID 1), which runs init.
4. The init process is responsible for starting all other user processes, services, and daemons, thus initializing user space.

Therefore, the Linux kernel does not directly launch user applications but hands over control to the first process init, which manages the startup of user services and applications.