Mastering Embedded Linux Programming - Second Edition

Early user space

In order to transition from kernel initialization to user space, the kernel has to mount a root filesystem and execute a program in that root filesystem. This can be achieved via a ramdisk or by mounting a real filesystem on a block device. The code for all of this is in init/main.c, starting with the function rest_init(), which creates the first thread with PID 1 and runs the code in kernel_init(). If there is a ramdisk, it will try to execute the program /init, which will take on the task of setting up the user space.

If fails to find and run /init, it tries to mount a filesystem by calling the function prepare_namespace() in init/do_mounts.c. This requires a root= command line to give the name of the block device to use ...