by H. Peter Alvin of Transmeta
As the Linux kernel developed, the root file system became more important and had to be able to live in more and more different places, like configurable locations on the disk or even on the network. Eventually even in RAM as initrd entered the scene. This caused all sorts of special cases needing handling to support all these variants. And all this is happening inside the kernel which is tough for development as testing is really painful. So the ideal case would be to be able to organize the booting process in user space.
The solution is to have a virtual root for the system, called rootfs using ramfs code. As the kernel starts, / is rootfs and the actual root filesystem becomes a simple overlay mount. This means that it is possible to use an initial ram disk and get rid of it later. As the kernel threads live in their own world (rootfs).
To simplify matters further, initrd gets replaced with initialramfs which is populated by loading one or several cpio archives. The cpio archives can come from the disk, from the net or even be compiled into the kernel itself. They provide the files required in early user space. To allow initialramfs to be small and still provide a useful environment, a special stripped down C library called klibc was developed to provide library code for this case.
Programming in this environment is almost as simple as normal userspace programming. Malloc works as expected, file and socket handling is there. Still there are restrictions, as all the rest of the system is not up yet. STDIO is available but it is very slow especially for reading, as klibc does not implement buffering to save space.
Candidates for early Userspace
With this infrastructure the possibilities become endless. The following candidates for early user space come to mind:
Partition detection
Determining the root filesystem type
Network booting
Caveat
Note that this is very much an ongoing development and only available in 2.5.x. (www.kernel.org ...) has more information.