Early userspace support
 =======================
 
 Last update: 2004-12-20 tlh
 
 
 "Early userspace" is a set of libraries and programs that provide
 various pieces of functionality that are important enough to be
 available while a Linux kernel is coming up, but that don't need to be
 run inside the kernel itself.
 
 It consists of several major infrastructure components:
 
 - gen_init_cpio, a program that builds a cpio-format archive
   containing a root filesystem image.  This archive is compressed, and
   the compressed image is linked into the kernel image.
 - initramfs, a chunk of code that unpacks the compressed cpio image
   midway through the kernel boot process.
 - klibc, a userspace C library, currently packaged separately, that is
   optimized for correctness and small size.
 
 The cpio file format used by initramfs is the "newc" (aka "cpio -c")
 format, and is documented in the file "buffer-format.txt".  There are
 two ways to add an early userspace image: specify an existing cpio
 archive to be used as the image or have the kernel build process build
 the image from specifications.
 
 CPIO ARCHIVE method
 
 You can create a cpio archive that contains the early userspace image.
 Your cpio archive should be specified in CONFIG_INITRAMFS_SOURCE and it
 will be used directly.  Only a single cpio file may be specified in
 CONFIG_INITRAMFS_SOURCE and directory and file names are not allowed in
 combination with a cpio archive.
 
 IMAGE BUILDING method
 
 The kernel build process can also build an early userspace image from
 source parts rather than supplying a cpio archive.  This method provides
 a way to create images with root-owned files even though the image was
 built by an unprivileged user.
 
 The image is specified as one or more sources in
 CONFIG_INITRAMFS_SOURCE.  Sources can be either directories or files -
 cpio archives are *not* allowed when building from sources.
 
 A source directory will have it and all of it's contents packaged.  The
 specified directory name will be mapped to '/'.  When packaging a
 directory, limited user and group ID translation can be performed.
 INITRAMFS_ROOT_UID can be set to a user ID that needs to be mapped to
 user root (0).  INITRAMFS_ROOT_GID can be set to a group ID that needs
 to be mapped to group root (0).
 
 A source file must be directives in the format required by the
 usr/gen_init_cpio utility (run 'usr/gen_init_cpio --help' to get the
 file format).  The directives in the file will be passed directly to
 usr/gen_init_cpio.
 
 When a combination of directories and files are specified then the
 initramfs image will be an aggregate of all of them.  In this way a user
 can create a 'root-image' directory and install all files into it.
 Because device-special files cannot be created by a unprivileged user,
 special files can be listed in a 'root-files' file.  Both 'root-image'
 and 'root-files' can be listed in CONFIG_INITRAMFS_SOURCE and a complete
 early userspace image can be built by an unprivileged user.
 
 As a technical note, when directories and files are specified, the
 entire CONFIG_INITRAMFS_SOURCE is passed to
 scripts/gen_initramfs_list.sh.  This means that CONFIG_INITRAMFS_SOURCE
 can really be interpreted as any legal argument to
 gen_initramfs_list.sh.  If a directory is specified as an argument then
 the contents are scanned, uid/gid translation is performed, and
 usr/gen_init_cpio file directives are output.  If a directory is
 specified as an arugemnt to scripts/gen_initramfs_list.sh then the
 contents of the file are simply copied to the output.  All of the output
 directives from directory scanning and file contents copying are
 processed by usr/gen_init_cpio.
 
 See also 'scripts/gen_initramfs_list.sh -h'.
 
 Where's this all leading?
 =========================
 
 The klibc distribution contains some of the necessary software to make
 early userspace useful.  The klibc distribution is currently
 maintained separately from the kernel, but this may change early in
 the 2.7 era (it missed the boat for 2.5).
 
 You can obtain somewhat infrequent snapshots of klibc from
 ftp://ftp.kernel.org/pub/linux/libs/klibc/

 
 For active users, you are better off using the klibc BitKeeper
 repositories, at http://klibc.bkbits.net/
 
 The standalone klibc distribution currently provides three components,
 in addition to the klibc library:
 
 - ipconfig, a program that configures network interfaces.  It can
   configure them statically, or use DHCP to obtain information

   dynamically (aka "IP autoconfiguration").
 - nfsmount, a program that can mount an NFS filesystem.
 - kinit, the "glue" that uses ipconfig and nfsmount to replace the old
   support for IP autoconfig, mount a filesystem over NFS, and continue
   system boot using that filesystem as root.
 

 kinit is built as a single statically linked binary to save space.
 
 Eventually, several more chunks of kernel functionality will hopefully
 move to early userspace:
 
 - Almost all of init/do_mounts* (the beginning of this is already in
   place)

 - ACPI table parsing
 - Insert unwieldy subsystem that doesn't really need to be in kernel
   space here
 
 If kinit doesn't meet your current needs and you've got bytes to burn,
 the klibc distribution includes a small Bourne-compatible shell (ash)

 and a number of other utilities, so you can replace kinit and build
 custom initramfs images that meet your needs exactly.
 
 For questions and help, you can sign up for the early userspace
 mailing list at http://www.zytor.com/mailman/listinfo/klibc
 

 How does it work?
 =================
 
 The kernel has currently 3 ways to mount the root filesystem:
 
 a) all required device and filesystem drivers compiled into the kernel, no
    initrd.  init/main.c:init() will call prepare_namespace() to mount the

    final root filesystem, based on the root= option and optional init= to run
    some other init binary than listed at the end of init/main.c:init().
 
 b) some device and filesystem drivers built as modules and stored in an
    initrd.  The initrd must contain a binary '/linuxrc' which is supposed to
    load these driver modules.  It is also possible to mount the final root

    filesystem via linuxrc and use the pivot_root syscall.  The initrd is
    mounted and executed via prepare_namespace().
 
 c) using initramfs.  The call to prepare_namespace() must be skipped.
    This means that a binary must do all the work.  Said binary can be stored
    into initramfs either via modifying usr/gen_init_cpio.c or via the new

    initrd format, an cpio archive.  It must be called "/init".  This binary
    is responsible to do all the things prepare_namespace() would do.
 
    To remain backwards compatibility, the /init binary will only run if it
    comes via an initramfs cpio archive.  If this is not the case,
    init/main.c:init() will run prepare_namespace() to mount the final root

    and exec one of the predefined init binaries.
 
 Bryan O'Sullivan <bos@serpentine.com>