linux/Documentation/early-userspace/README
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   1Early userspace support
   2=======================
   3
   4Last update: 2004-12-20 tlh
   5
   6
   7"Early userspace" is a set of libraries and programs that provide
   8various pieces of functionality that are important enough to be
   9available while a Linux kernel is coming up, but that don't need to be
  10run inside the kernel itself.
  11
  12It consists of several major infrastructure components:
  13
  14- gen_init_cpio, a program that builds a cpio-format archive
  15  containing a root filesystem image.  This archive is compressed, and
  16  the compressed image is linked into the kernel image.
  17- initramfs, a chunk of code that unpacks the compressed cpio image
  18  midway through the kernel boot process.
  19- klibc, a userspace C library, currently packaged separately, that is
  20  optimized for correctness and small size.
  21
  22The cpio file format used by initramfs is the "newc" (aka "cpio -H newc")
  23format, and is documented in the file "buffer-format.txt".  There are
  24two ways to add an early userspace image: specify an existing cpio
  25archive to be used as the image or have the kernel build process build
  26the image from specifications.
  27
  28CPIO ARCHIVE method
  29
  30You can create a cpio archive that contains the early userspace image.
  31Your cpio archive should be specified in CONFIG_INITRAMFS_SOURCE and it
  32will be used directly.  Only a single cpio file may be specified in
  33CONFIG_INITRAMFS_SOURCE and directory and file names are not allowed in
  34combination with a cpio archive.
  35
  36IMAGE BUILDING method
  37
  38The kernel build process can also build an early userspace image from
  39source parts rather than supplying a cpio archive.  This method provides
  40a way to create images with root-owned files even though the image was
  41built by an unprivileged user.
  42
  43The image is specified as one or more sources in
  44CONFIG_INITRAMFS_SOURCE.  Sources can be either directories or files -
  45cpio archives are *not* allowed when building from sources.
  46
  47A source directory will have it and all of its contents packaged.  The
  48specified directory name will be mapped to '/'.  When packaging a
  49directory, limited user and group ID translation can be performed.
  50INITRAMFS_ROOT_UID can be set to a user ID that needs to be mapped to
  51user root (0).  INITRAMFS_ROOT_GID can be set to a group ID that needs
  52to be mapped to group root (0).
  53
  54A source file must be directives in the format required by the
  55usr/gen_init_cpio utility (run 'usr/gen_init_cpio --help' to get the
  56file format).  The directives in the file will be passed directly to
  57usr/gen_init_cpio.
  58
  59When a combination of directories and files are specified then the
  60initramfs image will be an aggregate of all of them.  In this way a user
  61can create a 'root-image' directory and install all files into it.
  62Because device-special files cannot be created by a unprivileged user,
  63special files can be listed in a 'root-files' file.  Both 'root-image'
  64and 'root-files' can be listed in CONFIG_INITRAMFS_SOURCE and a complete
  65early userspace image can be built by an unprivileged user.
  66
  67As a technical note, when directories and files are specified, the
  68entire CONFIG_INITRAMFS_SOURCE is passed to
  69scripts/gen_initramfs_list.sh.  This means that CONFIG_INITRAMFS_SOURCE
  70can really be interpreted as any legal argument to
  71gen_initramfs_list.sh.  If a directory is specified as an argument then
  72the contents are scanned, uid/gid translation is performed, and
  73usr/gen_init_cpio file directives are output.  If a directory is
  74specified as an arugemnt to scripts/gen_initramfs_list.sh then the
  75contents of the file are simply copied to the output.  All of the output
  76directives from directory scanning and file contents copying are
  77processed by usr/gen_init_cpio.
  78
  79See also 'scripts/gen_initramfs_list.sh -h'.
  80
  81Where's this all leading?
  82=========================
  83
  84The klibc distribution contains some of the necessary software to make
  85early userspace useful.  The klibc distribution is currently
  86maintained separately from the kernel, but this may change early in
  87the 2.7 era (it missed the boat for 2.5).
  88
  89You can obtain somewhat infrequent snapshots of klibc from
  90ftp://ftp.kernel.org/pub/linux/libs/klibc/
  91
  92For active users, you are better off using the klibc git
  93repository, at http://git.kernel.org/?p=libs/klibc/klibc.git
  94
  95The standalone klibc distribution currently provides three components,
  96in addition to the klibc library:
  97
  98- ipconfig, a program that configures network interfaces.  It can
  99  configure them statically, or use DHCP to obtain information
 100  dynamically (aka "IP autoconfiguration").
 101- nfsmount, a program that can mount an NFS filesystem.
 102- kinit, the "glue" that uses ipconfig and nfsmount to replace the old
 103  support for IP autoconfig, mount a filesystem over NFS, and continue
 104  system boot using that filesystem as root.
 105
 106kinit is built as a single statically linked binary to save space.
 107
 108Eventually, several more chunks of kernel functionality will hopefully
 109move to early userspace:
 110
 111- Almost all of init/do_mounts* (the beginning of this is already in
 112  place)
 113- ACPI table parsing
 114- Insert unwieldy subsystem that doesn't really need to be in kernel
 115  space here
 116
 117If kinit doesn't meet your current needs and you've got bytes to burn,
 118the klibc distribution includes a small Bourne-compatible shell (ash)
 119and a number of other utilities, so you can replace kinit and build
 120custom initramfs images that meet your needs exactly.
 121
 122For questions and help, you can sign up for the early userspace
 123mailing list at http://www.zytor.com/mailman/listinfo/klibc
 124
 125How does it work?
 126=================
 127
 128The kernel has currently 3 ways to mount the root filesystem:
 129
 130a) all required device and filesystem drivers compiled into the kernel, no
 131   initrd.  init/main.c:init() will call prepare_namespace() to mount the
 132   final root filesystem, based on the root= option and optional init= to run
 133   some other init binary than listed at the end of init/main.c:init().
 134
 135b) some device and filesystem drivers built as modules and stored in an
 136   initrd.  The initrd must contain a binary '/linuxrc' which is supposed to
 137   load these driver modules.  It is also possible to mount the final root
 138   filesystem via linuxrc and use the pivot_root syscall.  The initrd is
 139   mounted and executed via prepare_namespace().
 140
 141c) using initramfs.  The call to prepare_namespace() must be skipped.
 142   This means that a binary must do all the work.  Said binary can be stored
 143   into initramfs either via modifying usr/gen_init_cpio.c or via the new
 144   initrd format, an cpio archive.  It must be called "/init".  This binary
 145   is responsible to do all the things prepare_namespace() would do.
 146
 147   To maintain backwards compatibility, the /init binary will only run if it
 148   comes via an initramfs cpio archive.  If this is not the case,
 149   init/main.c:init() will run prepare_namespace() to mount the final root
 150   and exec one of the predefined init binaries.
 151
 152Bryan O'Sullivan <bos@serpentine.com>
 153
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