The Design and Implementation of the FreeBSD Operating System, Second Edition
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FreeBSD/Linux Kernel Cross Reference
sys/Documentation/early-userspace/

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Name Size Last modified (GMT) Description
Back Parent directory 2013-02-02 13:13:18
File README 6882 bytes 2008-04-29 18:09:44
File buffer-format.txt 4606 bytes 2006-12-13 11:48:12

    1 Early userspace support
    2 =======================
    3 
    4 Last update: 2004-12-20 tlh
    5 
    6 
    7 "Early userspace" is a set of libraries and programs that provide
    8 various pieces of functionality that are important enough to be
    9 available while a Linux kernel is coming up, but that don't need to be
   10 run inside the kernel itself.
   11 
   12 It 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 
   22 The cpio file format used by initramfs is the "newc" (aka "cpio -H newc")
   23 format, and is documented in the file "buffer-format.txt".  There are
   24 two ways to add an early userspace image: specify an existing cpio
   25 archive to be used as the image or have the kernel build process build
   26 the image from specifications.
   27 
   28 CPIO ARCHIVE method
   29 
   30 You can create a cpio archive that contains the early userspace image.
   31 Your cpio archive should be specified in CONFIG_INITRAMFS_SOURCE and it
   32 will be used directly.  Only a single cpio file may be specified in
   33 CONFIG_INITRAMFS_SOURCE and directory and file names are not allowed in
   34 combination with a cpio archive.
   35 
   36 IMAGE BUILDING method
   37 
   38 The kernel build process can also build an early userspace image from
   39 source parts rather than supplying a cpio archive.  This method provides
   40 a way to create images with root-owned files even though the image was
   41 built by an unprivileged user.
   42 
   43 The image is specified as one or more sources in
   44 CONFIG_INITRAMFS_SOURCE.  Sources can be either directories or files -
   45 cpio archives are *not* allowed when building from sources.
   46 
   47 A source directory will have it and all of its contents packaged.  The
   48 specified directory name will be mapped to '/'.  When packaging a
   49 directory, limited user and group ID translation can be performed.
   50 INITRAMFS_ROOT_UID can be set to a user ID that needs to be mapped to
   51 user root (0).  INITRAMFS_ROOT_GID can be set to a group ID that needs
   52 to be mapped to group root (0).
   53 
   54 A source file must be directives in the format required by the
   55 usr/gen_init_cpio utility (run 'usr/gen_init_cpio --help' to get the
   56 file format).  The directives in the file will be passed directly to
   57 usr/gen_init_cpio.
   58 
   59 When a combination of directories and files are specified then the
   60 initramfs image will be an aggregate of all of them.  In this way a user
   61 can create a 'root-image' directory and install all files into it.
   62 Because device-special files cannot be created by a unprivileged user,
   63 special files can be listed in a 'root-files' file.  Both 'root-image'
   64 and 'root-files' can be listed in CONFIG_INITRAMFS_SOURCE and a complete
   65 early userspace image can be built by an unprivileged user.
   66 
   67 As a technical note, when directories and files are specified, the
   68 entire CONFIG_INITRAMFS_SOURCE is passed to
   69 scripts/gen_initramfs_list.sh.  This means that CONFIG_INITRAMFS_SOURCE
   70 can really be interpreted as any legal argument to
   71 gen_initramfs_list.sh.  If a directory is specified as an argument then
   72 the contents are scanned, uid/gid translation is performed, and
   73 usr/gen_init_cpio file directives are output.  If a directory is
   74 specified as an arugemnt to scripts/gen_initramfs_list.sh then the
   75 contents of the file are simply copied to the output.  All of the output
   76 directives from directory scanning and file contents copying are
   77 processed by usr/gen_init_cpio.
   78 
   79 See also 'scripts/gen_initramfs_list.sh -h'.
   80 
   81 Where's this all leading?
   82 =========================
   83 
   84 The klibc distribution contains some of the necessary software to make
   85 early userspace useful.  The klibc distribution is currently
   86 maintained separately from the kernel, but this may change early in
   87 the 2.7 era (it missed the boat for 2.5).
   88 
   89 You can obtain somewhat infrequent snapshots of klibc from
   90 ftp://ftp.kernel.org/pub/linux/libs/klibc/
   91 
   92 For active users, you are better off using the klibc git
   93 repository, at http://git.kernel.org/?p=libs/klibc/klibc.git
   94 
   95 The standalone klibc distribution currently provides three components,
   96 in 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 
  106 kinit is built as a single statically linked binary to save space.
  107 
  108 Eventually, several more chunks of kernel functionality will hopefully
  109 move 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 
  117 If kinit doesn't meet your current needs and you've got bytes to burn,
  118 the klibc distribution includes a small Bourne-compatible shell (ash)
  119 and a number of other utilities, so you can replace kinit and build
  120 custom initramfs images that meet your needs exactly.
  121 
  122 For questions and help, you can sign up for the early userspace
  123 mailing list at http://www.zytor.com/mailman/listinfo/klibc
  124 
  125 How does it work?
  126 =================
  127 
  128 The kernel has currently 3 ways to mount the root filesystem:
  129 
  130 a) 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 
  135 b) 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 
  141 c) 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 
  152 Bryan O'Sullivan <bos@serpentine.com>

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