ramfs-rootfs-initramfs.txt

linux 内核源代码

use in place of the above config file:

  #!/bin/sh

  # Copyright 2006 Rob Landley <[e-mail]> and TimeSys Corporation.
  # Licensed under GPL version 2

  if [ $# -ne 2 ]
  then
    echo "usage: mkinitramfs directory imagename.cpio.gz"
    exit 1
  fi

  if [ -d "$1" ]
  then
    echo "creating $2 from $1"
    (cd "$1"; find . | cpio -o -H newc | gzip) > "$2"
  else
    echo "First argument must be a directory"
    exit 1
  fi

Note: The cpio man page contains some bad advice that will break your initramfs
archive if you follow it.  It says "A typical way to generate the list
of filenames is with the find command; you should give find the -depth option
to minimize problems with permissions on directories that are unwritable or not
searchable."  Don't do this when creating initramfs.cpio.gz images, it won't
work.  The Linux kernel cpio extractor won't create files in a directory that
doesn't exist, so the directory entries must go before the files that go in
those directories.  The above script gets them in the right order.

External initramfs images:
--------------------------

If the kernel has initrd support enabled, an external cpio.gz archive can also
be passed into a 2.6 kernel in place of an initrd.  In this case, the kernel
will autodetect the type (initramfs, not initrd) and extract the external cpio
archive into rootfs before trying to run /init.

This has the memory efficiency advantages of initramfs (no ramdisk block
device) but the separate packaging of initrd (which is nice if you have
non-GPL code you'd like to run from initramfs, without conflating it with
the GPL licensed Linux kernel binary).

It can also be used to supplement the kernel's built-in initramfs image.  The
files in the external archive will overwrite any conflicting files in
the built-in initramfs archive.  Some distributors also prefer to customize
a single kernel image with task-specific initramfs images, without recompiling.

Contents of initramfs:
----------------------

An initramfs archive is a complete self-contained root filesystem for Linux.
If you don't already understand what shared libraries, devices, and paths
you need to get a minimal root filesystem up and running, here are some
references:
http://www.tldp.org/HOWTO/Bootdisk-HOWTO/
http://www.tldp.org/HOWTO/From-PowerUp-To-Bash-Prompt-HOWTO.html
http://www.linuxfromscratch.org/lfs/view/stable/

The "klibc" package (http://www.kernel.org/pub/linux/libs/klibc) is
designed to be a tiny C library to statically link early userspace
code against, along with some related utilities.  It is BSD licensed.

I use uClibc (http://www.uclibc.org) and busybox (http://www.busybox.net)
myself.  These are LGPL and GPL, respectively.  (A self-contained initramfs
package is planned for the busybox 1.3 release.)

In theory you could use glibc, but that's not well suited for small embedded
uses like this.  (A "hello world" program statically linked against glibc is
over 400k.  With uClibc it's 7k.  Also note that glibc dlopens libnss to do
name lookups, even when otherwise statically linked.)

A good first step is to get initramfs to run a statically linked "hello world"
program as init, and test it under an emulator like qemu (www.qemu.org) or
User Mode Linux, like so:

  cat > hello.c << EOF
  #include <stdio.h>
  #include <unistd.h>

  int main(int argc, char *argv[])
  {
    printf("Hello world!\n");
    sleep(999999999);
  }
  EOF
  gcc -static hello2.c -o init
  echo init | cpio -o -H newc | gzip > test.cpio.gz
  # Testing external initramfs using the initrd loading mechanism.
  qemu -kernel /boot/vmlinuz -initrd test.cpio.gz /dev/zero

When debugging a normal root filesystem, it's nice to be able to boot with
"init=/bin/sh".  The initramfs equivalent is "rdinit=/bin/sh", and it's
just as useful.

Why cpio rather than tar?
-------------------------

This decision was made back in December, 2001.  The discussion started here:

  http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1538.html

And spawned a second thread (specifically on tar vs cpio), starting here:

  http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1587.html

The quick and dirty summary version (which is no substitute for reading
the above threads) is:

1) cpio is a standard.  It's decades old (from the AT&T days), and already
   widely used on Linux (inside RPM, Red Hat's device driver disks).  Here's
   a Linux Journal article about it from 1996:

      http://www.linuxjournal.com/article/1213

   It's not as popular as tar because the traditional cpio command line tools
   require _truly_hideous_ command line arguments.  But that says nothing
   either way about the archive format, and there are alternative tools,
   such as:

     http://freshmeat.net/projects/afio/

2) The cpio archive format chosen by the kernel is simpler and cleaner (and
   thus easier to create and parse) than any of the (literally dozens of)
   various tar archive formats.  The complete initramfs archive format is
   explained in buffer-format.txt, created in usr/gen_init_cpio.c, and
   extracted in init/initramfs.c.  All three together come to less than 26k
   total of human-readable text.

3) The GNU project standardizing on tar is approximately as relevant as
   Windows standardizing on zip.  Linux is not part of either, and is free
   to make its own technical decisions.

4) Since this is a kernel internal format, it could easily have been
   something brand new.  The kernel provides its own tools to create and
   extract this format anyway.  Using an existing standard was preferable,
   but not essential.

5) Al Viro made the decision (quote: "tar is ugly as hell and not going to be
   supported on the kernel side"):

      http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1540.html

   explained his reasoning:

      http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1550.html
      http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1638.html

   and, most importantly, designed and implemented the initramfs code.

Future directions:
------------------

Today (2.6.16), initramfs is always compiled in, but not always used.  The
kernel falls back to legacy boot code that is reached only if initramfs does
not contain an /init program.  The fallback is legacy code, there to ensure a
smooth transition and allowing early boot functionality to gradually move to
"early userspace" (I.E. initramfs).

The move to early userspace is necessary because finding and mounting the real
root device is complex.  Root partitions can span multiple devices (raid or
separate journal).  They can be out on the network (requiring dhcp, setting a
specific MAC address, logging into a server, etc).  They can live on removable
media, with dynamically allocated major/minor numbers and persistent naming
issues requiring a full udev implementation to sort out.  They can be
compressed, encrypted, copy-on-write, loopback mounted, strangely partitioned,
and so on.

This kind of complexity (which inevitably includes policy) is rightly handled
in userspace.  Both klibc and busybox/uClibc are working on simple initramfs
packages to drop into a kernel build.

The klibc package has now been accepted into Andrew Morton's 2.6.17-mm tree.
The kernel's current early boot code (partition detection, etc) will probably
be migrated into a default initramfs, automatically created and used by the
kernel build.