slide 1: Introduction

  differences between initramfs and initrd:

    ramfs vs block
      - less memory
      - dynamic resizing, no limits
      - rootfs always mounted for same reason pid1 unkillable.

    archive vs image
      - no filesystem driver
        - ramfs tiny, always loaded in 2.6.  Wrapper around vfs cache.

    bundled vs unbundled
      - both available to ramfs

    init vs linuxrc
      - linuxrc sucky.
        - Not PID 1.
          - unpredictable PID, killable, zombies, etc.
        - strange flow control, unnecessarily complicated.
          - setup then return to kernel so it can mount root?  (nfs?)
          - you're right there already, just do it yourself.

slide 2: Introduction

  advantages of initramfs

    - fundamental sequencing problem: mount "/" before running init.
      - Device enumeration complicated.  (USB, anyone?  SCSI longstanding)
      - NFS mounts require dhcp and login.
      - encrypt, compress, 
      - modular filesystem driver, build into kernel.  Magic format.  Special.
        - root ext3.  Initrd ext2 _not_ ext3, journal on ramdisk!  Build in
          ext2 driver then can't get rid of it.
      - Binary only firmware for some devices (network cards).
        - Too cheap to spring for ram or flash.
          - Require a cross-compiler for mips to build your kernel?
      - initramfs has no chicken and egg problems ala nfs mounts
    - very simple implementation.  (Small, cross-platform.)
    - One file.  Bootloader simple to set up.
    - actually init
      - no assumptions about what you're going to do.  Stay running.
        - root mount not "magic".  Built-in root, ignorable.
    - moves more control to userspace
      - laptops, embedded systems, clusters, mainframes


slide 3: Build a "hello world" program to put in initramfs.

  #include <stdio.h>
  #include <unistd.h>

  int main(int argc, char *argv[])
  {
    printf("Hello world!\n");
    sleep(9999999);
    return 0;
  }

  gcc hello.c -o hello

  ./hello

  Sleep instead of returning avoids "kernel panic, tried to kill init" later.






slide 4: Get the "hello world" program to run in a chroot environment.

  Shared libraries:
    The executable doesn't work by itself: chroot . /hello

    Use the ldd command to find shared libraries.  (I made a script.)
      mkdir walrus
      ./mkchroot.sh walrus a.out
      chroot walrus /a.out

    It's big:
      du walrus: 1.3 megabytes.  Plus your kernel.  For hello world.
        gets gzipped down to 600k, but still.
      strip - library already stripped with --strip-unneeded
      -Os, we're optimizing the idle task here.

  Statically link:
    gcc hello.c -o hello -static -s
    A 400k hello world.  That's glibc for you.  Can we do better?




slide 5: Get the size down by building against uClibc.
  - gcc sucks, but no time to go into depth and breadth of gcc's suckage.

  - lots of ways to get a uClibc toolchain
    buildroot.uClibc.org, gentoo embedded, I did one (landley.net/code/firmware)

    Here are some prebuilt ones, via my day job:
      http://crossdev.timesys.com/download/latest/i686/toolchain/i686-linux-toolchain-fat.tar.gz
      Extract into root directory, as root.  (Yeah, I know.)
      They don't supply an ldd that works with my script.  (use readelf -d)

    PATH=/opt/timesys/toolchains/i686-linux/bin/:$PATH i686-uclibc-gcc \
      hello.c -o hello -Os -s

      hello + lib/ld-uClibc.so.0 + lib/libc.so.0
        252k: smaller than statically linked hello world from uClibc.

      Build statically against uClibc (add -static): 7k





slide 6: Build a User Mode Linux kernel to try out initramfs.

  Get current kernel source, configure (I'm using miniconfig), build UML:

    wget http://kernel.org/pub/linux/kernel/v2.6/linux-2.6.17.tar.bz2
    tar xjvf linux-2.6.17.tar.bz2
    patch -p1 -i ../miniconfig.patch
    make miniconfig MINICONFIG=../linux-miniconfig-uml ARCH=um
      optional: make ARCH=um menuconfig (general setup->small systems->load all)
    make ARCH=um
      note: CONFIG_HZ error means it's distclean time.  You have leftover
      ARCH=i386 symlinks in your build directory.  Try again.

  Boot User Mode Linux with hostfs, demonstrate basic User Mode Linux.
    ./linux rootfstype=hostfs rw init=/bin/sh
      whoami
      mount -t procfs /proc /proc
      cat /proc/cpuinfo
      ps
      cat/proc/mounts
    exit - panics the kernel when PID 1 exits, but who cares?


slide: Build our "hello world" initramfs into UML as directory.

  - Grab above statically linked hello.
    As a directory:
      echo 'CONFIG_INITRAMFS_SOURCE="../walrus"' >> ../linux-miniconfig-uml
      make ARCH=um miniconfig MINICONFIG=../linux-miniconfig-uml
      make ARCH=um

  - Make it actually run:
    ./linux
      - doesn't find it, because it's not called /init, so:
    ./linux rdinit=/hello
      - no console!

    Downside of the directory method, must make /dev/console as root.
      mknod dev/console c 5 1
      The default initramfs has a /dev/console in it.

    Note for my fellow ubuntu users, to fix a bug in 2.6.17 kernel:
      sed -i 's/gawk/awk/' scripts/gen_initramfs_list.sh

  - make ARCH=um; ./linux rdinit=/hello
     kill it from another window, UML puts console into raw mode, but PID 1
     not killable.

slide: Build our "hello world" initramfs into UML as directory.

  scripts/gen_initramfs_list.sh creates a file list.  Using file list to create
  archive doesn't require root.

  - Grab above statically linked hello.  Put in initramfs with directory
     method.  Show it.  Put in initramfs with script method.  Wheee.

slide: Build a static busybox for initramfs.

  - Build a statically linked busybox, run it as standalone shell.
    wget http://busybox.net/downloads/busybox-1.2.0.tar.bz2
    tar xvjf busybox-1.2.0.tar.bz2
    cd busybox-1.2.0
    make defconfig
    make

    Now run it as standalone shell under UML with rdinit=

  - Transfer to new RFS (hostfs) with switch_root, by hand.







Now boot a real kernel under QEMU.

  - Build a real kernel, run under QEMU.  Package external initramfs with cpio,
    show initrd=.  Show builtin too.  Build second initrd with data and
    show initrd overlay on builtin base.

  - Cross-compile for ARM (kernel, busybox, uClibc)

Get the size down:

  - Build dynamically linked, copy shared libraries, boot as #3.
    - Script to do this

  - Build against uClibc.  Talk about klibc.
    - uClibc full fledged replacement for glibc.  Build anything against it.
      klibc isn't.  Doesn't even build BusyBox.

  - In many ways, gcc sucks.
      The uClibc guys recommend building gcc from source.
        (I run sed against the source code.)
      There's a wrapper too, and various ways it can break.  Newer gccs break
      less, but can still break.  (libgcc_s) Klibc also uses the wrapper.

Download http://crossdev.timesys.com
PATH=/opt/timesys/toolchains/i686-linux/bin:$PATH make 

Do real things with it:

  - shared libraries, directory layout, /dev, /proc, /sys

  - Setup quick init script.  mdev -s, reproduce root=

  - The firmware Firmware Linux initramfs.

  - encfs

Bonus:
  Extracting initramfs image from existing kernel.