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Merge pull request #1 from stevesbrain/master

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Spelling/Grammar Fixes
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Michiel Derhaeg
2017-05-04 18:55:36 +02:00
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parent bc2113cd3c 6558a2f366
commit 9ef418211b
1 changed files with 36 additions and 35 deletions
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README.md
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@@ -87,8 +87,9 @@ Busybox Userspace
All these tools you know and love like ``ls``, ``echo``, ``cat`` ``mv``, and
``rm`` and so on are commonly referred to as the 'coreutils'. Busybox has that
and alot more, like utilities from ``util-linux`` so we can do stuff like
``mount`` and even a complete init system. Basicly most tools to expect to be
present on a Linux system only are these somewhat simplified.
``mount`` and even a complete init system. Basically, it contains most tools
you expect to be present on a Linux system, except they are a slightly
simplified version of the regular ones.
You can get the source from [busybox.net](https://busybox.net/). They also
provide prebuilt binaries which will do just fine for most use-cases. But just
@@ -110,7 +111,7 @@ The C standard library is more important to the operating system than you might
think. It provides some useful functions and an interface to the kernel. But it
also handles DNS requests and provides a dynamic linker. We don't really have to
pay attention to any of this, we can just statically link the one we are using
right know which is probably 'glibc'. This means the following part is optional.
right now which is probably 'glibc'. This means the following part is optional.
But I thought this would make it more interesting and it also makes us able to
build smaller binaries.
@@ -120,33 +121,33 @@ on Ubuntu or simply ``musl`` on Arch Linux. Now we can link binaries to musl
instead of glibc by using ``musl-gcc`` instead of ``gcc``.
Before we can build busybox with musl, we need sanitized kernel headers for use
with musl. You get get that from [this github
with musl. You get that from [this github
repo](https://github.com/sabotage-linux/kernel-headers). And set
``CONFIG_EXTRA_CFLAGS`` in your busybox config to
``CONFIG_EXTRA_CFLAGS="-I/path/to/kernel-headers/x86_64/include"`` to use them.
Obviously change ``/path/to`` to the location where you put the headers repo,
can be relative from within the busybox source directory.
which can be relative from within the busybox source directory.
If you run ``make CC=musl-gcc`` now, the busybox executable will be
significantly smaller because we are statically linking a much smaller libc.
Be aware that even though there is a libc standard, musl is not always a
drop-in replacement from glibc if the application you're compiling uses glibc
drop-in replacement for glibc if the application you're compiling uses glibc
specific things.
Building the Disk Image
-----------------------
Installing a OS on a file instead of a real disk complicates things but this
Installing an OS on a file instead of a real disk complicates things but this
makes development and testing easier.
So let's start by allocating a new file of size 100M by doing ``fallocate -l100M
image``(some distro's don't have ``fallocate`` so you can do ``dd if=/dev/zero
of=image bs=1M count=100`` instead). And then we format it like we would format
a disk with ``fdisk image``. It automatically creates a MBR partition table for
a disk with ``fdisk image``. It automatically creates an MBR partition table for
us and we'll create just one partition filling the whole image by pressing 'n' and
afterwards just use the default options for everything and keep spamming 'enter'
untill you're done. Finally press 'w' exit and to write the changes to the
until you're done. Finally press 'w' exit and to write the changes to the
image.
```bash
$ fdisk image
@@ -191,8 +192,8 @@ my case. Let's make a filesystem on it.
```bash
$ mkfs.ext4 /dev/loop0p1
```
If you want to use something else than ext4, be sure to enable it when
configuring your kernel. Now that we have done that, we can mount it start
If you want to use something other than ext4, be sure to enable it when
configuring your kernel. Now that we have done that, we can mount it and start
putting everything in place.
```bash
$ mkdir image_root
@@ -217,7 +218,7 @@ We'll copy our binaries over.
$ cp /path/to/busybox usr/bin/busybox
$ cp /path/to/bzImage boot/bzImage
```
You can call every busybox utility by supplying the utility as argument, like
You can call every busybox utility by supplying the utility as an argument, like
so: ``busybox ls --help``. But busybox also detects by what name it is called
and then executes that utility. So you can put symlinks for each utility and
busybox can figure out which utility you want by the symlink's name.
@@ -231,7 +232,7 @@ These symlinks might be incorrect from outside the system because of the
absolute path, but they work just fine from within the booted system.
Lastly, we'll copy some files from ``../filesystem`` to the image that will be
some use to us later.
of some use to us later.
```bash
$ cp ../filesystem/{passwd,shadow,group,issue,profile,locale.sh,hosts,fstab} etc
$ install -Dm755 ../filesystem/simple.script usr/share/udhcpc/default.script
@@ -245,7 +246,7 @@ a binary keymap file I use for belgian azerty.
The Boot Loader
---------------
The next step is to install the bootloader, the program that loads our kernel in
The next step is to install the bootloader - the program that loads our kernel in
memory and starts it. For this we use GRUB, one of the most widely used
bootloaders. It has a ton of features but we are going to keep it very simple.
Installing it is very simple, we just do this:
@@ -256,7 +257,7 @@ grub-install --modules=part_msdos \
/dev/loop0
```
The ``--target=i386-pc`` tells grub to use the simple msdos MBR bootloader. This
is often the default but this can vary from machine to machine so you better
is often the default, but this can vary from machine to machine so you better
specify it here. The ``--boot-directory`` options tells grub to install the grub
files in /boot inside the image instead of the /boot of your current system.
``--modules=part_msdos`` is a workaround for a bug in Ubuntu's grub. When you
@@ -265,7 +266,7 @@ think it needs to support msdos partition tables and won't be able to find the
root partition.
Now we just have to configure grub and then our system should be able to boot.
This basicly means telling grub how to load the kernel. This config is located
This basically means telling grub how to load the kernel. This config is located
at ``boot/grub/grub.cfg`` (some distro's use ``/boot/grub2``). This file needs
to be created first, but before we do that, we need to figure something out
first. If you look at ``/proc/cmdline`` on your own machine you might see
@@ -277,14 +278,14 @@ BOOT_IMAGE=/boot/vmlinuz-4.4.0-71-generic root=UUID=83066fa6-cf94-4de3-9803-ace8
These are the arguments passed to your kernel when it's booted. The 'root'
option tells our kernel which device holds the root filesystem that needs to be
mounted at '/'. The kernel needs to know this or it won't be able to boot. There
are different ways of identifying your the root filesystem. Using a UUID is a
are different ways of identifying your root filesystem. Using a UUID is a
good way because it is a unique identifier for the filesystem generated when you
do ``mkfs``. The issue with using this is that the kernel doesn't really
support it because it depends on the implementation of the filesystem. This
works on your system because it uses an initramfs. But we can't use it now. We
works on your system because it uses an initramfs, but we can't use it now. We
could do ``root=/dev/sda1``, this will probably work but it has some other problems.
The 'a' in 'sda' is can depend on the order the bios will load the disk and this
can change when you add a new disk or sometimes the order can change randomly.
The 'a' in 'sda' depends on the order the bios will load the disk and this
can change when you add a new disk, or for a variety of other reasons.
Or when you use a different type of interface/disk it can be something entirely
different. So we need something more robust. I suggest we use the PARTUUID. It's
a unique id for the partition (and not the filesystem like UUID) and this is a
@@ -294,9 +295,9 @@ a GPT thing). We'll find the id like this:
$ fdisk -l ../image | grep "Disk identifier"
Disk identifier: 0x4f4abda5
```
Then we drop the 0x and append the partition number as two digit hexidecimal. A
Then we drop the 0x and append the partition number as two digit hexidecimal. An
MBR only has 4 partitions max so that it's hexidecimal or decimal doesn't really
matter but that's what the standard says. So the grub.cfg should look like this:
matter, but that's what the standard says. So the grub.cfg should look like this:
```
linux /boot/bzImage quiet init=/bin/sh root=PARTUUID=4f4abda5-01
boot
@@ -352,14 +353,14 @@ everything down because it's the first and last process to live.
This also makes this ``init`` process very suitable to start and manage services
as is the case with the very common ``sysvinit`` and the more modern
``systemd``. But this isn't strictly necessary and some other process can cary
``systemd``. But this isn't strictly necessary and some other process can carry
the burden of service supervision, which is the case with the
[runit](http://smarden.org/runit/)-like ``init`` that is included with
``busybox``.
Unless you passed the ``rw`` kernel parameter the root filesystem is mounted as
read-only. So before we can make changes to our running system we have to
remount it as read-write first. And before we can do any mounting at all we have
remount it as read-write first. Before we can do any mounting at all we have
to mount the ``proc`` pseudo filesystem that serves as an interface to kernel.
```bash
$ mount -t proc proc /proc
@@ -370,14 +371,14 @@ $ mount / -o remount,rw
are not confortable using either of those you could always shutdown the VM,
mount the image again, and use your favorite text editor on your host machine.
If you don't use an qwerty keyboard you might have noticed that the VM uses a
qwerty layout which is the default, you might want to change it to azerty with
If you don't use a qwerty keyboard, you might have noticed that the VM uses a
qwerty layout as this is the default. You might want to change it to azerty with
``loadkmap < /usr/share/keymaps/be-latin1.bmap``. You can dump the layout you
are using on your host machine with ``busybox dumpkmap > keymap.bmap`` in a
virtual console (not in X) and put this on your image instead.
First, we'll create a script that handles the initialisation of the system
itself like mounting filesystems and configuring devices, etc. You could call it
itself (like mounting filesystems and configuring devices, etc). You could call it
``startup`` and put it in the ``/etc/init.d`` directory (create this first).
Don't forget to ``chmod +x`` this file when you're done.
```bash
@@ -457,17 +458,17 @@ after ``sysinit`` and will be restarted when they exit. We'll put some
it's correct. If you don't care for user login and passwords, you could instead
of the ``getty``'s do ``::askfirst:-/bin/sh``. ``askfirst`` does the same as
``respawn`` but asks you to press enter first. If no tty is specified it will
figure out what the console is. And the ``-`` infront of ``-/bin/sh`` means that
figure out what the console is. The ``-`` infront of ``-/bin/sh`` means that
the shell is started as a login shell. ``/bin/login`` usually does this for us
but we have to specify it here. Starting the shell as a login shell means that
it configures certain things it otherwise assumes already to be configured. E.g.
it sources ``/etc/profile``.
We can now start our system with ``init``. You can remove the ``init=/bin/sh``
entry in ``/boot/grub/grub.cfg`` because it defaults to ``/sbin/init``. And if
entry in ``/boot/grub/grub.cfg`` because it defaults to ``/sbin/init``. If
you reboot the system you should see a login screen. But if you run ``reboot``,
you'll notice it won't do anything. This happens because normally ``reboot``
tells the running ``init`` to reboot. You know, the ``init`` that isn't running
tells the running ``init`` to reboot. You know - the ``init`` that isn't running
right now. So we have two options, we could run ``reboot -f`` which skips the
``init``, or we could do this:
```bash
@@ -489,10 +490,10 @@ from how the more common ``sysvinit`` does things but it'll give you a
feel for which problems it's supposed to solve and how.
A basic service consists of a directory containing a ``run`` executable, usually
a script. This ``run`` script usually starts the daemon and doesn't exit untill
the daemon does. If ``run`` exits ``runit`` will thinks the service itself has
a script. This ``run`` script usually starts the daemon and doesn't exit until
the daemon does. If ``run`` exits ``runit`` will think the service itself has
stopped and if it wasn't supposed to stop, ``runit`` will try to restart it. So
be careful with forking daemons. Startin the service is done with ``runsv``.
be careful with forking daemons. Starting the service is done with ``runsv``.
This is the process that actually monitors the service and restarts it if
necessary. Usually you won't run it manually but doing so is useful for testing
services.
@@ -580,7 +581,7 @@ exec udhcpc -f -S
$ chmod +x /etc/init.d/udhcpc/run
$ ln -s /etc/init.d/udhcpc /etc/rc.d
```
And we're done. Yes it's that simple. Note that udhcpc just asks for a lease
Now we're done. Yes - it's that simple. Note that udhcpc just asks for a lease
from the DHCP server and that's it. When it has a lease it executes
``/usr/share/udhcpc/default.script`` to configure the system. We already copied
this script to this location. This script is included with the busybox source.
@@ -592,4 +593,4 @@ Epilogue
--------
That's it! We're done for now. Thanks for reading. I hope you learned something
useful, I certainly did while making this.
useful. I certainly did while making this.