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grub done

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Michiel Derhaeg
2017-04-02 22:08:45 +02:00
parent 5244a1dcfe
commit 31e6850e18
1 changed files with 42 additions and 6 deletions
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README.md
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@@ -201,6 +201,18 @@ 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
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.
```bash
for util in $(./usr/bin/busybox --list-full); do
ln -s /usr/bin/busybox $util
done
```
These symlinks might be incorrect from outside the system, but they work just
fine from within the booted system.
The Boot Loader
---------------
@@ -232,7 +244,7 @@ $ cat /proc/cmdline
BOOT_IMAGE=/boot/vmlinuz-4.4.0-71-generic root=UUID=83066fa6-cf94-4de3-9803-ace841e5066c ro
```
These are the arguments passed to your kernel when it's booted. The 'root'
options tells our kernel which device holds the root filesystem that needs to be
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
good way because it is a unique identifier for the filesystem generated when you
@@ -245,16 +257,40 @@ can change when you add a new disk or sometimes the order can change randomly.
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
somewhat recent addition to the kernel for msdos partition tables (GPT had this
for a while since it is quite essential for it's functionality). We'll find the
id like this:
somewhat recent addition to the kernel for msdos partition tables (it's actually
a GPT thing). We'll find the id like this:
```bash
$ fdisk -l ../image | grep "Disk identifier"
Disk identifier: 0x4f4abda5
```
So the grub.cfg should look like this:
Then we drop the 0x and append the partition number as two digit hexidecimal. A
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:
```
linux /boot/bzImage quiet init=/bin/sh root=PARTUUID=4f4abda5-01
boot
```
The ``defconfig`` kernel is actually a debug build so it's very verbose, so to
make it shut up you can add the ``quiet`` option. This stops it from being
printed to the console, you can still read it with the ``dmesg`` utility.
``init`` specifies the first process that will be started when the kernel is
booted. For now we just start a shell, we'll configure a real init when it's
booted.
So now we should be able to boot the system. You can umount the image, exit root
and start a VM to test it out. The simplest way of doing this is using QEMU.
The Ubuntu package is ``qemu-kvm``, and just ``qemu`` on Arch Linux.
```bash
$ cd ../
$ umount image_root
$ exit
$ qemu-system-x86_64 -enable-kvm image
```
And if everything went right you should now be dropped in a shell in our
homemade operating system.
PID 1, The Init
---------------
% TODO