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Source Mage ISO boot design
Benoît Papillault, 2004-10-05
Presentation
We would like to build a bootable CD-ROM (on x86 and ppc platforms at
least). To do so, we will use syslinux (x86) and yaboot (ppc). This
both tools allow to load a linux kernel as well as an initial
ramdisk. This article will show you the goals of this CD-ROM (what it
could be used for) and a general description of a way to implement it,
along with some explanations about technical points.
Goals
Our three goals are:
- First goal is to use the CD-ROM to install a SourceMage
distribution. This mean that the CD-ROM will be used as a root
filesystem.
- The second goal is to use the CD-ROM as a rescue CD-ROM to boot
an already installed distribution on the hard drive. The location of
this distribution will be given by the
root= command
line parameter.
- The third goal is a variant of the second one. In some cases,
it is possible that the kernel used need additional modules to be
able to mount the root filesystem. To do so, we will use an initial
ramdisk.
How an initial ramdisk works
How an initial ramdisk works is quite simple, but some important
details need to be described. First of all, a ramdisk is a file
(either compressed with gzip or not) that contains a
filesystem (ext3, ...). Advantage of using an initial ramdisk is that
it is loaded in memory directly by the bootloader and not by the linux
kernel. Such file can be used in two ways:
- If the boot parameters have
root=/dev/ram0.
In this case, the ramdisk will be used exactly as a root filesystem
and the executable /sbin/init present in this ramdisk
will be launched with a process id (PID) of 1. This property allows
to mount another filesytem, and to switch to this new filesystem
as if the new filesystem was the root filesystem.
mount /dev/hda2 /mnt
cd /mnt
pivot_root . initrd
# mount /dev if needed
exec chroot . /sbin/init "$@" < /dev/console > /dev/console 2>&1
- If the boot parameters does not have
root=/dev/ram0.
In this case, the ramdisk is temporarily used as the root
filesystem and the /linuxrc executable present in the
ramdisk will be launched. Its process id (PID) will not be 1. When
the /linuxrc execution ends, the real root filesystem
is mounted and /sbin/init is launched.
Implementation
To implement our goals, the bootloader will have three entries:
- install
This is our first goal. The ramdisk will be loaded with
root=/dev/ram0. If we had used devfs, we
could have used root=/dev/cdroms/cdrom0. But this will
not work on a system which has several CD-ROM drives because only
the first one will be used. Thus, the ramdisk will deal with
scanning all available CD-ROM drives and detecting the SourceMage
CD-ROM with the help of /etc/sourcemage_version
existing on it.
- linux root=/dev/hda1
This is our second goal. No ramdisk is loaded. The user must add a
root= on the boot command line.
- initrd root=/dev/hda1
This is our third goal. A ramdisk is used only to load needed
modules to access the root filesystem. As the second entry, the user
must add a root= on the boot command line.
Documentations
Here are some sources of informations to go further:
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