Linux.Desktop.Hacks [Electronic resources] نسخه متنی

Hack 3. Bypass the Boot Manager

Save your system from a broken or missing
kernel.

Of all the fantastic things
you can do with your Linux box, one
of the most nerve-wracking activities is changing or upgrading a
kernel. This not only involves the archaic process of compiling the
kernel itself [Hack #88], but it
also requires you to update your bootloader so that you can actually
use the new kernel. Updating the bootloader often involves editing a
complex configuration file in which one wrong change can lead to a
system that won't boot. Perhaps nothing is more
worrisome in the Linux world than a computer containing all your
important work and files that doesn't boot, because
you screwed up the bootloader. Worrisome as it is, there is a useful
hack you can use to get around the bootloader to boot a working
kernel, fix the broken configuration file, and restart a healthy,
working system.

1.4.1. Bypassing the Installer

The key to restoring your sanity
when confronted with a broken kernel
is to use the installation disk you used to install Linux on your
computer in the first place. Every installation disk boots a generic
kernel from the CD-ROM to perform the installation routine. Many of
the install disks for distributions such as Debian, Ubuntu, Mandrake,
Red Hat, and others include a slim-line shell called BusyBox that you
can use to sneak behind the installer and fix your system.

When you run the CD-ROM-based installation program, nothing on your
hard disk is affected until you get to the partitioning part of the
program. Just before this section, jump to another virtual terminal
by pressing Ctrl-Alt-F(n) (such as Ctrl-Alt-F2).
If you are installing Linux on a Mac, this can be a little trickier,
because you need to press the Apple Option
Key-F(n)-F(n). These key
combinations switch you to a blank screen where you see a root shell.
This shell is a substantially cut-down version of a normal shell, but
it has the essential commands to navigate directories, mount disks,
and use an editor to change files.

1.4.2. Mounting Disks

Before you can fix the
bootloader, you need to identify the
problem that is preventing your system from booting. In many cases,
the problem occurs because the kernel files in
/boot do not have the same name/location as
specified in the bootloader configuration. This can occur if, for
example, you specify in your bootloader configuration that your
kernel is called vmlinux, but when you installed
a new kernel, the original was automatically renamed by the installer
to vmlinux.old, thus breaking the bootloader
configuration because the bootloader no longer contains the correct
kernel name. When this occurs, first you need to access
/boot and check the names of your kernel and
associated files.

To do this, mount /boot. First, create a new
directory for the mount point on your hard disk:

foo@bar:~# mkdir /bootmnt

Now you can mount your hard disk's boot partition
using the notation /dev/discs/disc[disk
number]/part[partition number]. For example, mount the
third partition on the first disk with the following:

foo@bar:~# mount /dev/discs/disc0/part3 /bootmnt

With your boot partition mounted to /bootmnt you
can note the names of your kernels and associated files (such as
initrd.img files). Now you need to mount the
main root partition so that you can modify your bootloader
configuration file. First, create the mount point:

foo@bar:~# mkdir /rootmnt

Now mount it:

foo@bar:~# mount /dev/discs/disc0/
part4
/rootmnt

Of course, your root partition might be something other than
part4 as specified in this command. With
the partition mounted in /rootmnt, you need to
change the root partition to that mount point so that you can specify
a different root partition from the one the installer is running.
This will enable you to access your root partition and run commands
from it. To do this, use the chroot command on
/rootmnt:

foo@bar:~# chroot /rootmnt

When you have run this command, take a look at the directory
structure with ls, and you will see that your
normal root partition is visible.

At this point in this process, you have access to your normal range
of applications, and if you have a network connection, you can use
this opportunity to copy important files and directories to another
computer, just in case you can't work out how to fix
your kernel or bootloader problem. You can find more information on
making network backups with scp and rsync in [Hack #83] .

To restore your system to its previous working form, you can enter
/etc and edit the configuration file for the
bootloader using the notes of the filenames that you took from the
/boot partition earlier. Then run the command to
re-create the bootloader (such as lilo or
ybin) and reboot the machine.

1.4.3. Hacking the Hack

If you system is failing to
boot the kernel, it could be
that your kernel has become corrupted or is incorrectly installed. At
this point, you should try to reinstall the kernel from a package if
one is available. As root, you can do this with an RPM file using the
following code:

foo@bar:~# rpm -Uvh kernel.rpm

You can reinstall a Debian package using this code:

foo@bar:~# dpkg -i kernel.deb

Instructions to compile a kernel are in [Hack #88] . You can compile a new kernel
from the chrooted environment you used earlier in
this hack.