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Repairing a damaged Grub


If you have read the previous chapter, you know that Grub requires multiple stages to be installed correctly. They are all installed in different locations of your hard-disk. If one of these sectors is overwritten then Grub will stop working. Fortunately there are solutions to recover your linux installation by reinstalling Grub, and you don’t have to reinstall Linux entirely.

All the administration programs required to re-enable Grub are part of the standard Linux installations, so the main problem is to access Linux the first time. The first solution is to start the Linux system which is on your hard-disk by booting from SystemRescueCd. The second solution is to boot from a rescue disk such as SystemRescueCd normally and to run the Grub installation command from a chroot environment. These two solutions are described below and they should work for both Grub1 (Grub Legacy) and Grub2.

Why Grub may break

Grub may stop working for many reasons. Here are examples of what can break it:

  • installation of another operating system on your computer: the installation program is likely to overwrite the boot code in the MBR where Grub can store its first stage.
  • modifications of the disk partitioning: Grub sometimes uses a static list of sectors to remember where the next stage is located. It can be installed at the beginning of a partition when the filesystem of that partition leaves some free space at the beginning (eg: ReiserFS does not use the first 64KB). If you move that partition, then the sectors where Grub is installed will move with it, and the bad sectors will be referenced.
  • disk cloning: you may want to clone your partitions to another disk or another computer for some reason. If you just clone the partitions of the disks, the sectors where Grub is installed may not be copied (eg: sectors before the first partition, or MBR).
  • modification of the boot priority of the hard-disks: If you have multiple hard-disks you can specify which one you want to boot from in the BIOS settings. In general Grub is installed on only one disk, so it will stop working if the BIOS tries to boot from another disk.

Identification of the boot and root filesystems

To repair Grub, you may need to know the name of the boot and root partitions where Linux is installed. If you have many partitions on your disk you may not remember which one it is. You can run fsarchiver probe simple or fsarchiver probe detailed from SystemRescuecd to show the list of filesystems of your computer. Here is an example:

root@sysresccd /root % fsarchiver probe simple
[=====DEVICE=====] [==FILESYS==] [=====LABEL=====] [====SIZE====] [MAJ] [MIN]
[/dev/sda1       ] [ext3       ] [boot           ] [   256.00 MB] [  8] [  1]
[/dev/sda2       ] [reiserfs   ] [debian         ] [    16.00 GB] [  8] [  2]
[/dev/sda3       ] [ntfs       ] [winxp32        ] [    16.00 GB] [  8] [  3]
[/dev/sda4       ] [ext3       ] [data           ] [   898.56 GB] [  8] [  4]

There we can see that /dev/sda1 is the boot partition, and /dev/sda2 is the root filesystem for Linux-Debian. This is obvious because the labels are appropriate, but it’s not always that simple. All linux root filesystems are supposed to have their init program in /sbin/init. You can check that Linux is installed on a partition by checking if that file exists:

root@sysresccd /root % mkdir -p /mnt/linux
root@sysresccd /root % mount -r /dev/sda2 /mnt/linux
root@sysresccd /root % ls -l /mnt/linux/sbin/init
-rwxr-xr-x 1 root root 37384 2008-08-12 15:20 /mnt/linux/sbin/init
root@sysresccd /root % umount /mnt/linux

The /boot directory is where the linux kernel image (vmlinuz) and the associated initramfs (initrd) and grub files are installed. This directory is either part of the root filesystem or on a separate partition. You can identify the boot partition because it’s quite small in general (between 50MB and 300MB), and it’s often the first partition of the hard drive. You can mount the boot partition and check that it contains the files we expect (vmlinuz and initrd):

root@sysresccd /root % mkdir -p /mnt/boot
root@sysresccd /root % mount -r /dev/sda1 /mnt/boot
root@sysresccd /root % ls -l /mnt/boot
lrwxrwxrwx 1 root root        1 2008-08-05 22:46 boot -> .
-rw-r--r-- 1 root root    98203 2009-10-27 10:05 config-2.6.30-bpo.2-amd64
drwxr-xr-x 2 root root     2800 2009-11-12 19:38 grub
-rw-r--r-- 1 root root  8198587 2009-11-08 14:59 initrd.img-2.6.30-bpo.2-amd64
drwx------ 2 root root       48 2006-11-25 15:55 lost+found
-rw-r--r-- 1 root root  1508757 2009-10-27 10:05
-rw-r--r-- 1 root root  2224064 2009-10-27 10:04 vmlinuz-2.6.30-bpo.2-amd64
root@sysresccd /root % umount /mnt/boot

Solution 1: Booting your Linux installation from SystemRescueCd

SystemRescueCd allows you to boot a Linux system installed on the disk even if grub is broken. You have to boot SystemRescueCd either from the cdrom, usb stick or the network. The purpose is to just have access to your system so that you can reinstall Grub from your original Linux installation.

When the first screen shows up (with the ACSII-art logo), you will have to boot with specific boot options so that it starts the system which is installed on the disk. The root=/dev/xxx option can be used either with the name of the root partition of your Linux installation, or with auto. In the first case (eg: rescuecd root=/dev/sda2) the SystemRescueCd initialization script will mount the specified partition. If you type rescuecd root=auto then SystemRescueCd will use the first valid root partition where a Linux installation has been detected.

If your Linux system uses 64bit binaries, you have to use a 64bit kernel, so the complete command can be something like rescue64 root=auto if you want SystemRescueCd to find and boot the first valid 64bit installation of Linux. You can use a 64bit kernel even if you have a 32bit installation of Linux, as long as your hardware supports 64bit programs (which is the case for all the recent Intel & AMD processors).

Your Linux system should then boot using the kernel from SystemRescueCd. the consequence is that it may complain about missing kernel modules (No such file or directory errors). This is because each specific kernel version has its own kernel modules, and the linux distribution you have installed does not provide the modules of that particular kernel. It should not be a problem since all the filesystems and disk drivers should be loaded at that stage. This is all you need to reinstall Grub.

Once you have access to your system, you have to get a shell to run commands. You have to identify the name of the disk where Grub has to be installed. In general, it will be /dev/sda. If you have more than one disk, it may also be /dev/sdb, /dev/sdc, … You can use fsarchiver probe simple to get the list of the partitions, and then you can guess the name of the disk. Here is an example of a computer with two disks (/dev/sda and /dev/sdb):

root@debian /root % fsarchiver probe simple
[=====DEVICE=====] [==FILESYS==] [=====LABEL=====] [====SIZE====] [MAJ] [MIN]
[/dev/sda1       ] [ext3       ] [boot           ] [   256.00 MB] [  8] [  1]
[/dev/sda2       ] [reiserfs   ] [debian         ] [    16.00 GB] [  8] [  2]
[/dev/sda3       ] [ntfs       ] [winxp32        ] [    16.00 GB] [  8] [  3]
[/dev/sda4       ] [ext3       ] [data           ] [   898.56 GB] [  8] [  4]
[/dev/sdb1       ] [ext3       ] [boot           ] [   976.55 MB] [  8] [ 17]
[/dev/sdb2       ] [reiserfs   ] [gentoo         ] [    16.00 GB] [  8] [ 18]
[/dev/sdb3       ] [LVM2_member] [<unknown>      ] [   866.56 GB] [  8] [ 19]

Then you can use grub-install with the name of the disk where it has to be installed. Here is what you can expect:

root@debian /root % grub-install /dev/sda
Installation finished. No error reported.
This is the contents of the device map /boot/grub/
Check if this is correct or not. If any of the lines is incorrect,
fix it and re-run the script grub-install.
(hd0)   /dev/sda

Grub should now be fixed on your disk, and you can reboot your computer.

Solution 2: Reinstallation of Grub using chroot

The second option is to repair Grub by running grub-install another way. The command we are using is still part of your Linux installation. The difference is that we will start SystemRescueCd normally, and we will access your Linux installation from chroot. The first thing to do is to start SystemRescueCd normally. You just have to boot with a 64bit kernel (eg: rescue64 or raltker64) if your Linux installation is based on 64bit binaries.

Next, you can run fsarchiver probe simple to identify your boot and root filesystems (see the section about detection of the boot and root filesystems).

root@sysresccd /root % fsarchiver probe simple
[=====DEVICE=====] [==FILESYS==] [=====LABEL=====] [====SIZE====] [MAJ] [MIN]
[/dev/sda1       ] [ext3       ] [/boot          ] [   196.08 MB] [  8] [  1]
[/dev/sda2       ] [ext3       ] [fedora11       ] [     6.05 GB] [  8] [  2]
[/dev/sda3       ] [ext3       ] [data           ] [     2.07 GB] [  8] [  3]

Now you have to mount the partition that contains the root filesystem. In that example /dev/sda2 is the partition where Fedora-Linux is installed. You also have to mount the proc, dev and sys virtual filesystems this way:

root@sysresccd /root % mkdir /mnt/linux
root@sysresccd /root % mount /dev/sda2 /mnt/linux
root@sysresccd /root % mount -o bind /proc /mnt/linux/proc
root@sysresccd /root % mount -o bind /dev /mnt/linux/dev
root@sysresccd /root % mount -o bind /sys /mnt/linux/sys

The mount -o bind command makes something that looks like a symbolic link. For instance the directory /mnt/linux/proc is an access to the real primary directory which is /proc

Now we have to chroot to /mnt/linux. Chroot is a very powerful command: it gives the programs the illusion that the root of the system is /mnt/linux. This means that each time a program reads a file such as /bin/ls it will use /mnt/linux/bin/ls instead. Chroot is required because we want to execute commands from the Linux installation from the disk, as if it was the current root. Chroot only has an effect on the current shell and on all the commands that you will run from that shell. In has no effect on the other programs which are already running from SystemRescueCd.

root@sysresccd /root % chroot /mnt/linux /bin/bash

If the contents of /boot is on a separate partition (if /boot is currently empty) you have to mount it:

[root@sysresccd /]# ls -l /boot/
total 0
[root@sysresccd /]# mount /dev/sda1 /boot/
[root@sysresccd /]# ls -l /boot/
-rw-r--r--. 1 root root   97567 2009-05-27 22:25 config-
drwxr-xr-x. 2 root root    1024 2009-11-14 18:57 grub
-rw-------. 1 root root 2944107 2009-06-14 11:08 initrd-
-rw-r--r--. 1 root root 1257178 2009-05-27 22:25
-rwxr-xr-x. 1 root root 3035056 2009-05-27 22:25 vmlinuz-

Now you can run grub-install to repair Grub. The first argument it takes is the name of the disk where to reinstall Grub. See the previous section for more details.

[root@sysresccd /]# grub-install /dev/sda
Installation finished. No error reported.
This is the contents of the device map /boot/grub/
Check if this is correct or not. If any of the lines is incorrect,
fix it and re-run the script `grub-install'.
# this device map was generated by anaconda
(hd0)     /dev/sda

Once Grub has been reinstalled, you can type exit to leave the chroot environment.

[root@sysresccd /]# exit

Then unmount all the filesystems properly:

root@sysresccd /root % umount /mnt/linux/{dev,proc,sys}
root@sysresccd /root % umount /mnt/linux/boot
root@sysresccd /root % umount /mnt/linux/

You can now reboot your computer, Grub should work.

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