Kernel/grub : how override root parameter inside initrd script

Question

I have an old kernel ( 2.4.37.9 ) and I want to override or substitute the root=XXXXX parameter to send to the kernel inside the initrd script.

I already made some attempt to do that but it seems that at the end of initrd grub alway pass to kernel the root parameter define inside the menu.lst file, while I'm tryng to define a dynamic value ( ex. hda1 or hdc1 ) depending of the layout of th mother board.

title Linux-2.4.37.9_CCL_20130122 with INITRD
    root (hd0,0)
    kernel /boot/vmlinuz-2.4.37.9_CCL_20130122  ro root=XXXXXX console=ttyS0,9600 console=tty0 apm=off
    initrd /boot/initrd-CCL.img.gz

Any suggestions ?

Answer 1

You can't, it's not supported.

What you can do is extract initrd /boot/initrd-CCL.img.gz, find the part that mounts root, and modify the corresponding part, add the script to detect hard drive layout, and mount the right drive.

The recommended way is set a UUID for your root drive, and change the root= part to root=UUID=XXX instead, I'm not sure if it's supported on old distros. In that way, no matter how device layout changes, it always mount the right device.

Answer 2

This is not the more polite solution to this problem, but it works and mybe should be helpfull to some other so I brifly describe here how I solved my problem to have a DOM capable to change it boot device automatically.

Inside the linuxrc, the script of initrd, I detect wich device is available and based on that result I set the default startup option used by grub

my linuxrx i something like this

#!/bin/ash

restart=0
mntdev=""

target="hda"
echo "--> check fdisk ${target} "

mount -t ext2 /dev/${target}1 /mnt/tmp
if [ -f /mnt/tmp/etc/slackware-release ]; then
  echo "Found $target "
  mntdev="/dev/${target}1"
      olddef=$( cat /mnt/tmp/boot/grub/default )
      if [ $olddef -ne 0 ]; then
          echo "0" > /mnt/tmp/boot/grub/default
          restart=1
      fi
fi
umount /mnt/tmp
# ================================
if [ -z $dskroot ]; then
    target="hdc"
    echo "--> check fdisk ${target} "

    mount -t ext2 /dev/${target}1 /mnt/tmp
    if [ -f /mnt/tmp/etc/slackware-release ]; then
      echo "Found $target  "
      mntdev="/dev/${target}1"
      olddef=$( cat /mnt/tmp/boot/grub/default )
      if [ $olddef -ne 1 ]; then
          echo "1" > /mnt/tmp/boot/grub/default
          restart=1
      fi
    fi
    umount /mnt/tmp
fi
# ================================

if [ $restart -eq 1 ]; then
  echo "Changed grub default : Rebooting PC "
  echo "===================================="
  sleep 2
  mount -t ext2 $mntdev /mnt/tmp
  chroot /mnt/tmp <<EOF
  /sbin/reboot -f
EOF
fi

And inside the grub menu I reserver the first two entry , the 0 for device hda and 1 for device hdc

default saved

title Linux-2.4.37.9_CCL_20130122 with INITRD hda pos 0
        root (hd0,0)
        kernel /boot/vmlinuz-2.4.37.9_CCL_20130122  ro root=/dev/hda1 console=ttyS0,9600 console=tty0 apm=off
        initrd /boot/initrd-CCL.img.gz

title Linux-2.4.37.9_CCL_20130122 with INITRD hdc pos 1
        root (hd0,0)
        kernel /boot/vmlinuz-2.4.37.9_CCL_20130122  ro root=/dev/hdc1 console=ttyS0,9600 console=tty0 apm=off
        initrd /boot/initrd-CCL.img.gz