Given vmlinuz and initrd.gz, how do I find out, where the kernel is going to load / (root) file system from?

Question

I'm trying various ways of Linux installation (from iso, flash, iso on flash, kernel on flash, root FS in iso-file on flash...) and want to understand what's going on.

My question is: is it possible, given the built kernel and ramfs files from a distribution (vmlinuz and initrd), to find out, where they are going to look for the "/" file system? Is it possible to configure this without re-compiling the kernel?

And one more: when kernel loads the root filesystem from loopback device, created from .iso-filesystem, how can I configure this process? Thanks!

Answer 1

It is given at boot time by your bootloader, for example Grub.

To see with which arguments your kernel was started, do this:

$ cat /proc/cmdline

For me, this ouputs:

BOOT_IMAGE=/vmlinuz-3.5.0-13-generic root=/dev/mapper/crypt-precise--root ro

So the initrd/initramfs will try to mount my /dev/mapper/crypt-precise--root (encrypted LVM) logical volume as /.

You can re-configure Grub to load other operating systems from your harddrive using the same kernel (multi-boot) or edit this line runtime by pressing e while selecting (not yet booting) the Grub entry.

For recent Debian-based distributions, changing it permanently works like this:
(be careful, you may not be able to boot into your original operating system again!)

In the file /etc/default/grub set some GRUB_CMDLINE_LINUX="root=/dev/mydevice" yourself and update Grub by doing update-grub.

However, I recommend you to configure multiboot, otherwise it's not possible to change or update your Grub configuration again easily.

Answer 2

On most installations, you specify the root device to the bootloader. The root device is passed to the kernel as a parameter called root. Depending on the bootloader, this may be part of the kernel command line arguments (which may look like ro quiet root=/dev/sda1) or a separate setting. For some common bootloaders:

• Lilo: in the configuration file (/etc/lilo.conf), there is a root directive to indicate the Linux root device.
• Grub legacy (versions 0.9x): in the configuration file (typically /boot/grub/menu.lst), the root device is passed as part of the kernel command line (kernel directive). Note that there is a different root directive for Grub itself; that tells Grub where to look for files such as the kernel. On many systems, the Grub configuration file is generated by the update-grub script which fills in the root device from the installed system (using /boot/grub/device.map).
• Grub 2 (versions 1.9x): in the configuration file (typically /boot/grub/grub.cfg), the root device is passed as part of the kernel command line (linux directive). Note that there is a different root variable (set root=(hd0,1)) that tells Grub where to look for files such as the kernel. On many systems, the Grub configuration file is generated by the update-grub script which fills in the root device from the installed system (using /boot/grub/device.map).
• U-Boot: the root device is passed as part of the command line arguments in the bootargs environment variable. The U-Boot binary contains a default value baked in at compile time, which can be overridden by a setting in flash memory (if the device permits).

In the absence of a root command line argument, the initrd or initramfs, if used, may try to find a root partition to mount. Even in the presence of a root argument, the initrd or initramfs may decide to use a different device. The code calls switch_root to switch from the initrd to the actual root.

The default root device when there is no root command line argument is determined at compile time. You can change that value in a binary image with the rdev utility.

On a booted kernel, you can see the root parameter (if any) that was passed to the kernel in /proc/cmdline. You can see the actual device mounted on / in /proc/mounts.

Answer 3

Given the boot image, you could find internal kernel version string with file command:

%> file /boot/vmlinuz-linux
/boot/vmlinuz-linux: Linux kernel x86 boot executable bzImage, version 3.5.3-1-NAME (X@Y) #1 SMP PREEMPT WEEK MON DAY, RO-rootFS, swap_dev 0x5, Normal VGA