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Sachin Patil

Sachin Patil

Free Software Developer | GNU Emacs Hacker

Guess what?

User mode Linux (UML)
Published on Apr 28, 2014 by Sachin.

UML is a port of linux to linux as referred by Jeff Dike, the man behind UML. It is a virtual machine which runs on linux and used to debug new kernel builds and kernel modules. In this post, I will show how to compile, and run UML

Compile

Download latest kernel source from kernel.org, as of this writing, the kernel version was 3.14.2 

wget -c https://www.kernel.org/pub/linux/kernel/v3.x/linux-3.14.2.tar.xz

Un-compress and visit the directory 

1: tar -xvJf linux-3.14.2.tar.xz
2: cd linux-3.14.2

Clean previous configs, blobs using 

make mrproper; make mrproper ARCH=um; make clean

Load default host config 

make defconfig ARCH=um

If you don’t start with a defconfig, then the kernel build will be that of the host(it will find a config file in /boot), which is not appropriate for UML and will produce a UML that lacks vital drivers and won’t boot. 

make menuconfig ARCH=um

Select host processor type and features

Linux kernel menuconfig

Figure 1: Linux kernel menuconfig

Save and exit menuconfig window.

Finally, compile the kernel 

make linux ARCH=um

The result will be 2 binaries, vmlinuz & linux

Run

Boot your newly compiled kernel using 

./linux

This should die with a message something like 

Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(98,0)

UML kernel panic

Figure 2: UML kernel panic

  • Boot with rootfs
    • Now that the kernel was unable to find roofs, you can download rootfs from http://fs.devloop.org.uk/

    • Add a parameter ubda followed by your rootfs path. Optionally, you may also add a mem parameter to specify amount of RAM your UML should use. 

      ./linux ubda=/path/to/rootFS mem=128M

UML: Boot with rootfs

Figure 3: UML: Boot with rootfs

UML: login prompt

Figure 4: UML: login prompt