Val's User-Mode Linux tips

There is a summary of how to get, compile, and run UML at the end of this page, but I strongly suggest you read the whole thing first. It will save you a lot of time later.

UML Tips

• You don't need kernel patches any more. Any fairly current kernel has UML included, and it usually compiles and runs just fine.

• You will need a decent .config file. Here's .config file I used for doing file system development.

  UML .config for fs development

  This has, among other things, the setting to require synchronous I/O to the file containing the file system image. This is a requirement for doing file system development, otherwise it is like having a disk that lies continually about whether it has written your data.

• Don't forget to always always always run compile commands with ARCH=um. For example:

  $ make menuconfig ARCH=um
  $ make linux ARCH=um
  $ make clean ARCH=um
  

• If you accidentally run a command without ARCH=um, this will build a .config or the kernel or set up the headers for the wrong ARCH. To fix this, you'll have to do a:

  $ make mrproper
  

  If you get tired of forgetting this, you can edit the ARCH= line in the Makefile, but that produces unsightly extra bits when you make patches.

• The root file system available on the UML web site didn't quite work for me, due to issues with /dev/udb0 becoming /dev/udb/0 in 2.6 and various other reasons. Here is an edited version of the Slackware-based root file system that worked for me.

  UML root fs for 2.6

  Download this file and unzip it. It is an image of an ext2 file system, mountable as a block device from UML.

• To actually use this file system image, you will need to tell UML where it is located when it boots. This seems relatively simple on first sight:

  $ ./linux ubd0=/path/to/unzipped/root_fs_val
  

  However, you will find yourself continually mistyping this by transposing the 'b' and the 'd' in the "ubd" part. UML helpfully prints out a little message saying that you have probably mistyped this, which promptly scrolls up and off the screen, leaving you to wonder why your root file system can't be mounted. If you say "UML Block Device" out loud several times, you will remember which way to type this. (I would prefer UML just recognized both forms).

• Another excellent way to fail to mount your root file system is to only partially kill off a running UML instance, leaving several threads still off in the background doing something inscrutable. These threads still have the file containing your file system image open, and UML refuses to run unless it can get an exclusive lock on this file. I usually fix the problem thus:

  $ pkill -9 '^linux$'
  

  Be careful if you are running any other process on the system named linux; this command will kill that process too.

• One problem I haven't fixed yet is that if fsck wants to drop into manual mode, I can't do it. I supect the problem is the lack of password on this file system image; just hitting enter won't put you into a shell. If you figure out how to fix this, let me know. In the mean time, I fix this by setting up the image on the host as a loopback device and fscking that:

  $ sudo losetup -f /path/to/unzipped/root_val_fs
  $ sudo fsck /dev/loop0
  $ sudo losetup -d /dev/loop0
  

• If you want to edit the file system image on the host:

  $ sudo mount -o loop /path/to/unzipped/root_val_fs /mnt
  [edit files]
  $ sudo umount /mnt
  

• I still haven't fully figured out how to use UML under gdb, but this seems to work pretty well most of the time. I put the following commands into a file named gdb_commands:

  set args ubd0=/path/to/unzipped/root_fs_val
  handle SIGUSR1 nostop noprint
  

  Then run gdb as follows:

  $ gdb -x ../gdb_commands ./linux
  [wait for prompt, then type "run"]
  

Running UML in a nutshell

• Download the latest kernel from http://www.kernel.org/ - I suggest 2.6.16 since that matches the .config file.

• Unpack the kernel. Copy this .config file into the top-level directory.

• Download this file system image and uncompress it.

• Cd into the top-level directory and run:

  $ make oldconfig ARCH=um
  $ make linux ARCH=um
  $ ./linux ubd0=/path/to/unzipped/root_fs_val
  [wait for prompt, then login as "root", no password]