Using User-Mode-Linux in UTCS

Why User-Mode-Linux (UML)
How to run UML in UTCS
UML Networking
Compile Your Own UML Kernel
Project Ideas
More Questions

Why User-Mode-Linux (UML)

• See UML Homepage
• See my presentation (upcoming)

UML can run on any UTCS linux machines (e.g., "publinux" hosts). You can do the following and more without root access:

• Teaching/learning OS
• Doing OS/networking projects
• Kernel hacking (and gdb through your kernel!)
• Building your own network
• Breaking into your own systems

How to run UML in UTCS

	cp /projects/uml/fs/root_fs_utcs .

	/projects/uml/bin/linux ubd0=root_fs_utcs eth0=mcast

You can run multiple instances of UML, in the same or different linux hosts. However, each instance will need its own separate root_fs_utcs file because linux will write to it. (To save some space, see sharing file systems.)

This version of UML is 2.4.19-8um. I compiled it to include netfilter & tcpdump, but to exclude some uncommon modules or those needed root to set up (see config file /projects/uml/config/uml.config-2.4.19). The root file system is a version of Debian 3.0 tailored to UTCS's departmental file system structure. Here are the changes I made: I pre-mounted the host's / onto /mnt/host and created links for /v, /u, /stage, etc., so that you can access your UTCS home directory as well as /lusr (/usr/local). To save more space, it reuses /usr from the host as well. Obviously, you can only access those host files that you have permissions as an ordinary user in UTCS host (even though you are now root in UML). Their uid/gid may be numeric because you haven't had a record in your UML's /etc/passwd file yet.

You may also use the following utilities:

• uml_mconsole: /projects/uml/bin/uml_mconsole
• uml_moo: /projects/uml/bin/uml_moo
• uml_switch: /projects/uml/bin/uml_switch

Note that this instruction and root file system are suitable for use in UTCS only. If you want to run UML in your home computer, you may need a complete root file system and/or a different kernel. You can download those from original UML homepage.

UML Networking

        ifconfig eth0 192.168.1.1 up

	eth0=slirp,FE:FD:01:02:03:04,/projects/uml/bin/slirp

	ifconfig eth0 10.0.2.15 up
	route add default gw 10.0.2.15
	mv /etc/resolv.conf /etc/resolv.conf.local
	cp /mnt/host/etc/resolv.conf /etc/resolv.conf

Compile Your Own UML Kernel

• Download Linux 2.4.19 kernel source (e.g., from kernel.org) to your directory (e.g., ~/uml):

  	lynx -source http://www.kernel.org/pub/linux/kernel/v2.4/linux-2.4.19.tar.bz2 > linux-2.4.19.tar.bz2
  	bzcat linux-2.4.19.tar.bz2 | tar xf -
  

• Download the UML patch uml-patch-2.4.19-8.bz2 from SourceForge:

  	lynx -source http://unc.dl.sourceforge.net/user-mode-linux/uml-patch-2.4.19-8.bz2 > uml-patch-2.4.19-8.bz2
  

  and apply the patch:

  	cd linux-2.4.19
  	bzcat ../uml-patch-2.4.19-8.bz2 | patch -p1
  

• Compile the kernel as you usually would:

  	make xconfig ARCH=um
  	make dep ARCH=um
  	make linux ARCH=um
  

  The result is the binary file "linux" in this directory and you can use it in place of my linux-uml.
  
  
• If you configure to use kernel modules, you can compile with

  	make modules ARCH=um
  

  but you must install them inside UML -- run UML (using the old kernel), login as root, cd to the above UML source directory in the host file system, and do:

  	make modules_install ARCH=um
  

Project Ideas

• Make contribution to the ongoing UML projects.
• Turn uml_router (which is currently a single-segment ethernet switch) into a complex network emulator (e.g., wireless network).
• Add a user-level TCP/IP stack and NAT to uml_router so that we can connect applications in UML space with the real world (UTCS network or the Internet).

More Questions?

Here are a few commonly asked questions about running UML in UTCS:

• Q: there is not enough disk space in UML's file system
  A: UML's root file system has a limited free space -- about 5MB. However, since UML can access your home directory under /u/username, you should put your files there and run your program from there, instead of copying files to UML's root file system. Alternatively, /mnt/host/tmp is the /tmp in your host and it often has as much as 1GB space.

  There is a limitation which you use the host file system: any files you create while being root of UML will have your UTCS uid as the owner. That is, you cannot create files with root owner or chown to root. If the program you run has a problem with this, you may have to work on a file system "local to UML" instead of the host file system. (See below).

• Q: How to add a second file system to UML?
  A: As with the root file system, the second file system is also a file in your linux host. If you don't already have such a file-system file, you can create one with these commands:

  	dd if=/dev/zero bs=1M count=100 > root_fs_2
  	/sbin/mke2fs root_fs_2
  

  This creates a 100MB file-system file root_fs_2. If you want it bigger or smaller, change the number 100 (MB).

  Add an extra command-line option each time you run UML:   ubd1=root_fs_2

  Once UML is running, login as root and mount the file system:

  	mkdir /local
  	mount /dev/ubd/1 /local
  

  Do a "df -k" -- you now have 100MB in /local.

• Q: how do I get another window for login to this UML?
  A: If you have used the older version before, UML will pop up two xterm windows when it starts, but that has been disabled so UML can run in non-X environment. Nonetheless, UML still supports two virtual consoles that you can login (in addition to the main console).

  First you need to find out the device names at your host machine for these two UML virtual consoles. To do that you can login to UML as root and run "dmesg". Find the strings like "Virtual console N assigned device '/dev/pts/X'". Then, in your host machine, run "screen /dev/pts/X" to be connected to the virtual console. You can also do that with a new xterm as: "xterm -e screen /dev/pts/X"

• Q: I got an error like this: "xterm: Can't execvp port-helper: No such file or directory"
  A: port-helper is under /projects/uml/bin, but it needs to be in your PATH. Copy it or make a link.