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User Mode Linux HOWTO User Mode Linux Core Team Mon Nov 18 14:16:16 EST 2002 This document describes the use and abuse of Jeff Dike's User Mode Linux: a port of the Linux kernel as a normal Intel Linux process. ______________________________________________________________________ Table of Contents 1. Introduction 1.1 How is User Mode Linux Different? 1.2 Why Would I Want User Mode Linux? 2. Compiling the kernel and modules 2.1 Compiling the kernel 2.2 Compiling and installing kernel modules 2.3 Compiling and installing uml_utilities 3. Running UML and logging in 3.1 Running UML 3.2 Logging in 3.3 Examples 4. UML on 2G/2G hosts 4.1 Introduction 4.2 The problem 4.3 The solution 5. Setting up serial lines and consoles 5.1 Specifying the device 5.2 Specifying the channel 5.3 Examples 6. Setting up the network 6.1 General setup 6.2 Userspace daemons 6.3 Specifying ethernet addresses 6.4 UML interface setup 6.5 Multicast 6.6 TUN/TAP with the uml_net helper 6.7 TUN/TAP with a preconfigured tap device 6.8 Ethertap 6.9 The switch daemon 6.10 Slip 6.11 Slirp 6.12 pcap 6.13 Setting up the host yourself 7. Sharing Filesystems between Virtual Machines 7.1 A warning 7.2 Using layered block devices 7.3 Note! 7.4 Another warning 7.5 uml_moo : Merging a COW file with its backing file 8. Creating filesystems 8.1 Create the filesystem file 8.2 Assign the file to a UML device 8.3 Creating and mounting the filesystem 9. Host file access 9.1 Using hostfs 9.2 hostfs as the root filesystem 9.3 Building hostfs 10. The Management Console 10.1 version 10.2 halt and reboot 10.3 config 10.4 remove 10.5 sysrq 10.6 help 10.7 cad 10.8 stop 10.9 go 11. Kernel debugging 11.1 Starting the kernel under gdb 11.2 Examining sleeping processes 11.3 Running ddd on UML 11.4 Debugging modules 11.5 Attaching gdb to the kernel 11.6 Using alternate debuggers 12. Kernel debugging examples 12.1 The case of the hung fsck 12.2 Episode 2: The case of the hung fsck 13. What to do when UML doesn't work 13.1 Strange compilation errors when you build from source 13.2 (obsolete) 13.3 A variety of panics and hangs with /tmp on a reiserfs filesystem 13.4 The compile fails with errors about conflicting types for 'open', 'dup', and 'waitpid' 13.5 UML doesn't work when /tmp is an NFS filesystem 13.6 UML hangs on boot when compiled with gprof support 13.7 syslogd dies with a SIGTERM on startup 13.8 TUN/TAP networking doesn't work on a 2.4 host 13.9 You can network to the host but not to other machines on the net 13.10 I have no root and I want to scream 13.11 UML build conflict between ptrace.h and ucontext.h 13.12 The UML BogoMips is exactly half the host's BogoMips 13.13 When you run UML, it immediately segfaults 13.14 xterms appear, then immediately disappear 13.15 Any other panic, hang, or strange behavior 14. Diagnosing Problems 14.1 Case 1 : Normal kernel panics 14.2 Case 2 : Tracing thread panics 14.3 Case 3 : Tracing thread panics caused by other threads 14.4 Case 4 : Hangs 15. Thanks 15.1 Code and Documentation 15.2 Flushing out bugs 15.3 Buglets and clean-ups 15.4 Case Studies 15.5 Other contributions ______________________________________________________________________ 11.. IInnttrroodduuccttiioonn Welcome to User Mode Linux. It's going to be fun. 11..11.. HHooww iiss UUsseerr MMooddee LLiinnuuxx DDiiffffeerreenntt?? Normally, the Linux Kernel talks straight to your hardware (video card, keyboard, hard drives, etc), and any programs which run ask the kernel to operate the hardware, like so: +-----------+-----------+----+ | Process 1 | Process 2 | ...| +-----------+-----------+----+ | Linux Kernel | +----------------------------+ | Hardware | +----------------------------+ The User Mode Linux Kernel is different; instead of talking to the hardware, it talks to a `real' Linux kernel (called the `host kernel' from now on), like any other program. Programs can then run inside User-Mode Linux as if they were running under a normal kernel, like so: +----------------+ | Process 2 | ...| +-----------+----------------+ | Process 1 | User-Mode Linux| +----------------------------+ | Linux Kernel | +----------------------------+ | Hardware | +----------------------------+ 11..22.. WWhhyy WWoouulldd II WWaanntt UUsseerr MMooddee LLiinnuuxx?? 1. If User Mode Linux crashes, your host kernel is still fine. 2. You can run a usermode kernel as a non-root user. 3. You can debug the User Mode Linux like any normal process. 4. You can run gprof (profiling) and gcov (coverage testing). 5. You can play with your kernel without breaking things. 6. You can use it as a sandbox for testing new apps. 7. You can try new development kernels safely. 8. You can run different distributions simultaneously. 9. It's extremely fun. 22.. CCoommppiilliinngg tthhee kkeerrnneell aanndd mmoodduulleess 22..11.. CCoommppiilliinngg tthhee kkeerrnneell Compiling the user mode kernel is just like compiling any other kernel. Let's go through the steps, using 2.4.0-prerelease (current as of this writing) as an example: 1. Download the latest UML patch from the download page <http://user-mode-linux.sourceforge.net/dl- sf.html> In this example, the file is uml-patch-2.4.0-prerelease.bz2. 2. Download the matching kernel from your favourite kernel mirror, such as: ftp://ftp.ca.kernel.org/pub/kernel/v2.4/linux-2.4.0-prerelease.tar.bz2 <ftp://ftp.ca.kernel.org/pub/kernel/v2.4/linux-2.4.0-prerelease.tar.bz2> . 3. Make a directory and unpack the kernel into it. host% mkdir ~/uml host% cd ~/uml host% tar -xzvf linux-2.4.0-prerelease.tar.bz2 4. Apply the patch using host% cd ~/uml/linux host% bzcat uml-patch-2.4.0-prerelease.bz2 | patch -p1 5. Run your favorite config; `make xconfig ARCH=um' is the most convenient. `make config ARCH=um' and 'make menuconfig ARCH=um' will work as well. The defaults will give you a useful kernel. If you want to change something, go ahead, it probably won't hurt anything. Note: If the host is configured with a 2G/2G address space split rather than the usual 3G/1G split, then the packaged UML binaries will not run. They will immediately segfault. See ``UML on 2G/2G hosts'' for the scoop on running UML on your system. 6. Finish with `make linux ARCH=um': the result is a file called `linux' in the top directory of your source tree. Make sure that you don't build this kernel in /usr/src/linux. On some distributions, /usr/include/asm is a link into this pool. The user- mode build changes the other end of that link, and things that include <asm/anything.h> stop compiling. The sources are also available from cvs at the project's cvs page, which has directions on getting the sources. You can also browse the CVS pool from there. If you get the CVS sources, you will have to check them out into an empty directory. You will th