usermodelinux-howto.txt

linux 内核源代码

  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 then have to copy each file into the
  corresponding directory in the appropriate kernel pool.

  If you don't have the latest kernel pool, you can get the
  corresponding user-mode sources with


       host% cvs co -r v_2_3_x linux




  where 'x' is the version in your pool. Note that you will not get the
  bug fixes and enhancements that have gone into subsequent releases.


  22..22..  CCoommppiilliinngg aanndd iinnssttaalllliinngg kkeerrnneell mmoodduulleess

  UML modules are built in the same way as the native kernel (with the
  exception of the 'ARCH=um' that you always need for UML):


       host% make modules ARCH=um




  Any modules that you want to load into this kernel need to be built in
  the user-mode pool.  Modules from the native kernel won't work.

  You can install them by using ftp or something to copy them into the
  virtual machine and dropping them into /lib/modules/`uname -r`.

  You can also get the kernel build process to install them as follows:

  1. with the kernel not booted, mount the root filesystem in the top
     level of the kernel pool:


       host% mount root_fs mnt -o loop






  2. run


       host%
       make modules_install INSTALL_MOD_PATH=`pwd`/mnt ARCH=um






  3. unmount the filesystem


       host% umount mnt






  4. boot the kernel on it


  When the system is booted, you can use insmod as usual to get the
  modules into the kernel.  A number of things have been loaded into UML
  as modules, especially filesystems and network protocols and filters,
  so most symbols which need to be exported probably already are.
  However, if you do find symbols that need exporting, let  us
  <http://user-mode-linux.sourceforge.net/contacts.html>  know, and
  they'll be "taken care of".



  22..33..  CCoommppiilliinngg aanndd iinnssttaalllliinngg uummll__uuttiilliittiieess

  Many features of the UML kernel require a user-space helper program,
  so a uml_utilities package is distributed separately from the kernel
  patch which provides these helpers. Included within this is:

  +o  port-helper - Used by consoles which connect to xterms or ports

  +o  tunctl - Configuration tool to create and delete tap devices

  +o  uml_net - Setuid binary for automatic tap device configuration

  +o  uml_switch - User-space virtual switch required for daemon
     transport

     The uml_utilities tree is compiled with:


       host#
       make && make install




  Note that UML kernel patches may require a specific version of the
  uml_utilities distribution. If you don't keep up with the mailing
  lists, ensure that you have the latest release of uml_utilities if you
  are experiencing problems with your UML kernel, particularly when
  dealing with consoles or command-line switches to the helper programs








  33..  RRuunnnniinngg UUMMLL aanndd llooggggiinngg iinn



  33..11..  RRuunnnniinngg UUMMLL

  It runs on 2.2.15 or later, and all 2.4 kernels.


  Booting UML is straightforward.  Simply run 'linux': it will try to
  mount the file `root_fs' in the current directory.  You do not need to
  run it as root.  If your root filesystem is not named `root_fs', then
  you need to put a `ubd0=root_fs_whatever' switch on the linux command
  line.


  You will need a filesystem to boot UML from.  There are a number
  available for download from  here  <http://user-mode-
  linux.sourceforge.net/dl-sf.html> .  There are also  several tools
  <http://user-mode-linux.sourceforge.net/fs_making.html>  which can be
  used to generate UML-compatible filesystem images from media.
  The kernel will boot up and present you with a login prompt.


  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.



  33..22..  LLooggggiinngg iinn



  The prepackaged filesystems have a root account with password 'root'
  and a user account with password 'user'.  The login banner will
  generally tell you how to log in.  So, you log in and you will find