UserModeLinux-HOWTO.txt 135 KB
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  User Mode Linux HOWTO
  User Mode Linux Core Team
  Mon Jan 30 16:52:10 EST 2006

  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 What is User Mode Linux?
     1.2 How is User Mode Linux Different?
     1.3 How does UML Work?
     1.4 Why Would I Want UML?

  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 Moving a backing file
     7.6 uml_moo : Merging a COW file with its backing file
     7.7 uml_mkcow : Create a new COW 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 command line options
     9.3 hostfs as the root filesystem
     9.4 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
     10.10 log
     10.11 proc
     10.12 Making online backups
     10.13 Event notification

  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 UML hangs on boot after mounting devfs
     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 cannot set up thread-local storage
     13.16 Process segfaults with a modern (NPTL-using) filesystem
     13.17 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


  ______________________________________________________________________

  1.  Introduction

  Welcome to User Mode Linux.  It's going to be fun.


  1.1.  What is User Mode Linux?

  User Mode Linux lets you run Linux inside itself! With that comes the
  power to do all sorts of new things. It virtualises (or simulates, as
  some people call it) Linux so that you can run an entire Linux where
  once you would have only run a program.


  You might have heard of functionality like this before.  There are
  quite a few projects whose goal is to nest operating systems in one
  way or another: Linux on Linux, Windows on Linux, Linux on Windows,
  Linux/s390 on Linux/anythingelse, and so on. Or even just x86 on
  anything, where the 'x86' program can boot operating systems including
  Linux.


  Where x86 is involved there is the greatest concentration of efforts.
  At the end of this HOWTO you'll find a list of alternative projects.
  If all you want to do is run a copy of x86 Linux on another copy of
  x86 Linux as fast as possible with little control then quite possibly
  one of these other projects will so better than UML.


  1.2.  How is User Mode Linux Different?

  User Mode Linux (UML) is rather different from every other Linux
  virtualisation project, either free or commercial. UML strives to
  present itself as an ordinary program as much as possible. Here are
  some of the outcomes of that philosophy:



  1. Good speed with few compromises. UML compiles to native machine
     code that runs just like any other compiled application on the
     host. This makes it very much faster than portable virtualisation
     schemes that implement an entire hardware architecture in software.
     On the other hand, UML does not suffer from the extreme hardware
     specificity of virtualisation systems that rely on particular CPU
     features. UML runs applications inside itself with normally at
     worst a 20% slowdown compared to the host system, which modern
     hardware and clever system design can render negligable in real
     terms.

  2. Futureproof. Every time Linux gets improved so it can do something
     new and clever that benefits the programs it runs, UML
     automatically gets that facility.  Software suspend, fine-grained
     security control such as SE Linux, new filesystem features, support
     for bigger/faster hardware... the same is not true with those
     virtualisation systems that require major changes on the host
     computer.

  3. Flexible code. Normally an OS kernel is just that... a kernel. It
     talks to hardware or maybe some virtualised hardware. But UML can
     be viewed in many other ways. It would be possible to turn it into
     a shared library, for example, so that other programs could link to
     it to take advantage of things that Linux does very well. It can be
     started as a subshell of an existing application. It can use
     stin/stdout like any other program.

  4. Portable. Really portable. UML has only just started to be
     exploited for its portability, but there is promising evidence that
     ports to x86 Windows, PowerPC Linux, x86 BSD and other systems are
     very practical.

  5. Mature. UML has been in development since 1999. One indication of
     its robustness is that UML can be compiled to run within UML,
     making it 'self-hosting'. Production systems are running on UML.



  6. Free Software. UML is under the GPL (as it must be, being part of
     the Linux kernel.)



  1.3.  How does UML Work?

  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 UML 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           |
         +----------------------------+



  1.4.  Why Would I Want UML?


  1. If UML crashes, your host kernel is still fine.

  2. You can run a usermode kernel as a non-root user.

  3. You can debug the UML 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.

  2.  Compiling the kernel and modules



  2.1.  Compiling the kernel


  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: http://ftp.ca.kernel.org/linux/kernel/
     http://ftp.ca.kernel.org/linux/kernel/
     <http://ftp.ca.kernel.org/linux/kernel/> .


  3. Make a directory and unpack the kernel into it.



       host%
       mkdir ~/uml



       host%
       cd ~/uml



       host%
       tar -xjvf 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.

     You may notice that the final binary is pretty large (many 10's of
     megabytes for a debuggable UML).  This is almost entirely symbol
     information.  The actual binary is comparable in size to a native
     kernel.  You can run that huge binary, and only the actual code and
     data will be loaded into memory, so the symbols only consume disk
     space unless you are running UML under gdb.  You can strip UML:


       host% strip linux



  to see the true size of the UML kernel.

  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. You can  browse
  <http://www.user-mode-linux.org/cvs>  the CVS pool or access it
  anonymously via


        cvs -d:pserver:anonymous@www.user-mode-linux.org:/cvsroot/user-mode-linux
       cvs command



  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.


  If you build your own kernel, and want to boot it from one of the
  filesystems distributed from this site, then, in nearly all cases,
  devfs must be compiled into the kernel and mounted at boot time.  The
  exception is the tomsrtbt filesystem.  For this, devfs must either not
  be in the kernel at all, or "devfs=nomount" must be on the kernel
  command line.  Any disagreement between the kernel and the filesystem
  being booted about whether devfs is being used will result in the boot
  getting no further than single-user mode.


  If you don't want to use devfs, you can remove the need for it from a
  filesystem by copying /dev from someplace, making a bunch of /dev/ubd
  devices:


       UML#
       for i in 0 1 2 3 4 5 6 7; do mknod ubd$i b 98 $[ $i * 16 ]; done



  and changing /etc/fstab and /etc/inittab to refer to the non-devfs
  devices.



  2.2.  Compiling and installing kernel modules

  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.  If
  you notice that the modules you get are much larger than they are on
  the host, see the note above about the size of the final UML binary.

  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

  If you can't mount the root filesystem on the host for some reason
  (like it's a COW file), then an alternate approach is to mount the UML
  kernel tree from the host into the UML with  hostfs <http://user-mode-
  linux.sourceforge.net/hostfs.html>  and run the modules_install inside
  UML:

  1. With UML booted, mount the host kernel tree inside UML at the same
     location as on the host:


       UML# mount none -t hostfs path to UML pool -o
       path to UML pool



  2. Run make modules_install:


       UML# cd path to UML pool ; make modules_install



  The depmod at the end may complain about unresolved symbols because
  there is an incorrect or missing System.map installed in the UML
  filesystem.  This appears to be harmless.  insmod or modprobe should
  work fine at this point.



  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".



  If you try building an external module against a UML tree, you will
  find that it doesn't compile because of missing includes.  There are
  less obvious problems with the CFLAGS that the module Makefile or
  script provides which would make it not run even if it did build.  To
  get around this, you need to provide the same CFLAGS that the UML
  kernel build uses.


  A reasonably slick way of getting the UML CFLAGS is



       cd uml-tree ; make script 'SCRIPT=@echo $(CFLAGS)' ARCH=um



  If the module build process has something that looks like


        $(CC) $(CFLAGS) file



  then you can define CFLAGS in a script like this



       CFLAGS=`cd uml-tree ; make script 'SCRIPT=@echo $(CFLAGS)' ARCH=um`



  and like this in a Makefile



       CFLAGS=$(shell cd uml-tree ; make script 'SCRIPT=@echo
       $$(CFLAGS)' ARCH=um)



  2.3.  Compiling and installing uml_utilities

  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



  3.  Running UML and logging in



  3.1.  Running UML

  It runs on 2.2.15 or later, and all 2.4 and 2.6 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.



  3.2.  Logging in



  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
  yourself inside a little virtual machine. Our filesystems have a
  variety of commands and utilities installed (and it is fairly easy to
  add more), so you will have a lot of tools with which to poke around
  the system.

  There are a couple of other ways to log in:

  +o  On a virtual console



     Each virtual console that is configured (i.e. the device exists in
     /dev and /etc/inittab runs a getty on it) will come up in its own
     xterm.  If you get tired of the xterms, read ``Setting up serial
     lines and consoles''  to see how to attach the consoles to
     something else, like host ptys.



  +o  Over the serial line


     In the boot output, find a line that looks like:



       serial line 0 assigned pty /dev/ptyp1



  Attach your favorite terminal program to the corresponding tty.  I.e.
  for minicom, the command would be


       host% minicom -o -p /dev/ttyp1



  +o  Over the net


     If the network is running, then you can telnet to the virtual
     machine and log in to it.  See ``Setting up the network''  to learn
     about setting up a virtual network.


  When you're done using it, run halt, and the kernel will bring itself
  down and the process will exit.


  3.3.  Examples

  Here are some examples of UML in action:

  +o  A login session <http://user-mode-linux.sourceforge.net/login.html>

  +o  A virtual network <http://user-mode-linux.sourceforge.net/net.html>



  4.  UML on 2G/2G hosts



  4.1.  Introduction


  Most Linux machines are configured so that the kernel occupies the
  upper 1G (0xc0000000 - 0xffffffff) of the 4G address space and
  processes use the lower 3G (0x00000000 - 0xbfffffff).  However, some
  machine are configured with a 2G/2G split, with the kernel occupying
  the upper 2G (0x80000000 - 0xffffffff) and processes using the lower
  2G (0x00000000 - 0x7fffffff).



  4.2.  The problem


  The prebuilt UML binaries on this site will not run on 2G/2G hosts
  because UML occupies the upper .5G of the 3G process address space
  (0xa0000000 - 0xbfffffff).  Obviously, on 2G/2G hosts, this is right
  in the middle of the kernel address space, so UML won't even load - it
  will immediately segfault.



  4.3.  The solution


  The fix for this is to rebuild UML from source after enabling
  CONFIG_HOST_2G_2G (under 'General Setup').  This will cause UML to
  load itself in the top .5G of that smaller process address space,
  where it will run fine.  See ``Compiling the kernel and modules''  if
  you need help building UML from source.



  5.  Setting up serial lines and consoles


  It is possible to attach UML serial lines and consoles to many types
  of host I/O channels by specifying them on the command line.


  You can attach them to host ptys, ttys, file descriptors, and ports.
  This allows you to do things like

  +o  have a UML console appear on an unused host console,

  +o  hook two virtual machines together by having one attach to a pty
     and having the other attach to the corresponding tty

  +o  make a virtual machine accessible from the net by attaching a
     console to a port on the host.


  The general format of the command line option is device=channel.



  5.1.  Specifying the device

  Devices are specified with "con" or "ssl" (console or serial line,
  respectively), optionally with a device number if you are talking
  about a specific device.


  Using just "con" or "ssl" describes all of the consoles or serial
  lines.  If you want to talk about console #3 or serial line #10, they
  would be "con3" and "ssl10", respectively.


  A specific device name will override a less general "con=" or "ssl=".
  So, for example, you can assign a pty to each of the serial lines
  except for the first two like this:


        ssl=pty ssl0=tty:/dev/tty0 ssl1=tty:/dev/tty1



  The specificity of the device name is all that matters; order on the
  command line is irrelevant.



  5.2.  Specifying the channel

  There are a number of different types of channels to attach a UML
  device to, each with a different way of specifying exactly what to
  attach to.

  +o  pseudo-terminals - device=pty pts terminals - device=pts


     This will cause UML to allocate a free host pseudo-terminal for the
     device.  The terminal that it got will be announced in the boot
     log.  You access it by attaching a terminal program to the
     corresponding tty:

     +o  screen /dev/pts/n

     +o  screen /dev/ttyxx

     +o  minicom -o -p /dev/ttyxx - minicom seems not able to handle pts
        devices

     +o  kermit - start it up, 'open' the device, then 'connect'



  +o  terminals - device=tty:tty device file


     This will make UML attach the device to the specified tty (i.e


        con1=tty:/dev/tty3



  will attach UML's console 1 to the host's /dev/tty3).  If the tty that
  you specify is the slave end of a tty/pty pair, something else must
  have already opened the corresponding pty in order for this to work.



  +o  xterms - device=xterm


     UML will run an xterm and the device will be attached to it.



  +o  Port - device=port:port number


     This will attach the UML devices to the specified host port.
     Attaching console 1 to the host's port 9000 would be done like
     this:


        con1=port:9000



  Attaching all the serial lines to that port would be done similarly:


        ssl=port:9000



  You access these devices by telnetting to that port.  Each active tel-
  net session gets a different device.  If there are more telnets to a
  port than UML devices attached to it, then the extra telnet sessions
  will block until an existing telnet detaches, or until another device
  becomes active (i.e. by being activated in /etc/inittab).


  This channel has the advantage that you can both attach multiple UML
  devices to it and know how to access them without reading the UML boot
  log.  It is also unique in allowing access to a UML from remote
  machines without requiring that the UML be networked.  This could be
  useful in allowing public access to UMLs because they would be
  accessible from the net, but wouldn't need any kind of network
  filtering or access control because they would have no network access.


  If you attach the main console to a portal, then the UML boot will
  appear to hang.  In reality, it's waiting for a telnet to connect, at
  which point the boot will proceed.



  +o  already-existing file descriptors - device=file descriptor


     If you set up a file descriptor on the UML command line, you can
     attach a UML device to it.  This is most commonly used to put the
     main console back on stdin and stdout after assigning all the other
     consoles to something else:


        con0=fd:0,fd:1 con=pts


  +o  Nothing - device=null


     This allows the device to be opened, in contrast to 'none', but
     reads will block, and writes will succeed and the data will be
     thrown out.



  +o  None - device=none


     This causes the device to disappear.  If you are using devfs, the
     device will not appear in /dev.  If not, then attempts to open it
     will return -ENODEV.



  You can also specify different input and output channels for a device
  by putting a comma between them:


        ssl3=tty:/dev/tty2,xterm



  will cause serial line 3 to accept input on the host's /dev/tty3 and
  display output on an xterm.  That's a silly example - the most common
  use of this syntax is to reattach the main console to stdin and stdout
  as shown above.


  If you decide to move the main console away from stdin/stdout, the
  initial boot output will appear in the terminal that you're running
  UML in.  However, once the console driver has been officially
  initialized, then the boot output will start appearing wherever you
  specified that console 0 should be.  That device will receive all
  subsequent output.



  5.3.  Examples

  There are a number of interesting things you can do with this
  capability.


  First, this is how you get rid of those bleeding console xterms by
  attaching them to host ptys:


        con=pty con0=fd:0,fd:1



  This will make a UML console take over an unused host virtual console,
  so that when you switch to it, you will see the UML login prompt
  rather than the host login prompt:


        con1=tty:/dev/tty6

  You can attach two virtual machines together with what amounts to a
  serial line as follows:

  Run one UML with a serial line attached to a pty -


        ssl1=pty



  Look at the boot log to see what pty it got (this example will assume
  that it got /dev/ptyp1).

  Boot the other UML with a serial line attached to the corresponding
  tty -


        ssl1=tty:/dev/ttyp1



  Log in, make sure that it has no getty on that serial line, attach a
  terminal program like minicom to it, and you should see the login
  prompt of the other virtual machine.



  6.  Setting up the network



  This page describes how to set up the various transports and to
  provide a UML instance with network access to the host, other machines
  on the local net, and the rest of the net.


  As of 2.4.5, UML networking has been completely redone to make it much
  easier to set up, fix bugs, and add new features.


  There is a new helper, uml_net, which does the host setup that
  requires root privileges.


  There are currently five transport types available for a UML virtual
  machine to exchange packets with other hosts:

  +o  ethertap

  +o  TUN/TAP

  +o  Multicast

  +o  a switch daemon

  +o  slip

  +o  slirp


  +o  pcap

     The TUN/TAP, ethertap, slip, and slirp transports allow a UML
     instance to exchange packets with the host.  They may be directed
     to the host or the host may just act as a router to provide access
     to other physical or virtual machines.


  The pcap transport is a synthetic read-only interface, using the
  libpcap binary to collect packets from interfaces on the host and
  filter them.  This is useful for building preconfigured traffic
  monitors or sniffers.


  The daemon and multicast transports provide a completely virtual
  network to other virtual machines.  This network is completely
  disconnected from the physical network unless one of the virtual
  machines on it is acting as a gateway.


  With so many host transports, which one should you use?  Here's when
  you should use each one:

  +o  ethertap - if you want access to the host networking and it is
     running 2.2

  +o  TUN/TAP - if you want access to the host networking and it is
     running 2.4.  Also, the TUN/TAP transport is able to use a
     preconfigured device, allowing it to avoid using the setuid uml_net
     helper, which is a security advantage.

  +o  Multicast - if you want a purely virtual network and you don't want
     to set up anything but the UML

  +o  a switch daemon - if you want a purely virtual network and you
     don't mind running the daemon in order to get somewhat better
     performance

  +o  slip - there is no particular reason to run the slip backend unless
     ethertap and TUN/TAP are just not available for some reason

  +o  slirp - if you don't have root access on the host to setup
     networking, or if you don't want to allocate an IP to your UML

  +o  pcap - not much use for actual network connectivity, but great for
     monitoring traffic on the host

     Ethertap is available on 2.4 and works fine.  TUN/TAP is preferred
     to it because it has better performance and ethertap is officially
     considered obsolete in 2.4.  Also, the root helper only needs to
     run occasionally for TUN/TAP, rather than handling every packet, as
     it does with ethertap.  This is a slight security advantage since
     it provides fewer opportunities for a nasty UML user to somehow
     exploit the helper's root privileges.


  6.1.  General setup

  First, you must have the virtual network enabled in your UML.  If are
  running a prebuilt kernel from this site, everything is already
  enabled.  If you build the kernel yourself, under the "Network device
  support" menu, enable "Network device support", and then the three
  transports.


  The next step is to provide a network device to the virtual machine.
  This is done by describing it on the kernel command line.

  The general format is


       eth <n> = <transport> , <transport args>



  For example, a virtual ethernet device may be attached to a host
  ethertap device as follows:


       eth0=ethertap,tap0,fe:fd:0:0:0:1,192.168.0.254



  This sets up eth0 inside the virtual machine to attach itself to the
  host /dev/tap0, assigns it an ethernet address, and assigns the host
  tap0 interface an IP address.



  Note that the IP address you assign to the host end of the tap device
  must be different than the IP you assign to the eth device inside UML.
  If you are short on IPs and don't want to comsume two per UML, then
  you can reuse the host's eth IP address for the host ends of the tap
  devices.  Internally, the UMLs must still get unique IPs for their eth
  devices.  You can also give the UMLs non-routable IPs (192.168.x.x or
  10.x.x.x) and have the host masquerade them.  This will let outgoing
  connections work, but incoming connections won't without more work,
  such as port forwarding from the host.


  Also note that when you configure the host side of an interface, it is
  only acting as a gateway.  It will respond to pings sent to it
  locally, but is not useful to do that since it's a host interface.
  You are not talking to the UML when you ping that interface and get a
  response.


  You can also add devices to a UML and remove them at runtime.  See the
  ``The Management Console''  page for details.


  The sections below describe this in more detail.


  Once you've decided how you're going to set up the devices, you boot
  UML, log in, configure the UML side of the devices, and set up routes
  to the outside world.  At that point, you will be able to talk to any
  other machines, physical or virtual, on the net.


  If ifconfig inside UML fails and the network refuses to come up, run
  tell you what went wrong.



  6.2.  Userspace daemons

  You will likely need the setuid helper, or the switch daemon, or both.
  They are both installed with the RPM and deb, so if you've installed
  either, you can skip the rest of this section.
  If not, then you need to check them out of CVS, build them, and
  install them.  The helper is uml_net, in CVS /tools/uml_net, and the
  daemon is uml_switch, in CVS /tools/uml_router.  They are both built
  with a plain 'make'.  Both need to be installed in a directory that's
  in your path - /usr/bin is recommend.  On top of that, uml_net needs
  to be setuid root.



  6.3.  Specifying ethernet addresses

  Below, you will see that the TUN/TAP, ethertap, and daemon interfaces
  allow you to specify hardware addresses for the virtual ethernet
  devices.  This is generally not necessary.  If you don't have a
  specific reason to do it, you probably shouldn't.  If one is not
  specified on the command line, the driver will assign one based on the
  device IP address.  It will provide the address fe:fd:nn:nn:nn:nn
  where nn.nn.nn.nn is the device IP address.  This is nearly always
  sufficient to guarantee a unique hardware address for the device.  A
  couple of exceptions are:

  +o  Another set of virtual ethernet devices are on the same network and
     they are assigned hardware addresses using a different scheme which
     may conflict with the UML IP address-based scheme

  +o  You aren't going to use the device for IP networking, so you don't
     assign the device an IP address

     If you let the driver provide the hardware address, you should make
     sure that the device IP address is known before the interface is
     brought up.  So, inside UML, this will guarantee that:


       UML#
       ifconfig eth0 192.168.0.250 up



  If you decide to assign the hardware address yourself, make sure that
  the first byte of the address is even.  Addresses with an odd first
  byte are broadcast addresses, which you don't want assigned to a
  device.



  6.4.  UML interface setup

  Once the network devices have been described on the command line, you
  should boot UML and log in.


  The first thing to do is bring the interface up:


       UML# ifconfig ethn ip-address up



  You should be able to ping the host at this point.


  To reach the rest of the world, you should set a default route to the
  host:

  UML# route add default gw host ip



  Again, with host ip of 192.168.0.4:


       UML# route add default gw 192.168.0.4



  This page used to recommend setting a network route to your local net.
  This is wrong, because it will cause UML to try to figure out hardware
  addresses of the local machines by arping on the interface to the
  host.  Since that interface is basically a single strand of ethernet
  with two nodes on it (UML and the host) and arp requests don't cross
  networks, they will fail to elicit any responses.  So, what you want
  is for UML to just blindly throw all packets at the host and let it
  figure out what to do with them, which is what leaving out the network
  route and adding the default route does.


  Note: If you can't communicate with other hosts on your physical
  ethernet, it's probably because of a network route that's
  automatically set up.  If you run 'route -n' and see a route that
  looks like this:


       Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
       192.168.0.0     0.0.0.0         255.255.255.0   U     0      0      0   eth0



  with a mask that's not 255.255.255.255, then replace it with a route
  to your host:


       UML#
       route del -net 192.168.0.0 dev eth0 netmask 255.255.255.0



       UML#
       route add -host 192.168.0.4 dev eth0



  This, plus the default route to the host, will allow UML to exchange
  packets with any machine on your ethernet.



  6.5.  Multicast

  The simplest way to set up a virtual network between multiple UMLs is
  to use the mcast transport.  This was written by Harald Welte and is
  present in UML version 2.4.5-5um and later.  Your system must have
  multicast enabled in the kernel and there must be a multicast-capable
  network device on the host.  Normally, this is eth0, but if there is
  no ethernet card on the host, then you will likely get strange error
  messages when you bring the device up inside UML.


  To use it, run two UMLs with


        eth0=mcast



  on their command lines.  Log in, configure the ethernet device in each
  machine with different IP addresses:


       UML1# ifconfig eth0 192.168.0.254



       UML2# ifconfig eth0 192.168.0.253



  and they should be able to talk to each other.


  The full set of command line options for this transport are



       ethn=mcast,ethernet address,multicast
       address,multicast port,ttl



  Harald's original README is here <http://user-mode-linux.source-
  forge.net/text/mcast.txt>  and explains these in detail, as well as
  some other issues.



  6.6.  TUN/TAP with the uml_net helper

  TUN/TAP is the preferred mechanism on 2.4 to exchange packets with the
  host.  The TUN/TAP backend has been in UML since 2.4.9-3um.


  The easiest way to get up and running is to let the setuid uml_net
  helper do the host setup for you.  This involves insmod-ing the tun.o
  module if necessary, configuring the device, and setting up IP
  forwarding, routing, and proxy arp.  If you are new to UML networking,
  do this first.  If you're concerned about the security implications of
  the setuid helper, use it to get up and running, then read the next
  section to see how to have UML use a preconfigured tap device, which
  avoids the use of uml_net.


  If you specify an IP address for the host side of the device, the
  uml_net helper will do all necessary setup on the host - the only
  requirement is that TUN/TAP be available, either built in to the host
  kernel or as the tun.o module.

  The format of the command line switch to attach a device to a TUN/TAP
  device is


       eth <n> =tuntap,,, <host IP address>



  For example, this argument will attach the UML's eth0 to the next
  available tap device, assign the IP address 192.168.0.254 to the host
  side of the tap device, and assign an ethernet address to it based on
  the IP address assigned to it by ifconfig inside UML.


       eth0=tuntap,,,192.168.0.254



  If you using the uml_net helper to set up the host side of the
  networking, as in this example, note that changing the UML IP address
  will cause uml_net to change the host routing and arping to match.
  This is one reason you should not be using uml_net if there is any
  possibility that the user inside the UML may be unfriendly.  This
  feature is convenient, but can be used to make the UML pretend to be
  something like your name server or mail server, and the host will
  steal packets intended for those servers and forward them to the UML.
  See the next section for setting up networking in a secure manner.


  There are a couple potential problems with running the TUN/TAP
  transport on a 2.4 host kernel

  +o  TUN/TAP seems not to work on 2.4.3 and earlier.  Upgrade the host
     kernel or use the ethertap transport.

  +o  With an upgraded kernel, TUN/TAP may fail with


       File descriptor in bad state



  This is due to a header mismatch between the upgraded kernel and the
  kernel that was originally installed on the machine.  The fix is to
  make sure that /usr/src/linux points to the headers for the running
  kernel.

  These were pointed out by Tim Robinson <timro at trkr dot net> in
  <http://www.geocrawler.com/lists/3/SourceForge/597/0/> name="this uml-
  user post"> .



  6.7.  TUN/TAP with a preconfigured tap device

  If you prefer not to have UML use uml_net (which is somewhat
  insecure), with UML 2.4.17-11, you can set up a TUN/TAP device
  beforehand.  The setup needs to be done as root, but once that's done,
  there is no need for root assistance.  Setting up the device is done
  as follows:

  +o  Create the device with tunctl (available from the UML utilities
     tarball)



       host#  tunctl -u uid



  where uid is the user id or username that UML will be run as.  This
  will tell you what device was created.

  +o  Configure the device IP (change IP addresses and device name to
     suit)



       host#  ifconfig tap0 192.168.0.254 up



  +o  Set up routing and arping if desired - this is my recipe, there are
     other ways of doing the same thing


       host#
       bash -c 'echo 1 > /proc/sys/net/ipv4/ip_forward'



       host#
       route add -host 192.168.0.253 dev tap0



       host#
       bash -c 'echo 1 > /proc/sys/net/ipv4/conf/tap0/proxy_arp'



       host#
       arp -Ds 192.168.0.253 eth0 pub



  Note that this must be done every time the host boots - this configu-
  ration is not stored across host reboots.  So, it's probably a good
  idea to stick it in an rc file.  An even better idea would be a little
  utility which reads the information from a config file and sets up
  devices at boot time.

  +o  Rather than using up two IPs and ARPing for one of them, you can
     also provide direct access to your LAN by the UML by using a
     bridge.


       host#
       brctl addbr br0



       host#
       ifconfig eth0 0.0.0.0 promisc up



       host#
       ifconfig tap0 0.0.0.0 promisc up



       host#
       ifconfig br0 192.168.0.1 netmask 255.255.255.0 up



       host#
       brctl stp br0 off



       host#
       brctl setfd br0 1



       host#
       brctl sethello br0 1



       host#
       brctl addif br0 eth0

       host#
       brctl addif br0 tap0



  Note that 'br0' should be setup using ifconfig with the existing IP
  address of eth0, as eth0 no longer has its own IP.

  +o


     Also, the /dev/net/tun device must be writable by the user running
     UML in order for the UML to use the device that's been configured
     for it.  The simplest thing to do is


       host#  chmod 666 /dev/net/tun



  Making it world-writeable looks bad, but it seems not to be
  exploitable as a security hole.  However, it does allow anyone to cre-
  ate useless tap devices (useless because they can't configure them),
  which is a DOS attack.  A somewhat more secure alternative would to be
  to create a group containing all the users who have preconfigured tap
  devices and chgrp /dev/net/tun to that group with mode 664 or 660.


  +o  Once the device is set up, run UML with


        eth0=tuntap,devicename



  i.e.


        eth0=tuntap,tap0



  on the command line (or do it with the mconsole config command).

  +o  Bring the eth device up in UML and you're in business.

     If you don't want that tap device any more, you can make it non-
     persistent with


       host#  tunctl -d tap device



  Finally, tunctl has a -b (for brief mode) switch which causes it to
  output only the name of the tap device it created.  This makes it
  suitable for capture by a script:


       host#  TAP=`tunctl -u 1000 -b`



  6.8.  Ethertap

  Ethertap is the general mechanism on 2.2 for userspace processes to
  exchange packets with the kernel.



  To use this transport, you need to describe the virtual network device
  on the UML command line.  The general format for this is


       eth <n> =ethertap, <device> , <ethernet address> , <host IP address>



  So, the previous example


       eth0=ethertap,tap0,fe:fd:0:0:0:1,192.168.0.254



  attaches the UML eth0 device to the host /dev/tap0, assigns it the
  ethernet address fe:fd:0:0:0:1, and assigns the IP address
  192.168.0.254 to the host side of the tap device.



  The tap device is mandatory, but the others are optional.  If the
  ethernet address is omitted, one will be assigned to it.


  The presence of the tap IP address will cause the helper to run and do
  whatever host setup is needed to allow the virtual machine to
  communicate with the outside world.  If you're not sure you know what
  you're doing, this is the way to go.


  If it is absent, then you must configure the tap device and whatever
  arping and routing you will need on the host.  However, even in this
  case, the uml_net helper still needs to be in your path and it must be
  setuid root if you're not running UML as root.  This is because the
  tap device doesn't support SIGIO, which UML needs in order to use
  something as a source of input.  So, the helper is used as a
  convenient asynchronous IO thread.

  If you're using the uml_net helper, you can ignore the following host
  setup - uml_net will do it for you.  You just need to make sure you
  have ethertap available, either built in to the host kernel or
  available as a module.


  If you want to set things up yourself, you need to make sure that the
  appropriate /dev entry exists.  If it doesn't, become root and create
  it as follows (the $[ ... ] is bash syntax for adding 16 to the minor
  number) :


       mknod /dev/tap <minor>  c 36 $[  <minor>  + 16 ]



  For example, this is how to create /dev/tap0:
       mknod /dev/tap0 c 36 $[ 0 + 16 ]



  You also need to make sure that the host kernel has ethertap support.
  If ethertap is enabled as a module, you apparently need to insmod
  ethertap once for each ethertap device you want to enable.  So,


       host#
       insmod ethertap



  will give you the tap0 interface.  To get the tap1 interface, you need
  to run


       host#
       insmod ethertap unit=1 -o ethertap1



  6.9.  The switch daemon

  Note: This is the daemon formerly known as uml_router, but which was
  renamed so the network weenies of the world would stop growling at me.


  The switch daemon, uml_switch, provides a mechanism for creating a
  totally virtual network.  By default, it provides no connection to the
  host network (but see -tap, below).


  The first thing you need to do is run the daemon.  Running it with no
  arguments will make it listen on a default unix domain socket.


  If you want it to listen on a different socket, use


        -unix socket



  If you want it to act as a hub rather than a switch, use


        -hub



  If you're planning on putting it in hub mode so you can sniff UML
  traffic from a tap device on the host, it appears that you need to
  assign the tap an IP address before you'll see any packets on it.


  If you want the switch to be connected to host networking (allowing
  the umls to get access to the outside world through the host), use


        -tap tap0



  Note that the tap device must be preconfigured (see "TUN/TAP with a
  preconfigured tap device", above).  If you're using a different tap
  device than tap0, specify that instead of tap0.


  uml_switch can be backgrounded as follows


       host%
       uml_switch [ options ] < /dev/null > /dev/null



  The reason it doesn't background by default is that it listens to
  stdin for EOF.  When it sees that, it exits.


  The general format of the kernel command line switch is



       ethn=daemon,ethernet address,socket type,socket



  You can leave off everything except the 'daemon'.  You only need to
  specify the ethernet address if the one that will be assigned to it
  isn't acceptable for some reason.  The rest of the arguments describe
  how to communicate with the daemon.  You should only specify them if
  you told the daemon to use different sockets than the default.  So, if
  you ran the daemon with no arguments, running the UML on the same
  machine with



       eth0=daemon



  will cause the eth0 driver to attach itself to the daemon correctly.
  The socket argument is the filename of a Unix domain socket which is
  used for communications between uml_switch and the UMLs on its net-
  work.  If you do specify a different socket from the default, which
  you will need to do if you want multiple, separate uml_switch networks
  on the host, you need to make sure that you name the same path for the
  socket on both the uml_switch and UML command lines.


  Currently the only supported value for the socket type is "unix".



  6.10.  Slip

  Slip is another, less general, mechanism for a process to communicate
  with the host networking.  In contrast to the ethertap interface,
  which exchanges ethernet frames with the host and can be used to
  transport any higher-level protocol, it can only be used to transport
  IP.


  The general format of the command line switch is



       ethn=slip,slip IP



  The slip IP argument is the IP address that will be assigned to the
  host end of the slip device.  If it is specified, the helper will run
  and will set up the host so that the virtual machine can reach it and
  the rest of the network.


  There are some oddities with this interface that you should be aware
  of.  You should only specify one slip device on a given virtual
  machine, and its name inside UML will be 'umn', not 'eth0' or whatever
  you specified on the command line.  These problems will be fixed at
  some point.



  6.11.  Slirp

  slirp uses an external program, usually /usr/bin/slirp, to provide IP
  only networking connectivity through the host. This is similar to IP
  masquerading with a firewall, although the translation is performed in
  user-space, rather than by the kernel.  As slirp does not set up any
  interfaces on the host, or changes routing, slirp does not require
  root access or setuid binaries on the host.


  The general format of the command line switch for slirp is:



       ethn=slirp,ethernet address,slirp path



  The ethernet address is optional, as UML will set up the interface
  with an ethernet address based upon the initial IP address of the
  interface.  The slirp path is generally /usr/bin/slirp, although it
  will depend on distribution.


  The slirp program can have a number of options passed to the command
  line and we can't add them to the UML command line, as they will be
  parsed incorrectly.  Instead, a wrapper shell script can be written or
  the options inserted into the  /.slirprc file.  More information on
  all of the slirp options can be found in its man pages.


  The eth0 interface on UML should be set up with the IP 10.2.0.15,
  although you can use anything as long as it is not used by a network
  you will be connecting to. The default route on UML should be set to
  use


       UML#
       route add default dev eth0



  slirp provides a number of useful IP addresses which can be used by
  UML, such as 10.0.2.3 which is an alias for the DNS server specified
  in /etc/resolv.conf on the host or the IP given in the 'dns' option
  for slirp.


  Even with a baudrate setting higher than 115200, the slirp connection
  is limited to 115200. If you need it to go faster, the slirp binary
  needs to be compiled with FULL_BOLT defined in config.h.



  6.12.  pcap

  The pcap transport is attached to a UML ethernet device on the command
  line or with uml_mconsole with the following syntax:



       ethn=pcap,host interface,filter
       expression,option1,option2



  The expression and options are optional.


  The interface is whatever network device on the host you want to
  sniff.  The expression is a pcap filter expression, which is also what
  tcpdump uses, so if you know how to specify tcpdump filters, you will
  use the same expressions here.  The options are up to two of
  'promisc', control whether pcap puts the host interface into
  promiscuous mode. 'optimize' and 'nooptimize' control whether the pcap
  expression optimizer is used.


  Example:



       eth0=pcap,eth0,tcp

       eth1=pcap,eth0,!tcp



  will cause the UML eth0 to emit all tcp packets on the host eth0 and
  the UML eth1 to emit all non-tcp packets on the host eth0.



  6.13.  Setting up the host yourself

  If you don't specify an address for the host side of the ethertap or
  slip device, UML won't do any setup on the host.  So this is what is
  needed to get things working (the examples use a host-side IP of
  192.168.0.251 and a UML-side IP of 192.168.0.250 - adjust to suit your
  own network):

  +o  The device needs to be configured with its IP address.  Tap devices
     are also configured with an mtu of 1484.  Slip devices are
     configured with a point-to-point address pointing at the UML ip
     address.


       host#  ifconfig tap0 arp mtu 1484 192.168.0.251 up



       host#
       ifconfig sl0 192.168.0.251 pointopoint 192.168.0.250 up



  +o  If a tap device is being set up, a route is set to the UML IP.


       UML# route add -host 192.168.0.250 gw 192.168.0.251



  +o  To allow other hosts on your network to see the virtual machine,
     proxy arp is set up for it.


       host#  arp -Ds 192.168.0.250 eth0 pub



  +o  Finally, the host is set up to route packets.


       host#  echo 1 > /proc/sys/net/ipv4/ip_forward



  7.  Sharing Filesystems between Virtual Machines



  7.1.  A warning

  Don't attempt to share filesystems simply by booting two UMLs from the
  same file.  That's the same thing as booting two physical machines
  from a shared disk.  It will result in filesystem corruption.



  7.2.  Using layered block devices

  The way to share a filesystem between two virtual machines is to use
  the copy-on-write (COW) layering capability of the ubd block driver.
  As of 2.4.6-2um, the driver supports layering a read-write private
  device over a read-only shared device.  A machine's writes are stored
  in the private device, while reads come from either device - the
  private one if the requested block is valid in it, the shared one if
  not.  Using this scheme, the majority of data which is unchanged is
  shared between an arbitrary number of virtual machines, each of which
  has a much smaller file containing the changes that it has made.  With
  a large number of UMLs booting from a large root filesystem, this
  leads to a huge disk space saving.  It will also help performance,
  since the host will be able to cache the shared data using a much
  smaller amount of memory, so UML disk requests will be served from the
  host's memory rather than its disks.



  To add a copy-on-write layer to an existing block device file, simply
  add the name of the COW file to the appropriate ubd switch:


        ubd0=root_fs_cow,root_fs_debian_22



  where 'root_fs_cow' is the private COW file and 'root_fs_debian_22' is
  the existing shared filesystem.  The COW file need not exist.  If it
  doesn't, the driver will create and initialize it.  Once the COW file
  has been initialized, it can be used on its own on the command line:


        ubd0=root_fs_cow



  The name of the backing file is stored in the COW file header, so it
  would be redundant to continue specifying it on the command line.



  7.3.  Note!

  When checking the size of the COW file in order to see the gobs of
  space that you're saving, make sure you use 'ls -ls' to see the actual
  disk consumption rather than the length of the file.  The COW file is
  sparse, so the length will be very different from the disk usage.
  Here is a 'ls -l' of a COW file and backing file from one boot and
  shutdown:
       host% ls -l cow.debian debian2.2
       -rw-r--r--    1 jdike    jdike    492504064 Aug  6 21:16 cow.debian
       -rwxrw-rw-    1 jdike    jdike    537919488 Aug  6 20:42 debian2.2



  Doesn't look like much saved space, does it?  Well, here's 'ls -ls':


       host% ls -ls cow.debian debian2.2
          880 -rw-r--r--    1 jdike    jdike    492504064 Aug  6 21:16 cow.debian
       525832 -rwxrw-rw-    1 jdike    jdike    537919488 Aug  6 20:42 debian2.2



  Now, you can see that the COW file has less than a meg of disk, rather
  than 492 meg.



  7.4.  Another warning

  Once a filesystem is being used as a readonly backing file for a COW
  file, do not boot directly from it or modify it in any way.  Doing so
  will invalidate any COW files that are using it.  The mtime and size
  of the backing file are stored in the COW file header at its creation,
  and they must continue to match.  If they don't, the driver will
  refuse to use the COW file.



  If you attempt to evade this restriction by changing either the
  backing file or the COW header by hand, you will get a corrupted
  filesystem.



  Among other things, this means that upgrading the distribution in a
  backing file and expecting that all of the COW files using it will see
  the upgrade will not work.



  7.5.  Moving a backing file

  Because UML stores the backing file name and its mtime in the COW
  header, if you move the backing file, that information becomes
  invalid.  So, the procedure for moving a backing file is

  +o  Move it in a way that preserves timestamps.  Usually, this is a
     "-p" switch.  "cp -a" works because "-a" implies "-p".

  +o  Update the COW header by booting UML on it, specifying both the COW
     file and the new location of the backing file


       host% ubd0=COW file,new backing file
       location



  UML will notice the mismatch between the command line and COW header,
  check the size and mtime of the new backing file path, and update the
  COW header to reflect it if it checks out.

  If you forget to preserve the timestamps when you move the backing
  file, you can fix the mtime by hand as follows


       host%
       mtime=whatever UML says mtime should be ; \
       touch --date="`date -d 1970-01-01\ UTC\ $mtime\ seconds`" backing file



  Note that if you do this on a backing file that has truly been
  changed, and not just moved, then you will get file corruption and you
  will lose the filesystem.



  7.6.  uml_moo : Merging a COW file with its backing file

  Depending on how you use UML and COW devices, it may be advisable to
  merge the changes in the COW file into the backing file every once in
  a while.



  The utility that does this is uml_moo.  Its usage is


       host% uml_moo COW file new backing file



  There's no need to specify the backing file since that information is
  already in the COW file header.  If you're paranoid, boot the new
  merged file, and if you're happy with it, move it over the old backing
  file.



  uml_moo creates a new backing file by default as a safety measure.  It
  also has a destructive merge option which will merge the COW file
  directly into its current backing file.  This is really only usable
  when the backing file only has one COW file associated with it.  If
  there are multiple COWs associated with a backing file, a -d merge of
  one of them will invalidate all of the others.  However, it is
  convenient if you're short of disk space, and it should also be
  noticably faster than a non-destructive merge.  This usage is


       host% uml_moo -d COW file



  uml_moo is installed with the UML deb and RPM.  If you didn't install
  UML from one of those packages, you can also get it from the UML
  utilities <http://user-mode-linux.sourceforge.net/dl-sf.html#UML
  utilities>  tar file in tools/moo.



  7.7.  uml_mkcow : Create a new COW file


  The normal way to create a COW file is to specify a non-existant COW
  file on the UML command line, and let UML create it for you.  However,
  sometimes you want a new COW file, and you don't want to boot UML in
  order to get it.  This can be done with uml_mkcow, which is a little
  standalone utility by Steve Schnepp.


  The standard usage is


       host% uml_mkcow new COW file existing
       backing file



  If you want to destroy an existing COW file, then there is a -f switch
  to force the overwriting of the old COW file


       host% uml_mkcow -f existing COW file existing
       backing file



  uml_mkcow is available from the  UML utilities <http://user-mode-
  linux.sourceforge.net/dl-sf.html#UML utilities>  tar file in
  tools/moo.



  8.  Creating filesystems


  You may want to create and mount new UML filesystems, either because
  your root filesystem isn't large enough or because you want to use a
  filesystem other than ext2.


  This was written on the occasion of reiserfs being included in the
  2.4.1 kernel pool, and therefore the 2.4.1 UML, so the examples will
  talk about reiserfs.  This information is generic, and the examples
  should be easy to translate to the filesystem of your choice.


  8.1.  Create the filesystem file

  dd is your friend.  All you need to do is tell dd to create an empty
  file of the appropriate size.  I usually make it sparse to save time
  and to avoid allocating disk space until it's actually used.  For
  example, the following command will create a sparse 100 meg file full
  of zeroes.


       host%
       dd if=/dev/zero of=new_filesystem seek=100 count=1 bs=1M



  8.2.  Assign the file to a UML device

  Add an argument like the following to the UML command line:



       ubd4=new_filesystem



  making sure that you use an unassigned ubd device number.



  8.3.  Creating and mounting the filesystem

  Make sure that the filesystem is available, either by being built into
  the kernel, or available as a module, then boot up UML and log in.  If
  the root filesystem doesn't have the filesystem utilities (mkfs, fsck,
  etc), then get them into UML by way of the net or hostfs.


  Make the new filesystem on the device assigned to the new file:


       host#  mkreiserfs /dev/ubd/4


       <----------- MKREISERFSv2 ----------->

       ReiserFS version 3.6.25
       Block size 4096 bytes
       Block count 25856
       Used blocks 8212
               Journal - 8192 blocks (18-8209), journal header is in block 8210
               Bitmaps: 17
               Root block 8211
       Hash function "r5"
       ATTENTION: ALL DATA WILL BE LOST ON '/dev/ubd/4'! (y/n)y
       journal size 8192 (from 18)
       Initializing journal - 0%....20%....40%....60%....80%....100%
       Syncing..done.



  Now, mount it:


       UML#
       mount /dev/ubd/4 /mnt



  and you're in business.



  9.  Host file access


  If you want to access files on the host machine from inside UML, you
  can treat it as a separate machine and either nfs mount directories
  from the host or copy files into the virtual machine with scp or rcp.
  However, since UML is running on the the host, it can access those
  files just like any other process and make them available inside the
  virtual machine without needing to use the network.


  This is now possible with the hostfs virtual filesystem.  With it, you
  can mount a host directory into the UML filesystem and access the
  files contained in it just as you would on the host.


  Note that hostfs is currently not available on 2.5.  The reason is
  that there was an fs.h rework early in 2.5 which required filesystem
  changes, and I haven't got around to updating hostfs to those changes.


  9.1.  Using hostfs

  To begin with, make sure that hostfs is available inside the virtual
  machine with


       UML# cat /proc/filesystems



  hostfs should be listed.  If it's not, either rebuild the kernel with
  hostfs configured into it or make sure that hostfs is built as a mod-
  ule and available inside the virtual machine, and insmod it.


  Now all you need to do is run mount:


       UML# mount none /mnt/host -t hostfs



  will mount the host's / on the virtual machine's /mnt/host.


  If you don't want to mount the host root directory, then you can
  specify a subdirectory to mount with the -o switch to mount:


       UML# mount none /mnt/home -t hostfs -o /home



  will mount the hosts's /home on the virtual machine's /mnt/home.



  9.2.  hostfs command line options

  There is a hostfs option available on the UML command line which can
  be used confine all hostfs mounts to a host directory hierarchy or to
  prevent a hostfs user from destroying data on the host.  The format is


        hostfs=directory,options



  The only option available at present is 'append', which forces all
  files to be opened in append mode and disallows any deletion of files.


  To specify append mode without confining hostfs to a host directory,
  just leave out the directory name so that the argument begins with a
  comma:


        hostfs=,append



  9.3.  hostfs as the root filesystem

  It's possible to boot from a directory hierarchy on the host using
  hostfs rather than using the standard filesystem in a file.

  To start, you need that hierarchy.  The easiest way is to loop mount
  an existing root_fs file:


       host#  mount root_fs uml_root_dir -o loop



  You need to change the filesystem type of / in etc/fstab to be
  'hostfs', so that line looks like this:


       none    /       hostfs defaults 1 1



  Then you need to chown to yourself all the files in that directory
  that are owned by root.  This worked for me:


       host#  find . -uid 0 -exec chown jdike {} \;



  If you don't want to do that because that's a filesystem image that
  you boot as a disk, then run UML as root instead.
  Next, make sure that your UML kernel has hostfs compiled in, not as a
  module.  Then run UML with the following arguments added to the
  command line:


        root=/dev/root rootflags=/path/to/uml/root rootfstype=hostfs



  UML should then boot as it does normally.


  9.4.  Building hostfs

  If you need to build hostfs because it's not in your kernel, you have
  two choices:



  +o  Compiling hostfs into the kernel:


     Reconfigure the kernel and set the 'Host filesystem' option under


  +o  Compiling hostfs as a module:


     Reconfigure the kernel and set the 'Host filesystem' option under
     be in arch/um/fs/hostfs/hostfs.o.  Install that in
     /lib/modules/`uname -r`/fs in the virtual machine, boot it up, and


       UML# insmod hostfs



  10.  The Management Console



  The UML management console is a low-level interface to the kernel,
  somewhat like the i386 SysRq interface.  Since there is a full-blown
  operating system under UML, there is much greater flexibility possible
  than with the SysRq mechanism.


  There are a number of things you can do with the mconsole interface:

  +o  get the kernel version

  +o  add and remove devices

  +o  halt or reboot the machine


  +o  send SysRq commands

  +o  pause and resume the UML

  +o  make online backups without shutting down the UML

  +o  receive notifications of events of interest from within UML

  +o  monitor the internal state of the UML


  You need the mconsole client (uml_mconsole) which is present in CVS
  (/tools/mconsole) in 2.4.5-9um and later, and will be in the RPM in
  2.4.6.


  You also need CONFIG_MCONSOLE (under 'General Setup') enabled in UML.
  When you boot UML, you'll see a line like:


       mconsole initialized on /home/jdike/.uml/umlNJ32yL/mconsole



  If you specify a unique machine id one the UML command line, i.e.


        umid=debian



  you'll see this


       mconsole initialized on /home/jdike/.uml/debian/mconsole



  That file is the socket that uml_mconsole will use to communicate with
  UML.  Run it with either the umid or the full path as its argument:


       host% uml_mconsole debian



  or


       host% uml_mconsole /home/jdike/.uml/debian/mconsole



  You'll get a prompt, at which you can run one of these commands:

  +o  version

  +o  halt

  +o  reboot

  +o  config

  +o  remove

  +o  sysrq

  +o  help

  +o  cad

  +o  stop

  +o  go

  +o  log

  +o  proc



  10.1.  version

  This takes no arguments.  It prints the UML version.


       (mconsole)  version
       OK Linux usermode 2.4.5-9um #1 Wed Jun 20 22:47:08 EDT 2001 i686



  There are a couple actual uses for this.  It's a simple no-op which
  can be used to check that a UML is running.  It's also a way of
  sending an interrupt to the UML.  This is sometimes useful on SMP
  hosts, where there's a bug which causes signals to UML to be lost,
  often causing it to appear to hang.  Sending such a UML the mconsole
  version command is a good way to 'wake it up' before networking has
  been enabled, as it does not do anything to the function of the UML.



  10.2.  halt and reboot

  These take no arguments.  They shut the machine down immediately, with
  no syncing of disks and no clean shutdown of userspace.  So, they are
  pretty close to crashing the machine.


       (mconsole)  halt
       OK



  10.3.  config

  "config" adds a new device to the virtual machine or queries the
  configuration of an existing device.


  Currently the ubd and network drivers support pulling devices.  It
  takes one argument, which is the device to add, with the same syntax
  as the kernel command line.

  (mconsole)
  config ubd3=/home/jdike/incoming/roots/root_fs_debian22

  OK
  (mconsole)  config eth1=mcast
  OK



  Querying the configuration of a device is handy when you don't know
  before the boot what host device the UML device will attach to.  This
  is a problem with attaching consoles and serial lines to host pty or
  pts devices.  You have no way of knowing how to access them without
  parsing the kernel messages.  So, the syntax for this is the same as
  above, except you don't specify a configuration


       (mconsole)  config ssl0
       OK pty:/dev/ptyp0
       (mconsole)  config ubd0
       OK /home/jdike/roots/cow.debian,/home/jdike/roots/debian_22



  This is supported by the console, serial line, and ubd drivers.  As
  yet, the network drivers don't support this.



  10.4.  remove

  "remove" deletes a device from the system.  Its argument is just the
  name of the device to be removed. The device must be idle in whatever
  sense the driver considers necessary.  In the case of the ubd driver,
  the removed block device must not be mounted, swapped on, or otherwise
  open, and in the case of the network driver, the device must be down.


       (mconsole)  remove ubd3
       OK
       (mconsole)  remove eth1
       OK



  10.5.  sysrq

  This takes one argument, which is a single letter.  It calls the
  generic kernel's SysRq driver, which does whatever is called for by
  that argument.  See the SysRq documentation in Documentation/sysrq.txt
  in your favorite kernel tree to see what letters are valid and what
  they do.



  10.6.  help

  "help" returns a string listing the valid commands and what each one
  does.


  10.7.  cad

  This invokes the Ctl-Alt-Del action on init.  What exactly this ends
  up doing is up to /etc/inittab.  Normally, it reboots the machine.
  With UML, this is usually not desired, so if a halt would be better,
  then find the section of inittab that looks like this


       # What to do when CTRL-ALT-DEL is pressed.
       ca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -r now



  and change the command to halt.



  10.8.  stop

  This puts the UML in a loop reading mconsole requests until a 'go'
  mconsole command is recieved. This is very useful for making backups
  of UML filesystems, as the UML can be stopped, then synced via 'sysrq
  s', so that everything is written to the filesystem. You can then copy
  the filesystem and then send the UML 'go' via mconsole.


  Note that a UML running with more than one CPU will have problems
  after you send the 'stop' command, as only one CPU will be held in a
  mconsole loop and all others will continue as normal.  This is a bug,
  and will be fixed.



  10.9.  go

  This resumes a UML after being paused by a 'stop' command. Note that
  when the UML has resumed, TCP connections may have timed out and if
  the UML is paused for a long period of time, crond might go a little
  crazy, running all the jobs it didn't do earlier.



  10.10.  log

  This takes a string as its argument, and will cause the UML to printk
  the string so that it ends up in the kernel message log.  This is
  intended for use in honeypots by allowing the UML-specific stuff in
  the kernel log to be replaced with messages that don't expose the
  machine as being a UML.



  10.11.  proc

  This takes a filename as its argument.  It will return the contents of
  the corresponding /proc file inside the UML.  Example:


       (mconsole)  proc uptime



  will return the contents of the UML's /proc/uptime.

  10.12.  Making online backups

  It is possible to make a backup of a UML's data without shutting it
  down.  The idea is to pause it, make it flush out its data, copy the
  filesystem to a safe place, and then resume it.  This should usually
  take seconds, while shutting down and rebooting the UML could take
  minutes.  The exact procedure is this:


       (mconsole)  stop



       (mconsole)  sysrq s



       host% # Copy the UML's filesystem someplace safe



       (mconsole)  go



  By causing UML to flush its data out to disk, the 'sysrq s' will cause
  the filesystem to be a clean image.  Of course, no guarantees are made
  for process data which hadn't been written back to the kernel, but the
  filesystem itself won't need an fsck if it's booted.



  10.13.  Event notification

  The mconsole interface also provides a mechanism for processes inside
  a UML to send messages to an mconsole client on the host.  The
  procedure is this:

  +o  Create a unix socket and pass that to UML on the command line as
     the mconsole notification socket


        mconsole=notify:<bf>socket



  +o  A /proc/mconsole file will be created inside UML

  +o  Anything that is written to it will be turned into an mconsole
     notification which your mconsole client should be listening for on
     the notification socket

     A common use for this mechanism is to have an rc script inside UML
     send a message out that the UML has booted to a certain stage, and
     that something on the host which depends on that can proceed.
     However, this is a completely general mechanism which can be used
     to communicate any information at all to the host.


  There is a demo mconsole notification client in the utilities tarball
  in mconsole/notify.pl.  This is only a demo, and as such, isn't very
  useful by itself.  It should be customized to fit into whatever
  environment you are setting up.



  11.  Kernel debugging


  This page describes kernel debugging with UML running in tt mode (go
  here <http://user-mode-linux.sourceforge.net/skas.html>  for the
  details on skas and tt mode).  Kernel debugging in skas mode is
  described here <http://user-mode-linux.sourceforge.net/debugging-
  skas.html> .


  Since the UML runs as a normal Linux process, it is possible to debug
  it with gdb almost like any other process.  It is slightly different
  because the kernel's threads are already being ptraced for system call
  interception, so gdb can't ptrace them.  However, a mechanism has been
  added to work around that problem.


  In order to debug the kernel, you need build it from source.  See
  ``Compiling the kernel and modules''  for information on doing that.
  Make sure that you enable CONFIG_DEBUGSYM and CONFIG_PT_PROXY during
  the config.  These will compile the kernel with -g, and enable the
  ptrace proxy so that gdb works with UML, respectively.



  11.1.  Starting the kernel under gdb

  You can have the kernel running under the control of gdb from the
  beginning by putting 'debug' on the command line.  You will get an
  xterm with gdb running inside it.  The kernel will send some commands
  to gdb which will leave it stopped at the beginning of start_kernel.
  At this point, you can get things going with 'next', 'step', or
  'cont'.


  There is a transcript of a debugging session  here <debug-
  session.html> , with breakpoints being set in the scheduler and in an
  interrupt handler.



  11.2.  Examining sleeping processes

  Not every bug is evident in the currently running process.  Sometimes,
  processes hang in the kernel when they shouldn't because they've
  deadlocked on a semaphore or something similar.  In this case, when
  you ^C gdb and get a backtrace, you will see the idle thread, which
  isn't very relevant.
  What you want is the stack of whatever process is sleeping when it
  shouldn't be.  You need to figure out which process that is, which is
  generally fairly easy.  Then you need to get its host process id,
  which you can do either by looking at ps on the host or at
  task.thread.extern_pid in gdb.


  Now what you do is this:

  +o  detach from the current thread


       (UML gdb)  det



  +o  attach to the thread you are interested in


       (UML gdb)  att <host pid>



  +o  look at its stack and anything else of interest


       (UML gdb)  bt



  Note that you can't do anything at this point that requires that a
  process execute, e.g. calling a function

  +o  when you're done looking at that process, reattach to the current
     thread and continue it


       (UML gdb)
       att 1



       (UML gdb)
       c



  Here, specifying any pid which is not the process id of a UML thread
  will cause gdb to reattach to the current thread.  I commonly use 1,
  but any other invalid pid would work.



  11.3.  Running ddd on UML

  ddd works on UML, but requires a special kludge.  The process goes
  like this:
  +o  Start ddd


       host% ddd linux



  +o  With ps, get the pid of the gdb that ddd started.  You can ask the
     gdb to tell you, but for some reason that confuses things and
     causes a hang.

  +o  run UML with 'debug=parent gdb-pid=<pid>' added to the command line
     - it will just sit there after you hit return

  +o  type 'att 1' to the ddd gdb and you will see something like


       0xa013dc51 in __kill ()


       (gdb)



  +o  At this point, type 'c', UML will boot up, and you can use ddd just
     as you do on any other process.



  11.4.  Debugging modules

  gdb has support for debugging code which is dynamically loaded into
  the process.  This support is what is needed to debug kernel modules
  under UML.


  Using that support is somewhat complicated.  You have to tell gdb what
  object file you just loaded into UML and where in memory it is.  Then,
  it can read the symbol table, and figure out where all the symbols are
  from the load address that you provided.  It gets more interesting
  when you load the module again (i.e. after an rmmod).  You have to
  tell gdb to forget about all its symbols, including the main UML ones
  for some reason, then load then all back in again.


  There's an easy way and a hard way to do this.  The easy way is to use
  the umlgdb expect script written by Chandan Kudige.  It basically
  automates the process for you.


  First, you must tell it where your modules are.  There is a list in
  the script that looks like this:


       set MODULE_PATHS {
       "fat" "/usr/src/uml/linux-2.4.18/fs/fat/fat.o"
       "isofs" "/usr/src/uml/linux-2.4.18/fs/isofs/isofs.o"
       "minix" "/usr/src/uml/linux-2.4.18/fs/minix/minix.o"
       }



  You change that to list the names and paths of the modules that you
  are going to debug.  Then you run it from the toplevel directory of
  your UML pool and it basically tells you what to do:



                   ******** GDB pid is 21903 ********
       Start UML as: ./linux <kernel switches> debug gdb-pid=21903



       GNU gdb 5.0rh-5 Red Hat Linux 7.1
       Copyright 2001 Free Software Foundation, Inc.
       GDB is free software, covered by the GNU General Public License, and you are
       welcome to change it and/or distribute copies of it under certain conditions.
       Type "show copying" to see the conditions.
       There is absolutely no warranty for GDB.  Type "show warranty" for details.
       This GDB was configured as "i386-redhat-linux"...
       (gdb) b sys_init_module
       Breakpoint 1 at 0xa0011923: file module.c, line 349.
       (gdb) att 1



  After you run UML and it sits there doing nothing, you hit return at
  the 'att 1' and continue it:


       Attaching to program: /home/jdike/linux/2.4/um/./linux, process 1
       0xa00f4221 in __kill ()
       (UML gdb)  c
       Continuing.



  At this point, you debug normally.  When you insmod something, the
  expect magic will kick in and you'll see something like:



   *** Module hostfs loaded ***
  Breakpoint 1, sys_init_module (name_user=0x805abb0 "hostfs",
      mod_user=0x8070e00) at module.c:349
  349             char *name, *n_name, *name_tmp = NULL;
  (UML gdb)  finish
  Run till exit from #0  sys_init_module (name_user=0x805abb0 "hostfs",
      mod_user=0x8070e00) at module.c:349
  0xa00e2e23 in execute_syscall (r=0xa8140284) at syscall_kern.c:411
  411             else res = EXECUTE_SYSCALL(syscall, regs);
  Value returned is $1 = 0
  (UML gdb)
  p/x (int)module_list + module_list->size_of_struct

  $2 = 0xa9021054
  (UML gdb)  symbol-file ./linux
  Load new symbol table from "./linux"? (y or n) y
  Reading symbols from ./linux...
  done.
  (UML gdb)
  add-symbol-file /home/jdike/linux/2.4/um/arch/um/fs/hostfs/hostfs.o 0xa9021054

  add symbol table from file "/home/jdike/linux/2.4/um/arch/um/fs/hostfs/hostfs.o" at
          .text_addr = 0xa9021054
   (y or n) y

  Reading symbols from /home/jdike/linux/2.4/um/arch/um/fs/hostfs/hostfs.o...
  done.
  (UML gdb)  p *module_list
  $1 = {size_of_struct = 84, next = 0xa0178720, name = 0xa9022de0 "hostfs",
    size = 9016, uc = {usecount = {counter = 0}, pad = 0}, flags = 1,
    nsyms = 57, ndeps = 0, syms = 0xa9023170, deps = 0x0, refs = 0x0,
    init = 0xa90221f0 <init_hostfs>, cleanup = 0xa902222c <exit_hostfs>,
    ex_table_start = 0x0, ex_table_end = 0x0, persist_start = 0x0,
    persist_end = 0x0, can_unload = 0, runsize = 0, kallsyms_start = 0x0,
    kallsyms_end = 0x0,
    archdata_start = 0x1b855 <Address 0x1b855 out of bounds>,
    archdata_end = 0xe5890000 <Address 0xe5890000 out of bounds>,
    kernel_data = 0xf689c35d <Address 0xf689c35d out of bounds>}
  >> Finished loading symbols for hostfs ...



  That's the easy way.  It's highly recommended.  The umlgdb script is
  available in the  UML utilities tarball <http://user-mode-
  linux.sourceforge.net/dl-sf#UML utilities>  in tools/umlgdb/umlgdb.
  The hard way is described below in case you're interested in what's
  going on.


  Boot the kernel under the debugger and load the module with insmod or
  modprobe.  With gdb, do:


       (UML gdb)  p module_list



  This is a list of modules that have been loaded into the kernel, with
  the most recently loaded module first.  Normally, the module you want
  is at module_list.  If it's not, walk down the next links, looking at
  the name fields until find the module you want to debug.  Take the
  address of that structure, and add module.size_of_struct (which in
  2.4.10 kernels is 96 (0x60)) to it.  Gdb can make this hard addition
  for you :-):
       (UML gdb)
       printf "%#x\n", (int)module_list module_list->size_of_struct



  The offset from the module start occasionally changes (before 2.4.0,
  it was module.size_of_struct + 4), so it's a good idea to check the
  init and cleanup addresses once in a while, as describe below.  Now
  do:


       (UML gdb)
       add-symbol-file /path/to/module/on/host that_address



  Tell gdb you really want to do it, and you're in business.


  If there's any doubt that you got the offset right, like breakpoints
  appear not to work, or they're appearing in the wrong place, you can
  check it by looking at the module structure.  The init and cleanup
  fields should look like:


       init = 0x588066b0 <init_hostfs>, cleanup = 0x588066c0 <exit_hostfs>



  with no offsets on the symbol names.  If the names are right, but they
  are offset, then the offset tells you how much you need to add to the
  address you gave to add-symbol-file.


  When you want to load in a new version of the module, you need to get
  gdb to forget about the old one.  The only way I've found to do that
  is to tell gdb to forget about all symbols that it knows about:


       (UML gdb)  symbol-file



  Then reload the symbols from the kernel binary:


       (UML gdb)  symbol-file /path/to/kernel



  and repeat the process above.  You'll also need to re-enable break-
  points.  They were disabled when you dumped all the symbols because
  gdb couldn't figure out where they should go.



  11.5.  Attaching gdb to the kernel

  If you don't have the kernel running under gdb, you can attach gdb to
  it later by sending the tracing thread a SIGUSR1.  The first line of
  the console output identifies its pid:
       tracing thread pid = 20093



  When you send it the signal:


       host% kill -USR1 20093



  you will get an xterm with gdb running in it.


  If you have the mconsole compiled into UML, then the mconsole client
  can be used to start gdb:


       (mconsole)  (mconsole) config gdb=xterm



  will fire up an xterm with gdb running in it.



  11.6.  Using alternate debuggers

  UML has support for attaching to an already running debugger rather
  than starting gdb itself.  This is present in CVS as of 17 Apr 2001.
  I sent it to Alan for inclusion in the ac tree, and it will be in my
  2.4.4 release.


  This is useful when gdb is a subprocess of some UI, such as emacs or
  ddd.  It can also be used to run debuggers other than gdb on UML.
  Below is an example of using strace as an alternate debugger.


  To do this, you need to get the pid of the debugger and pass it in
  with the


  If you are using gdb under some UI, then tell it to 'att 1', and
  you'll find yourself attached to UML.


  If you are using something other than gdb as your debugger, then
  you'll need to get it to do the equivalent of 'att 1' if it doesn't do
  it automatically.


  An example of an alternate debugger is strace.  You can strace the
  actual kernel as follows:

  +o  Run the following in a shell


       host%
       sh -c 'echo pid=$$; echo -n hit return; read x; exec strace -p 1 -o strace.out'



  +o  Run UML with 'debug' and 'gdb-pid=<pid>' with the pid printed out
     by the previous command

  +o  Hit return in the shell, and UML will start running, and strace
     output will start accumulating in the output file.

     Note that this is different from running


       host% strace ./linux



  That will strace only the main UML thread, the tracing thread, which
  doesn't do any of the actual kernel work.  It just oversees the vir-
  tual machine.  In contrast, using strace as described above will show
  you the low-level activity of the virtual machine.



  12.  Kernel debugging examples

  12.1.  The case of the hung fsck

  When booting up the kernel, fsck failed, and dropped me into a shell
  to fix things up.  I ran fsck -y, which hung:



  Setting hostname uml                    [ OK ]
  Checking root filesystem
  /dev/fhd0 was not cleanly unmounted, check forced.
  Error reading block 86894 (Attempt to read block from filesystem resulted in short read) while reading indirect blocks of inode 19780.

  /dev/fhd0: UNEXPECTED INCONSISTENCY; RUN fsck MANUALLY.
          (i.e., without -a or -p options)
  [ FAILED ]

  *** An error occurred during the file system check.
  *** Dropping you to a shell; the system will reboot
  *** when you leave the shell.
  Give root password for maintenance
  (or type Control-D for normal startup):

  [root@uml /root]# fsck -y /dev/fhd0
  fsck -y /dev/fhd0
  Parallelizing fsck version 1.14 (9-Jan-1999)
  e2fsck 1.14, 9-Jan-1999 for EXT2 FS 0.5b, 95/08/09
  /dev/fhd0 contains a file system with errors, check forced.
  Pass 1: Checking inodes, blocks, and sizes
  Error reading block 86894 (Attempt to read block from filesystem resulted in short read) while reading indirect blocks of inode 19780.  Ignore error? yes

  Inode 19780, i_blocks is 1548, should be 540.  Fix? yes

  Pass 2: Checking directory structure
  Error reading block 49405 (Attempt to read block from filesystem resulted in short read).  Ignore error? yes

  Directory inode 11858, block 0, offset 0: directory corrupted
  Salvage? yes

  Missing '.' in directory inode 11858.
  Fix? yes

  Missing '..' in directory inode 11858.
  Fix? yes



  The standard drill in this sort of situation is to fire up gdb on the
  signal thread, which, in this case, was pid 1935.  In another window,
  I run gdb and attach pid 1935.



       ~/linux/2.3.26/um 1016: gdb linux
       GNU gdb 4.17.0.11 with Linux support
       Copyright 1998 Free Software Foundation, Inc.
       GDB is free software, covered by the GNU General Public License, and you are
       welcome to change it and/or distribute copies of it under certain conditions.
       Type "show copying" to see the conditions.
       There is absolutely no warranty for GDB.  Type "show warranty" for details.
       This GDB was configured as "i386-redhat-linux"...

       (gdb) att 1935
       Attaching to program `/home/dike/linux/2.3.26/um/linux', Pid 1935
       0x100756d9 in __wait4 ()



  Let's see what's currently running:



       (gdb) p current_task.pid
       $1 = 0



  It's the idle thread, which means that fsck went to sleep for some
  reason and never woke up.


  Let's guess that the last process in the process list is fsck:



       (gdb) p current_task.prev_task.comm
       $13 = "fsck.ext2\000\000\000\000\000\000"



  It is, so let's see what it thinks it's up to:



       (gdb) p current_task.prev_task.thread
       $14 = {extern_pid = 1980, tracing = 0, want_tracing = 0, forking = 0,
         kernel_stack_page = 0, signal_stack = 1342627840, syscall = {id = 4, args = {
             3, 134973440, 1024, 0, 1024}, have_result = 0, result = 50590720},
         request = {op = 2, u = {exec = {ip = 1350467584, sp = 2952789424}, fork = {
               regs = {1350467584, 2952789424, 0 <repeats 15 times>}, sigstack = 0,
               pid = 0}, switch_to = 0x507e8000, thread = {proc = 0x507e8000,
               arg = 0xaffffdb0, flags = 0, new_pid = 0}, input_request = {
               op = 1350467584, fd = -1342177872, proc = 0, pid = 0}}}}



  The interesting things here are the fact that its .thread.syscall.id
  is __NR_write (see the big switch in arch/um/kernel/syscall_kern.c or
  the defines in include/asm-um/arch/unistd.h), and that it never
  returned.  Also, its .request.op is OP_SWITCH (see
  arch/um/include/user_util.h).  These mean that it went into a write,
  and, for some reason, called schedule().


  The fact that it never returned from write means that its stack should
  be fairly interesting.  Its pid is 1980 (.thread.extern_pid).  That
  process is being ptraced by the signal thread, so it must be detached
  before gdb can attach it:



  (gdb) call detach(1980)

  Program received signal SIGSEGV, Segmentation fault.
  <function called from gdb>
  The program being debugged stopped while in a function called from GDB.
  When the function (detach) is done executing, GDB will silently
  stop (instead of continuing to evaluate the expression containing
  the function call).
  (gdb) call detach(1980)
  $15 = 0



  The first detach segfaults for some reason, and the second one
  succeeds.


  Now I detach from the signal thread, attach to the fsck thread, and
  look at its stack:


       (gdb) det
       Detaching from program: /home/dike/linux/2.3.26/um/linux Pid 1935
       (gdb) att 1980
       Attaching to program `/home/dike/linux/2.3.26/um/linux', Pid 1980
       0x10070451 in __kill ()
       (gdb) bt
       #0  0x10070451 in __kill ()
       #1  0x10068ccd in usr1_pid (pid=1980) at process.c:30
       #2  0x1006a03f in _switch_to (prev=0x50072000, next=0x507e8000)
           at process_kern.c:156
       #3  0x1006a052 in switch_to (prev=0x50072000, next=0x507e8000, last=0x50072000)
           at process_kern.c:161
       #4  0x10001d12 in schedule () at sched.c:777
       #5  0x1006a744 in __down (sem=0x507d241c) at semaphore.c:71
       #6  0x1006aa10 in __down_failed () at semaphore.c:157
       #7  0x1006c5d8 in segv_handler (sc=0x5006e940) at trap_user.c:174
       #8  0x1006c5ec in kern_segv_handler (sig=11) at trap_user.c:182
       #9  <signal handler called>
       #10 0x10155404 in errno ()
       #11 0x1006c0aa in segv (address=1342179328, is_write=2) at trap_kern.c:50
       #12 0x1006c5d8 in segv_handler (sc=0x5006eaf8) at trap_user.c:174
       #13 0x1006c5ec in kern_segv_handler (sig=11) at trap_user.c:182
       #14 <signal handler called>
       #15 0xc0fd in ?? ()
       #16 0x10016647 in sys_write (fd=3,
           buf=0x80b8800 <Address 0x80b8800 out of bounds>, count=1024)
           at read_write.c:159
       #17 0x1006d5b3 in execute_syscall (syscall=4, args=0x5006ef08)
           at syscall_kern.c:254
       #18 0x1006af87 in really_do_syscall (sig=12) at syscall_user.c:35
       #19 <signal handler called>
       #20 0x400dc8b0 in ?? ()



  The interesting things here are :

  +o  There are two segfaults on this stack (frames 9 and 14)

  +o  The first faulting address (frame 11) is 0x50000800

  (gdb) p (void *)1342179328
  $16 = (void *) 0x50000800



  The initial faulting address is interesting because it is on the idle
  thread's stack.  I had been seeing the idle thread segfault for no
  apparent reason, and the cause looked like stack corruption.  In hopes
  of catching the culprit in the act, I had turned off all protections
  to that stack while the idle thread wasn't running.  This apparently
  tripped that trap.


  However, the more immediate problem is that second segfault and I'm
  going to concentrate on that.  First, I want to see where the fault
  happened, so I have to go look at the sigcontent struct in frame 8:



       (gdb) up
       #1  0x10068ccd in usr1_pid (pid=1980) at process.c:30
       30        kill(pid, SIGUSR1);
       (gdb)
       #2  0x1006a03f in _switch_to (prev=0x50072000, next=0x507e8000)
           at process_kern.c:156
       156       usr1_pid(getpid());
       (gdb)
       #3  0x1006a052 in switch_to (prev=0x50072000, next=0x507e8000, last=0x50072000)
           at process_kern.c:161
       161       _switch_to(prev, next);
       (gdb)
       #4  0x10001d12 in schedule () at sched.c:777
       777             switch_to(prev, next, prev);
       (gdb)
       #5  0x1006a744 in __down (sem=0x507d241c) at semaphore.c:71
       71                      schedule();
       (gdb)
       #6  0x1006aa10 in __down_failed () at semaphore.c:157
       157     }
       (gdb)
       #7  0x1006c5d8 in segv_handler (sc=0x5006e940) at trap_user.c:174
       174       segv(sc->cr2, sc->err & 2);
       (gdb)
       #8  0x1006c5ec in kern_segv_handler (sig=11) at trap_user.c:182
       182       segv_handler(sc);
       (gdb) p *sc
       Cannot access memory at address 0x0.



  That's not very useful, so I'll try a more manual method:


       (gdb) p *((struct sigcontext *) (&sig + 1))
       $19 = {gs = 0, __gsh = 0, fs = 0, __fsh = 0, es = 43, __esh = 0, ds = 43,
         __dsh = 0, edi = 1342179328, esi = 1350378548, ebp = 1342630440,
         esp = 1342630420, ebx = 1348150624, edx = 1280, ecx = 0, eax = 0,
         trapno = 14, err = 4, eip = 268480945, cs = 35, __csh = 0, eflags = 66118,
         esp_at_signal = 1342630420, ss = 43, __ssh = 0, fpstate = 0x0, oldmask = 0,
         cr2 = 1280}



  The ip is in handle_mm_fault:


       (gdb) p (void *)268480945
       $20 = (void *) 0x1000b1b1
       (gdb) i sym $20
       handle_mm_fault + 57 in section .text



  Specifically, it's in pte_alloc:


       (gdb) i line *$20
       Line 124 of "/home/dike/linux/2.3.26/um/include/asm/pgalloc.h"
          starts at address 0x1000b1b1 <handle_mm_fault+57>
          and ends at 0x1000b1b7 <handle_mm_fault+63>.



  To find where in handle_mm_fault this is, I'll jump forward in the
  code until I see an address in that procedure:



       (gdb) i line *0x1000b1c0
       Line 126 of "/home/dike/linux/2.3.26/um/include/asm/pgalloc.h"
          starts at address 0x1000b1b7 <handle_mm_fault+63>
          and ends at 0x1000b1c3 <handle_mm_fault+75>.
       (gdb) i line *0x1000b1d0
       Line 131 of "/home/dike/linux/2.3.26/um/include/asm/pgalloc.h"
          starts at address 0x1000b1d0 <handle_mm_fault+88>
          and ends at 0x1000b1da <handle_mm_fault+98>.
       (gdb) i line *0x1000b1e0
       Line 61 of "/home/dike/linux/2.3.26/um/include/asm/pgalloc.h"
          starts at address 0x1000b1da <handle_mm_fault+98>
          and ends at 0x1000b1e1 <handle_mm_fault+105>.
       (gdb) i line *0x1000b1f0
       Line 134 of "/home/dike/linux/2.3.26/um/include/asm/pgalloc.h"
          starts at address 0x1000b1f0 <handle_mm_fault+120>
          and ends at 0x1000b200 <handle_mm_fault+136>.
       (gdb) i line *0x1000b200
       Line 135 of "/home/dike/linux/2.3.26/um/include/asm/pgalloc.h"
          starts at address 0x1000b200 <handle_mm_fault+136>
          and ends at 0x1000b208 <handle_mm_fault+144>.
       (gdb) i line *0x1000b210
       Line 139 of "/home/dike/linux/2.3.26/um/include/asm/pgalloc.h"
          starts at address 0x1000b210 <handle_mm_fault+152>
          and ends at 0x1000b219 <handle_mm_fault+161>.
       (gdb) i line *0x1000b220
       Line 1168 of "memory.c" starts at address 0x1000b21e <handle_mm_fault+166>
          and ends at 0x1000b222 <handle_mm_fault+170>.



  Something is apparently wrong with the page tables or vma_structs, so
  lets go back to frame 11 and have a look at them:



  #11 0x1006c0aa in segv (address=1342179328, is_write=2) at trap_kern.c:50
  50        handle_mm_fault(current, vma, address, is_write);
  (gdb) call pgd_offset_proc(vma->vm_mm, address)
  $22 = (pgd_t *) 0x80a548c



  That's pretty bogus.  Page tables aren't supposed to be in process
  text or data areas.  Let's see what's in the vma:


       (gdb) p *vma
       $23 = {vm_mm = 0x507d2434, vm_start = 0, vm_end = 134512640,
         vm_next = 0x80a4f8c, vm_page_prot = {pgprot = 0}, vm_flags = 31200,
         vm_avl_height = 2058, vm_avl_left = 0x80a8c94, vm_avl_right = 0x80d1000,
         vm_next_share = 0xaffffdb0, vm_pprev_share = 0xaffffe63,
         vm_ops = 0xaffffe7a, vm_pgoff = 2952789626, vm_file = 0xafffffec,
         vm_private_data = 0x62}
       (gdb) p *vma.vm_mm
       $24 = {mmap = 0x507d2434, mmap_avl = 0x0, mmap_cache = 0x8048000,
         pgd = 0x80a4f8c, mm_users = {counter = 0}, mm_count = {counter = 134904288},
         map_count = 134909076, mmap_sem = {count = {counter = 135073792},
           sleepers = -1342177872, wait = {lock = <optimized out or zero length>,
             task_list = {next = 0xaffffe63, prev = 0xaffffe7a},
             __magic = -1342177670, __creator = -1342177300}, __magic = 98},
         page_table_lock = {}, context = 138, start_code = 0, end_code = 0,
         start_data = 0, end_data = 0, start_brk = 0, brk = 0, start_stack = 0,
         arg_start = 0, arg_end = 0, env_start = 0, env_end = 0, rss = 1350381536,
         total_vm = 0, locked_vm = 0, def_flags = 0, cpu_vm_mask = 0, swap_cnt = 0,
         swap_address = 0, segments = 0x0}



  This also pretty bogus.  With all of the 0x80xxxxx and 0xaffffxxx
  addresses, this is looking like a stack was plonked down on top of
  these structures.  Maybe it's a stack overflow from the next page:



       (gdb) p vma
       $25 = (struct vm_area_struct *) 0x507d2434



  That's towards the lower quarter of the page, so that would have to
  have been pretty heavy stack overflow:



  (gdb) x/100x $25
  0x507d2434:     0x507d2434      0x00000000      0x08048000      0x080a4f8c
  0x507d2444:     0x00000000      0x080a79e0      0x080a8c94      0x080d1000
  0x507d2454:     0xaffffdb0      0xaffffe63      0xaffffe7a      0xaffffe7a
  0x507d2464:     0xafffffec      0x00000062      0x0000008a      0x00000000
  0x507d2474:     0x00000000      0x00000000      0x00000000      0x00000000
  0x507d2484:     0x00000000      0x00000000      0x00000000      0x00000000
  0x507d2494:     0x00000000      0x00000000      0x507d2fe0      0x00000000
  0x507d24a4:     0x00000000      0x00000000      0x00000000      0x00000000
  0x507d24b4:     0x00000000      0x00000000      0x00000000      0x00000000
  0x507d24c4:     0x00000000      0x00000000      0x00000000      0x00000000
  0x507d24d4:     0x00000000      0x00000000      0x00000000      0x00000000
  0x507d24e4:     0x00000000      0x00000000      0x00000000      0x00000000
  0x507d24f4:     0x00000000      0x00000000      0x00000000      0x00000000
  0x507d2504:     0x00000000      0x00000000      0x00000000      0x00000000
  0x507d2514:     0x00000000      0x00000000      0x00000000      0x00000000
  0x507d2524:     0x00000000      0x00000000      0x00000000      0x00000000
  0x507d2534:     0x00000000      0x00000000      0x507d25dc      0x00000000
  0x507d2544:     0x00000000      0x00000000      0x00000000      0x00000000
  0x507d2554:     0x00000000      0x00000000      0x00000000      0x00000000
  0x507d2564:     0x00000000      0x00000000      0x00000000      0x00000000
  0x507d2574:     0x00000000      0x00000000      0x00000000      0x00000000
  0x507d2584:     0x00000000      0x00000000      0x00000000      0x00000000
  0x507d2594:     0x00000000      0x00000000      0x00000000      0x00000000
  0x507d25a4:     0x00000000      0x00000000      0x00000000      0x00000000
  0x507d25b4:     0x00000000      0x00000000      0x00000000      0x00000000



  It's not stack overflow.  The only "stack-like" piece of this data is
  the vma_struct itself.


  At this point, I don't see any avenues to pursue, so I just have to
  admit that I have no idea what's going on.  What I will do, though, is
  stick a trap on the segfault handler which will stop if it sees any
  writes to the idle thread's stack.  That was the thing that happened
  first, and it may be that if I can catch it immediately, what's going
  on will be somewhat clearer.


  12.2.  Episode 2: The case of the hung fsck

  After setting a trap in the SEGV handler for accesses to the signal
  thread's stack, I reran the kernel.


  fsck hung again, this time by hitting the trap:



  Setting hostname uml                            [ OK ]
  Checking root filesystem
  /dev/fhd0 contains a file system with errors, check forced.
  Error reading block 86894 (Attempt to read block from filesystem resulted in short read) while reading indirect blocks of inode 19780.

  /dev/fhd0: UNEXPECTED INCONSISTENCY; RUN fsck MANUALLY.
          (i.e., without -a or -p options)
  [ FAILED ]

  *** An error occurred during the file system check.
  *** Dropping you to a shell; the system will reboot
  *** when you leave the shell.
  Give root password for maintenance
  (or type Control-D for normal startup):

  [root@uml /root]# fsck -y /dev/fhd0
  fsck -y /dev/fhd0
  Parallelizing fsck version 1.14 (9-Jan-1999)
  e2fsck 1.14, 9-Jan-1999 for EXT2 FS 0.5b, 95/08/09
  /dev/fhd0 contains a file system with errors, check forced.
  Pass 1: Checking inodes, blocks, and sizes
  Error reading block 86894 (Attempt to read block from filesystem resulted in short read) while reading indirect blocks of inode 19780.  Ignore error? yes

  Pass 2: Checking directory structure
  Error reading block 49405 (Attempt to read block from filesystem resulted in short read).  Ignore error? yes

  Directory inode 11858, block 0, offset 0: directory corrupted
  Salvage? yes

  Missing '.' in directory inode 11858.
  Fix? yes

  Missing '..' in directory inode 11858.
  Fix? yes

  Untested (4127) [100fe44c]: trap_kern.c line 31



  I need to get the signal thread to detach from pid 4127 so that I can
  attach to it with gdb.  This is done by sending it a SIGUSR1, which is
  caught by the signal thread, which detaches the process:


       kill -USR1 4127



  Now I can run gdb on it:



  ~/linux/2.3.26/um 1034: gdb linux
  GNU gdb 4.17.0.11 with Linux support
  Copyright 1998 Free Software Foundation, Inc.
  GDB is free software, covered by the GNU General Public License, and you are
  welcome to change it and/or distribute copies of it under certain conditions.
  Type "show copying" to see the conditions.
  There is absolutely no warranty for GDB.  Type "show warranty" for details.
  This GDB was configured as "i386-redhat-linux"...
  (gdb) att 4127
  Attaching to program `/home/dike/linux/2.3.26/um/linux', Pid 4127
  0x10075891 in __libc_nanosleep ()



  The backtrace shows that it was in a write and that the fault address
  (address in frame 3) is 0x50000800, which is right in the middle of
  the signal thread's stack page:


       (gdb) bt
       #0  0x10075891 in __libc_nanosleep ()
       #1  0x1007584d in __sleep (seconds=1000000)
           at ../sysdeps/unix/sysv/linux/sleep.c:78
       #2  0x1006ce9a in stop () at user_util.c:191
       #3  0x1006bf88 in segv (address=1342179328, is_write=2) at trap_kern.c:31
       #4  0x1006c628 in segv_handler (sc=0x5006eaf8) at trap_user.c:174
       #5  0x1006c63c in kern_segv_handler (sig=11) at trap_user.c:182
       #6  <signal handler called>
       #7  0xc0fd in ?? ()
       #8  0x10016647 in sys_write (fd=3, buf=0x80b8800 "R.", count=1024)
           at read_write.c:159
       #9  0x1006d603 in execute_syscall (syscall=4, args=0x5006ef08)
           at syscall_kern.c:254
       #10 0x1006af87 in really_do_syscall (sig=12) at syscall_user.c:35
       #11 <signal handler called>
       #12 0x400dc8b0 in ?? ()
       #13 <signal handler called>
       #14 0x400dc8b0 in ?? ()
       #15 0x80545fd in ?? ()
       #16 0x804daae in ?? ()
       #17 0x8054334 in ?? ()
       #18 0x804d23e in ?? ()
       #19 0x8049632 in ?? ()
       #20 0x80491d2 in ?? ()
       #21 0x80596b5 in ?? ()
       (gdb) p (void *)1342179328
       $3 = (void *) 0x50000800



  Going up the stack to the segv_handler frame and looking at where in
  the code the access happened shows that it happened near line 110 of
  block_dev.c:



  (gdb) up
  #1  0x1007584d in __sleep (seconds=1000000)
      at ../sysdeps/unix/sysv/linux/sleep.c:78
  ../sysdeps/unix/sysv/linux/sleep.c:78: No such file or directory.
  (gdb)
  #2  0x1006ce9a in stop () at user_util.c:191
  191       while(1) sleep(1000000);
  (gdb)
  #3  0x1006bf88 in segv (address=1342179328, is_write=2) at trap_kern.c:31
  31          KERN_UNTESTED();
  (gdb)
  #4  0x1006c628 in segv_handler (sc=0x5006eaf8) at trap_user.c:174
  174       segv(sc->cr2, sc->err & 2);
  (gdb) p *sc
  $1 = {gs = 0, __gsh = 0, fs = 0, __fsh = 0, es = 43, __esh = 0, ds = 43,
    __dsh = 0, edi = 1342179328, esi = 134973440, ebp = 1342631484,
    esp = 1342630864, ebx = 256, edx = 0, ecx = 256, eax = 1024, trapno = 14,
    err = 6, eip = 268550834, cs = 35, __csh = 0, eflags = 66070,
    esp_at_signal = 1342630864, ss = 43, __ssh = 0, fpstate = 0x0, oldmask = 0,
    cr2 = 1342179328}
  (gdb) p (void *)268550834
  $2 = (void *) 0x1001c2b2
  (gdb) i sym $2
  block_write + 1090 in section .text
  (gdb) i line *$2
  Line 209 of "/home/dike/linux/2.3.26/um/include/asm/arch/string.h"
     starts at address 0x1001c2a1 <block_write+1073>
     and ends at 0x1001c2bf <block_write+1103>.
  (gdb) i line *0x1001c2c0
  Line 110 of "block_dev.c" starts at address 0x1001c2bf <block_write+1103>
     and ends at 0x1001c2e3 <block_write+1139>.



  Looking at the source shows that the fault happened during a call to
  copy_to_user to copy the data into the kernel:


       107             count -= chars;
       108             copy_from_user(p,buf,chars);
       109             p += chars;
       110             buf += chars;



  p is the pointer which must contain 0x50000800, since buf contains
  0x80b8800 (frame 8 above).  It is defined as:


                       p = offset + bh->b_data;



  I need to figure out what bh is, and it just so happens that bh is
  passed as an argument to mark_buffer_uptodate and mark_buffer_dirty a
  few lines later, so I do a little disassembly:



  (gdb) disas 0x1001c2bf 0x1001c2e0
  Dump of assembler code from 0x1001c2bf to 0x1001c2d0:
  0x1001c2bf <block_write+1103>:  addl   %eax,0xc(%ebp)
  0x1001c2c2 <block_write+1106>:  movl   0xfffffdd4(%ebp),%edx
  0x1001c2c8 <block_write+1112>:  btsl   $0x0,0x18(%edx)
  0x1001c2cd <block_write+1117>:  btsl   $0x1,0x18(%edx)
  0x1001c2d2 <block_write+1122>:  sbbl   %ecx,%ecx
  0x1001c2d4 <block_write+1124>:  testl  %ecx,%ecx
  0x1001c2d6 <block_write+1126>:  jne    0x1001c2e3 <block_write+1139>
  0x1001c2d8 <block_write+1128>:  pushl  $0x0
  0x1001c2da <block_write+1130>:  pushl  %edx
  0x1001c2db <block_write+1131>:  call   0x1001819c <__mark_buffer_dirty>
  End of assembler dump.



  At that point, bh is in %edx (address 0x1001c2da), which is calculated
  at 0x1001c2c2 as %ebp + 0xfffffdd4, so I figure exactly what that is,
  taking %ebp from the sigcontext_struct above:


       (gdb) p (void *)1342631484
       $5 = (void *) 0x5006ee3c
       (gdb) p 0x5006ee3c+0xfffffdd4
       $6 = 1342630928
       (gdb) p (void *)$6
       $7 = (void *) 0x5006ec10
       (gdb) p *((void **)$7)
       $8 = (void *) 0x50100200



  Now, I look at the structure to see what's in it, and particularly,
  what its b_data field contains:


       (gdb) p *((struct buffer_head *)0x50100200)
       $13 = {b_next = 0x50289380, b_blocknr = 49405, b_size = 1024, b_list = 0,
         b_dev = 15872, b_count = {counter = 1}, b_rdev = 15872, b_state = 24,
         b_flushtime = 0, b_next_free = 0x501001a0, b_prev_free = 0x50100260,
         b_this_page = 0x501001a0, b_reqnext = 0x0, b_pprev = 0x507fcf58,
         b_data = 0x50000800 "", b_page = 0x50004000,
         b_end_io = 0x10017f60 <end_buffer_io_sync>, b_dev_id = 0x0,
         b_rsector = 98810, b_wait = {lock = <optimized out or zero length>,
           task_list = {next = 0x50100248, prev = 0x50100248}, __magic = 1343226448,
           __creator = 0}, b_kiobuf = 0x0}



  The b_data field is indeed 0x50000800, so the question becomes how
  that happened.  The rest of the structure looks fine, so this probably
  is not a case of data corruption.  It happened on purpose somehow.


  The b_page field is a pointer to the page_struct representing the
  0x50000000 page.  Looking at it shows the kernel's idea of the state
  of that page:



  (gdb) p *$13.b_page
  $17 = {list = {next = 0x50004a5c, prev = 0x100c5174}, mapping = 0x0,
    index = 0, next_hash = 0x0, count = {counter = 1}, flags = 132, lru = {
      next = 0x50008460, prev = 0x50019350}, wait = {
      lock = <optimized out or zero length>, task_list = {next = 0x50004024,
        prev = 0x50004024}, __magic = 1342193708, __creator = 0},
    pprev_hash = 0x0, buffers = 0x501002c0, virtual = 1342177280,
    zone = 0x100c5160}



  Some sanity-checking: the virtual field shows the "virtual" address of
  this page, which in this kernel is the same as its "physical" address,
  and the page_struct itself should be mem_map[0], since it represents
  the first page of memory:



       (gdb) p (void *)1342177280
       $18 = (void *) 0x50000000
       (gdb) p mem_map
       $19 = (mem_map_t *) 0x50004000



  These check out fine.


  Now to check out the page_struct itself.  In particular, the flags
  field shows whether the page is considered free or not:


       (gdb) p (void *)132
       $21 = (void *) 0x84



  The "reserved" bit is the high bit, which is definitely not set, so
  the kernel considers the signal stack page to be free and available to
  be used.


  At this point, I jump to conclusions and start looking at my early
  boot code, because that's where that page is supposed to be reserved.


  In my setup_arch procedure, I have the following code which looks just
  fine:



       bootmap_size = init_bootmem(start_pfn, end_pfn - start_pfn);
       free_bootmem(__pa(low_physmem) + bootmap_size, high_physmem - low_physmem);



  Two stack pages have already been allocated, and low_physmem points to
  the third page, which is the beginning of free memory.
  The init_bootmem call declares the entire memory to the boot memory
  manager, which marks it all reserved.  The free_bootmem call frees up
  all of it, except for the first two pages.  This looks correct to me.


  So, I decide to see init_bootmem run and make sure that it is marking
  those first two pages as reserved.  I never get that far.


  Stepping into init_bootmem, and looking at bootmem_map before looking
  at what it contains shows the following:



       (gdb) p bootmem_map
       $3 = (void *) 0x50000000



  Aha!  The light dawns.  That first page is doing double duty as a
  stack and as the boot memory map.  The last thing that the boot memory
  manager does is to free the pages used by its memory map, so this page
  is getting freed even its marked as reserved.


  The fix was to initialize the boot memory manager before allocating
  those two stack pages, and then allocate them through the boot memory
  manager.  After doing this, and fixing a couple of subsequent buglets,
  the stack corruption problem disappeared.



  13.  What to do when UML doesn't work



  13.1.  Strange compilation errors when you build from source

  As of test11, it is necessary to have "ARCH=um" in the environment or
  on the make command line for all steps in building UML, including
  clean, distclean, or mrproper, config, menuconfig, or xconfig, dep,