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authorLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
committerLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
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tree0bba044c4ce775e45a88a51686b5d9f90697ea9d /Documentation/uml
downloadlinux-linaro-stable-1da177e4c3f41524e886b7f1b8a0c1fc7321cac2.tar.gz
Linux-2.6.12-rc2v2.6.12-rc2
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
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+ User Mode Linux HOWTO
+ User Mode Linux Core Team
+ Mon Nov 18 14:16:16 EST 2002
+
+ This document describes the use and abuse of Jeff Dike's User Mode
+ Linux: a port of the Linux kernel as a normal Intel Linux process.
+ ______________________________________________________________________
+
+ Table of Contents
+
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+
+ 1. Introduction
+
+ 1.1 How is User Mode Linux Different?
+ 1.2 Why Would I Want User Mode Linux?
+
+ 2. Compiling the kernel and modules
+
+ 2.1 Compiling the kernel
+ 2.2 Compiling and installing kernel modules
+ 2.3 Compiling and installing uml_utilities
+
+ 3. Running UML and logging in
+
+ 3.1 Running UML
+ 3.2 Logging in
+ 3.3 Examples
+
+ 4. UML on 2G/2G hosts
+
+ 4.1 Introduction
+ 4.2 The problem
+ 4.3 The solution
+
+ 5. Setting up serial lines and consoles
+
+ 5.1 Specifying the device
+ 5.2 Specifying the channel
+ 5.3 Examples
+
+ 6. Setting up the network
+
+ 6.1 General setup
+ 6.2 Userspace daemons
+ 6.3 Specifying ethernet addresses
+ 6.4 UML interface setup
+ 6.5 Multicast
+ 6.6 TUN/TAP with the uml_net helper
+ 6.7 TUN/TAP with a preconfigured tap device
+ 6.8 Ethertap
+ 6.9 The switch daemon
+ 6.10 Slip
+ 6.11 Slirp
+ 6.12 pcap
+ 6.13 Setting up the host yourself
+
+ 7. Sharing Filesystems between Virtual Machines
+
+ 7.1 A warning
+ 7.2 Using layered block devices
+ 7.3 Note!
+ 7.4 Another warning
+ 7.5 uml_moo : Merging a COW file with its backing file
+
+ 8. Creating filesystems
+
+ 8.1 Create the filesystem file
+ 8.2 Assign the file to a UML device
+ 8.3 Creating and mounting the filesystem
+
+ 9. Host file access
+
+ 9.1 Using hostfs
+ 9.2 hostfs as the root filesystem
+ 9.3 Building hostfs
+
+ 10. The Management Console
+ 10.1 version
+ 10.2 halt and reboot
+ 10.3 config
+ 10.4 remove
+ 10.5 sysrq
+ 10.6 help
+ 10.7 cad
+ 10.8 stop
+ 10.9 go
+
+ 11. Kernel debugging
+
+ 11.1 Starting the kernel under gdb
+ 11.2 Examining sleeping processes
+ 11.3 Running ddd on UML
+ 11.4 Debugging modules
+ 11.5 Attaching gdb to the kernel
+ 11.6 Using alternate debuggers
+
+ 12. Kernel debugging examples
+
+ 12.1 The case of the hung fsck
+ 12.2 Episode 2: The case of the hung fsck
+
+ 13. What to do when UML doesn't work
+
+ 13.1 Strange compilation errors when you build from source
+ 13.2 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 Any other panic, hang, or strange behavior
+
+ 14. Diagnosing Problems
+
+ 14.1 Case 1 : Normal kernel panics
+ 14.2 Case 2 : Tracing thread panics
+ 14.3 Case 3 : Tracing thread panics caused by other threads
+ 14.4 Case 4 : Hangs
+
+ 15. Thanks
+
+ 15.1 Code and Documentation
+ 15.2 Flushing out bugs
+ 15.3 Buglets and clean-ups
+ 15.4 Case Studies
+ 15.5 Other contributions
+
+
+ ______________________________________________________________________
+
+ 11.. IInnttrroodduuccttiioonn
+
+ Welcome to User Mode Linux. It's going to be fun.
+
+
+
+ 11..11.. HHooww iiss UUsseerr MMooddee LLiinnuuxx DDiiffffeerreenntt??
+
+ Normally, the Linux Kernel talks straight to your hardware (video
+ card, keyboard, hard drives, etc), and any programs which run ask the
+ kernel to operate the hardware, like so:
+
+
+
+ +-----------+-----------+----+
+ | Process 1 | Process 2 | ...|
+ +-----------+-----------+----+
+ | Linux Kernel |
+ +----------------------------+
+ | Hardware |
+ +----------------------------+
+
+
+
+
+ The User Mode Linux Kernel is different; instead of talking to the
+ hardware, it talks to a `real' Linux kernel (called the `host kernel'
+ from now on), like any other program. Programs can then run inside
+ User-Mode Linux as if they were running under a normal kernel, like
+ so:
+
+
+
+ +----------------+
+ | Process 2 | ...|
+ +-----------+----------------+
+ | Process 1 | User-Mode Linux|
+ +----------------------------+
+ | Linux Kernel |
+ +----------------------------+
+ | Hardware |
+ +----------------------------+
+
+
+
+
+
+ 11..22.. WWhhyy WWoouulldd II WWaanntt UUsseerr MMooddee LLiinnuuxx??
+
+
+ 1. If User Mode Linux crashes, your host kernel is still fine.
+
+ 2. You can run a usermode kernel as a non-root user.
+
+ 3. You can debug the User Mode Linux like any normal process.
+
+ 4. You can run gprof (profiling) and gcov (coverage testing).
+
+ 5. You can play with your kernel without breaking things.
+
+ 6. You can use it as a sandbox for testing new apps.
+
+ 7. You can try new development kernels safely.
+
+ 8. You can run different distributions simultaneously.
+
+ 9. It's extremely fun.
+
+
+
+
+
+ 22.. CCoommppiilliinngg tthhee kkeerrnneell aanndd mmoodduulleess
+
+
+
+
+ 22..11.. CCoommppiilliinngg tthhee kkeerrnneell
+
+
+ Compiling the user mode kernel is just like compiling any other
+ kernel. Let's go through the steps, using 2.4.0-prerelease (current
+ as of this writing) as an example:
+
+
+ 1. Download the latest UML patch from
+
+ the download page <http://user-mode-linux.sourceforge.net/dl-
+ sf.html>
+
+ In this example, the file is uml-patch-2.4.0-prerelease.bz2.
+
+
+ 2. Download the matching kernel from your favourite kernel mirror,
+ such as:
+
+ ftp://ftp.ca.kernel.org/pub/kernel/v2.4/linux-2.4.0-prerelease.tar.bz2
+ <ftp://ftp.ca.kernel.org/pub/kernel/v2.4/linux-2.4.0-prerelease.tar.bz2>
+ .
+
+
+ 3. Make a directory and unpack the kernel into it.
+
+
+
+ host%
+ mkdir ~/uml
+
+
+
+
+
+
+ host%
+ cd ~/uml
+
+
+
+
+
+
+ host%
+ tar -xzvf linux-2.4.0-prerelease.tar.bz2
+
+
+
+
+
+
+ 4. Apply the patch using
+
+
+
+ host%
+ cd ~/uml/linux
+
+
+
+ host%
+ bzcat uml-patch-2.4.0-prerelease.bz2 | patch -p1
+
+
+
+
+
+
+ 5. Run your favorite config; `make xconfig ARCH=um' is the most
+ convenient. `make config ARCH=um' and 'make menuconfig ARCH=um'
+ will work as well. The defaults will give you a useful kernel. If
+ you want to change something, go ahead, it probably won't hurt
+ anything.
+
+
+ Note: If the host is configured with a 2G/2G address space split
+ rather than the usual 3G/1G split, then the packaged UML binaries
+ will not run. They will immediately segfault. See ``UML on 2G/2G
+ hosts'' for the scoop on running UML on your system.
+
+
+
+ 6. Finish with `make linux ARCH=um': the result is a file called
+ `linux' in the top directory of your source tree.
+
+ Make sure that you don't build this kernel in /usr/src/linux. On some
+ distributions, /usr/include/asm is a link into this pool. The user-
+ mode build changes the other end of that link, and things that include
+ <asm/anything.h> stop compiling.
+
+ The sources are also available from cvs at the project's cvs page,
+ which has directions on getting the sources. You can also browse the
+ CVS pool from there.
+
+ If you get the CVS sources, you will have to check them out into an
+ empty directory. You will then have to copy each file into the
+ corresponding directory in the appropriate kernel pool.
+
+ If you don't have the latest kernel pool, you can get the
+ corresponding user-mode sources with
+
+
+ host% cvs co -r v_2_3_x linux
+
+
+
+
+ where 'x' is the version in your pool. Note that you will not get the
+ bug fixes and enhancements that have gone into subsequent releases.
+
+
+ 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 SuSE 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; done
+
+
+
+
+ and changing /etc/fstab and /etc/inittab to refer to the non-devfs
+ devices.
+
+
+
+ 22..22.. CCoommppiilliinngg aanndd iinnssttaalllliinngg kkeerrnneell mmoodduulleess
+
+ UML modules are built in the same way as the native kernel (with the
+ exception of the 'ARCH=um' that you always need for UML):
+
+
+ host% make modules ARCH=um
+
+
+
+
+ Any modules that you want to load into this kernel need to be built in
+ the user-mode pool. Modules from the native kernel won't work.
+
+ You can install them by using ftp or something to copy them into the
+ virtual machine and dropping them into /lib/modules/`uname -r`.
+
+ You can also get the kernel build process to install them as follows:
+
+ 1. with the kernel not booted, mount the root filesystem in the top
+ level of the kernel pool:
+
+
+ host% mount root_fs mnt -o loop
+
+
+
+
+
+
+ 2. run
+
+
+ host%
+ make modules_install INSTALL_MOD_PATH=`pwd`/mnt ARCH=um
+
+
+
+
+
+
+ 3. unmount the filesystem
+
+
+ host% umount mnt
+
+
+
+
+
+
+ 4. boot the kernel on it
+
+
+ When the system is booted, you can use insmod as usual to get the
+ modules into the kernel. A number of things have been loaded into UML
+ as modules, especially filesystems and network protocols and filters,
+ so most symbols which need to be exported probably already are.
+ However, if you do find symbols that need exporting, let us
+ <http://user-mode-linux.sourceforge.net/contacts.html> know, and
+ they'll be "taken care of".
+
+
+
+ 22..33.. CCoommppiilliinngg aanndd iinnssttaalllliinngg uummll__uuttiilliittiieess
+
+ Many features of the UML kernel require a user-space helper program,
+ so a uml_utilities package is distributed separately from the kernel
+ patch which provides these helpers. Included within this is:
+
+ +o port-helper - Used by consoles which connect to xterms or ports
+
+ +o tunctl - Configuration tool to create and delete tap devices
+
+ +o uml_net - Setuid binary for automatic tap device configuration
+
+ +o uml_switch - User-space virtual switch required for daemon
+ transport
+
+ The uml_utilities tree is compiled with:
+
+
+ host#
+ make && make install
+
+
+
+
+ Note that UML kernel patches may require a specific version of the
+ uml_utilities distribution. If you don't keep up with the mailing
+ lists, ensure that you have the latest release of uml_utilities if you
+ are experiencing problems with your UML kernel, particularly when
+ dealing with consoles or command-line switches to the helper programs
+
+
+
+
+
+
+
+
+ 33.. RRuunnnniinngg UUMMLL aanndd llooggggiinngg iinn
+
+
+
+ 33..11.. RRuunnnniinngg UUMMLL
+
+ It runs on 2.2.15 or later, and all 2.4 kernels.
+
+
+ Booting UML is straightforward. Simply run 'linux': it will try to
+ mount the file `root_fs' in the current directory. You do not need to
+ run it as root. If your root filesystem is not named `root_fs', then
+ you need to put a `ubd0=root_fs_whatever' switch on the linux command
+ line.
+
+
+ You will need a filesystem to boot UML from. There are a number
+ available for download from here <http://user-mode-
+ linux.sourceforge.net/dl-sf.html> . There are also several tools
+ <http://user-mode-linux.sourceforge.net/fs_making.html> which can be
+ used to generate UML-compatible filesystem images from media.
+ The kernel will boot up and present you with a login prompt.
+
+
+ Note: If the host is configured with a 2G/2G address space split
+ rather than the usual 3G/1G split, then the packaged UML binaries will
+ not run. They will immediately segfault. See ``UML on 2G/2G hosts''
+ for the scoop on running UML on your system.
+
+
+
+ 33..22.. LLooggggiinngg iinn
+
+
+
+ The prepackaged filesystems have a root account with password 'root'
+ and a user account with password 'user'. The login banner will
+ generally tell you how to log in. So, you log in and you will find
+ 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.
+
+
+ 33..33.. EExxaammpplleess
+
+ 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>
+
+
+
+
+
+
+
+ 44.. UUMMLL oonn 22GG//22GG hhoossttss
+
+
+
+
+ 44..11.. IInnttrroodduuccttiioonn
+
+
+ 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).
+
+
+
+
+ 44..22.. TThhee pprroobblleemm
+
+
+ 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.
+
+
+
+
+ 44..33.. TThhee ssoolluuttiioonn
+
+
+ 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.
+
+
+
+
+
+
+
+
+
+
+ 55.. SSeettttiinngg uupp sseerriiaall lliinneess aanndd ccoonnssoolleess
+
+
+ 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.
+
+
+
+ 55..11.. SSppeecciiffyyiinngg tthhee ddeevviiccee
+
+ 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.
+
+
+
+ 55..22.. SSppeecciiffyyiinngg tthhee cchhaannnneell
+
+ 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.
+
+
+
+ 55..33.. EExxaammpplleess
+
+ 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.
+
+
+ 66.. SSeettttiinngg uupp tthhee nneettwwoorrkk
+
+
+
+ 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.
+
+
+ 66..11.. GGeenneerraall sseettuupp
+
+ 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.
+
+
+
+ 66..22.. UUsseerrssppaaccee ddaaeemmoonnss
+
+ 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.
+
+
+
+ 66..33.. SSppeecciiffyyiinngg eetthheerrnneett aaddddrreesssseess
+
+ 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.
+
+
+
+ 66..44.. UUMMLL iinntteerrffaaccee sseettuupp
+
+ 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.
+
+
+
+ 66..55.. MMuullttiiccaasstt
+
+ 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.
+
+
+
+ 66..66.. TTUUNN//TTAAPP wwiitthh tthhee uummll__nneett hheellppeerr
+
+ 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,,, <IP address>
+
+
+
+
+ For example, this argument will attach the UML's eth0 to the next
+ available tap device and assign an ethernet address to it based on its
+ IP address
+
+
+ eth0=tuntap,,,192.168.0.254
+
+
+
+
+
+
+ Note that the IP address that must be used for the eth device inside
+ UML is fixed by the routing and proxy arp that is set up on the
+ TUN/TAP device on the host. You can use a different one, but it won't
+ work because reply packets won't reach the UML. This is a feature.
+ It prevents a nasty UML user from doing things like setting the UML IP
+ to the same as the network's nameserver or mail server.
+
+
+ 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"> .
+
+
+
+ 66..77.. TTUUNN//TTAAPP wwiitthh aa pprreeccoonnffiigguurreedd ttaapp ddeevviiccee
+
+ 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,device name'
+ (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`
+
+
+
+
+
+
+ 66..88.. EEtthheerrttaapp
+
+ 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> , <tap 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 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:
+
+
+ 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
+
+
+
+
+
+
+
+ 66..99.. TThhee sswwiittcchh ddaaeemmoonn
+
+ NNoottee: 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 pair of unix domain
+ sockets.
+
+
+ If you want it to listen on a different pair of sockets, use
+
+
+ -unix control socket data socket
+
+
+
+
+
+ If you want it to act as a hub rather than a switch, use
+
+
+ -hub
+
+
+
+
+
+ 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,control socket,data 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.
+
+
+
+ 66..1100.. SSlliipp
+
+ 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.
+
+
+
+ 66..1111.. SSlliirrpp
+
+ 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.
+
+
+
+ 66..1122.. ppccaapp
+
+ 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.
+
+
+
+ 66..1133.. SSeettttiinngg uupp tthhee hhoosstt yyoouurrsseellff
+
+ 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
+
+
+
+
+
+
+
+
+
+
+ 77.. SShhaarriinngg FFiilleessyysstteemmss bbeettwweeeenn VViirrttuuaall MMaacchhiinneess
+
+
+
+
+ 77..11.. AA wwaarrnniinngg
+
+ 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.
+
+
+
+ 77..22.. UUssiinngg llaayyeerreedd bblloocckk ddeevviicceess
+
+ 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.
+
+
+
+ 77..33.. NNoottee!!
+
+ 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.
+
+
+
+ 77..44.. AAnnootthheerr wwaarrnniinngg
+
+ 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.
+
+
+
+
+ 77..55.. uummll__mmoooo :: MMeerrggiinngg aa CCOOWW ffiillee wwiitthh iittss bbaacckkiinngg ffiillee
+
+ 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.
+
+
+
+
+ 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.
+
+
+
+
+
+
+
+
+ 88.. CCrreeaattiinngg ffiilleessyysstteemmss
+
+
+ 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.
+
+
+ 88..11.. CCrreeaattee tthhee ffiilleessyysstteemm ffiillee
+
+ 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
+
+
+
+
+
+
+ 88..22.. AAssssiiggnn tthhee ffiillee ttoo aa UUMMLL ddeevviiccee
+
+ Add an argument like the following to the UML command line:
+
+ ubd4=new_filesystem
+
+
+
+
+ making sure that you use an unassigned ubd device number.
+
+
+
+ 88..33.. CCrreeaattiinngg aanndd mmoouunnttiinngg tthhee ffiilleessyysstteemm
+
+ 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.
+
+
+
+
+
+
+
+
+
+ 99.. HHoosstt ffiillee aacccceessss
+
+
+ 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.
+
+
+ 99..11.. UUssiinngg hhoossttffss
+
+ 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
+ module 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.
+
+
+
+ 99..22.. hhoossttffss aass tthhee rroooott ffiilleessyysstteemm
+
+ 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:
+
+ /dev/ubd/0 / 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 {} \;
+
+
+
+
+ Next, make sure that your UML kernel has hostfs compiled in, not as a
+ module. Then run UML with the boot device pointing at that directory:
+
+
+ ubd0=/path/to/uml/root/directory
+
+
+
+
+ UML should then boot as it does normally.
+
+
+ 99..33.. BBuuiillddiinngg hhoossttffss
+
+ 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
+
+
+
+
+
+
+
+
+
+
+
+
+ 1100.. TThhee MMaannaaggeemmeenntt CCoonnssoollee
+
+
+
+ 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
+
+
+ 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
+
+
+ 1100..11.. vveerrssiioonn
+
+ 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.
+
+
+
+ 1100..22.. hhaalltt aanndd rreebboooott
+
+ 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
+
+
+
+
+
+
+ 1100..33.. ccoonnffiigg
+
+ "config" adds a new device to the virtual machine. Currently the ubd
+ and network drivers support this. 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
+
+
+
+
+
+
+ 1100..44.. rreemmoovvee
+
+ "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
+
+
+
+
+
+
+ 1100..55.. ssyyssrrqq
+
+ 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.
+
+
+
+ 1100..66.. hheellpp
+
+ "help" returns a string listing the valid commands and what each one
+ does.
+
+
+
+ 1100..77.. ccaadd
+
+ 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.
+
+
+
+ 1100..88.. ssttoopp
+
+ This puts the UML in a loop reading mconsole requests until a 'go'
+ mconsole command is received. 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.
+
+
+
+ 1100..99.. ggoo
+
+ 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.
+
+
+
+
+
+
+
+
+ 1111.. KKeerrnneell ddeebbuuggggiinngg
+
+
+ NNoottee:: The interface that makes debugging, as described here, possible
+ is present in 2.4.0-test6 kernels and later.
+
+
+ Since the user-mode kernel 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.
+
+
+
+
+ 1111..11.. SSttaarrttiinngg tthhee kkeerrnneell uunnddeerr ggddbb
+
+ 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.
+ 1111..22.. EExxaammiinniinngg sslleeeeppiinngg pprroocceesssseess
+
+ 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.
+
+
+
+ 1111..33.. RRuunnnniinngg dddddd oonn UUMMLL
+
+ 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.
+
+
+
+ 1111..44.. DDeebbuuggggiinngg mmoodduulleess
+
+ 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 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.
+
+
+
+ 1111..55.. AAttttaacchhiinngg ggddbb ttoo tthhee kkeerrnneell
+
+ 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.
+
+
+
+ 1111..66.. UUssiinngg aalltteerrnnaattee ddeebbuuggggeerrss
+
+ 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.
+
+
+
+
+
+ 1122.. KKeerrnneell ddeebbuuggggiinngg eexxaammpplleess
+
+ 1122..11.. TThhee ccaassee ooff tthhee hhuunngg ffsscckk
+
+ 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.
+
+
+ 1122..22.. EEppiissooddee 22:: TThhee ccaassee ooff tthhee hhuunngg ffsscckk
+
+ 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.
+
+
+
+
+
+ 1133.. WWhhaatt ttoo ddoo wwhheenn UUMMLL ddooeessnn''tt wwoorrkk
+
+
+
+
+ 1133..11.. SSttrraannggee ccoommppiillaattiioonn eerrrroorrss wwhheenn yyoouu bbuuiilldd ffrroomm ssoouurrccee
+
+ 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,
+ and linux. If you forget for any of them, the i386 build seems to
+ contaminate the UML build. If this happens, start from scratch with
+
+
+ host%
+ make mrproper ARCH=um
+
+
+
+
+ and repeat the build process with ARCH=um on all the steps.
+
+
+ See ``Compiling the kernel and modules'' for more details.
+
+
+ Another cause of strange compilation errors is building UML in
+ /usr/src/linux. If you do this, the first thing you need to do is
+ clean up the mess you made. The /usr/src/linux/asm link will now
+ point to /usr/src/linux/asm-um. Make it point back to
+ /usr/src/linux/asm-i386. Then, move your UML pool someplace else and
+ build it there. Also see below, where a more specific set of symptoms
+ is described.
+
+
+
+ 1133..22.. UUMMLL hhaannggss oonn bboooott aafftteerr mmoouunnttiinngg ddeevvffss
+
+ The boot looks like this:
+
+
+ VFS: Mounted root (ext2 filesystem) readonly.
+ Mounted devfs on /dev
+
+
+
+
+ You're probably running a recent distribution on an old machine. I
+ saw this with the RH7.1 filesystem running on a Pentium. The shared
+ library loader, ld.so, was executing an instruction (cmove) which the
+ Pentium didn't support. That instruction was apparently added later.
+ If you run UML under the debugger, you'll see the hang caused by one
+ instruction causing an infinite SIGILL stream.
+
+
+ The fix is to boot UML on an older filesystem.
+
+
+
+ 1133..33.. AA vvaarriieettyy ooff ppaanniiccss aanndd hhaannggss wwiitthh //ttmmpp oonn aa rreeiisseerrffss ffiilleessyyss--
+ tteemm
+
+ I saw this on reiserfs 3.5.21 and it seems to be fixed in 3.5.27.
+ Panics preceded by
+
+
+ Detaching pid nnnn
+
+
+
+ are diagnostic of this problem. This is a reiserfs bug which causes a
+ thread to occasionally read stale data from a mmapped page shared with
+ another thread. The fix is to upgrade the filesystem or to have /tmp
+ be an ext2 filesystem.
+
+
+
+ 1133..44.. TThhee ccoommppiillee ffaaiillss wwiitthh eerrrroorrss aabboouutt ccoonnfflliiccttiinngg ttyyppeess ffoorr
+ ''ooppeenn'',, ''dduupp'',, aanndd ''wwaaiittppiidd''
+
+ This happens when you build in /usr/src/linux. The UML build makes
+ the include/asm link point to include/asm-um. /usr/include/asm points
+ to /usr/src/linux/include/asm, so when that link gets moved, files
+ which need to include the asm-i386 versions of headers get the
+ incompatible asm-um versions. The fix is to move the include/asm link
+ back to include/asm-i386 and to do UML builds someplace else.
+
+
+
+ 1133..55.. UUMMLL ddooeessnn''tt wwoorrkk wwhheenn //ttmmpp iiss aann NNFFSS ffiilleessyysstteemm
+
+ This seems to be a similar situation with the resierfs problem above.
+ Some versions of NFS seems not to handle mmap correctly, which UML
+ depends on. The workaround is have /tmp be non-NFS directory.
+
+
+ 1133..66.. UUMMLL hhaannggss oonn bboooott wwhheenn ccoommppiilleedd wwiitthh ggpprrooff ssuuppppoorrtt
+
+ If you build UML with gprof support and, early in the boot, it does
+ this
+
+
+ kernel BUG at page_alloc.c:100!
+
+
+
+
+ you have a buggy gcc. You can work around the problem by removing
+ UM_FASTCALL from CFLAGS in arch/um/Makefile-i386. This will open up
+ another bug, but that one is fairly hard to reproduce.
+
+
+
+ 1133..77.. ssyyssllooggdd ddiieess wwiitthh aa SSIIGGTTEERRMM oonn ssttaarrttuupp
+
+ The exact boot error depends on the distribution that you're booting,
+ but Debian produces this:
+
+
+ /etc/rc2.d/S10sysklogd: line 49: 93 Terminated
+ start-stop-daemon --start --quiet --exec /sbin/syslogd -- $SYSLOGD
+
+
+
+
+ This is a syslogd bug. There's a race between a parent process
+ installing a signal handler and its child sending the signal. See
+ this uml-devel post <http://www.geocrawler.com/lists/3/Source-
+ Forge/709/0/6612801> for the details.
+
+
+
+ 1133..88.. TTUUNN//TTAAPP nneettwwoorrkkiinngg ddooeessnn''tt wwoorrkk oonn aa 22..44 hhoosstt
+
+ There are a couple of problems which were
+ <http://www.geocrawler.com/lists/3/SourceForge/597/0/> name="pointed
+ out"> by Tim Robinson <timro at trkr dot net>
+
+ +o It doesn't work on hosts running 2.4.7 (or thereabouts) or earlier.
+ The fix is to upgrade to something more recent and then read the
+ next item.
+
+ +o If you see
+
+
+ File descriptor in bad state
+
+
+
+ when you bring up the device inside UML, you have a header mismatch
+ between the original kernel and the upgraded one. Make /usr/src/linux
+ point at the new headers. This will only be a problem if you build
+ uml_net yourself.
+
+
+
+ 1133..99.. YYoouu ccaann nneettwwoorrkk ttoo tthhee hhoosstt bbuutt nnoott ttoo ootthheerr mmaacchhiinneess oonn tthhee
+ nneett
+
+ If you can connect to the host, and the host can connect to UML, but
+ you can not connect to any other machines, then you may need to enable
+ IP Masquerading on the host. Usually this is only experienced when
+ using private IP addresses (192.168.x.x or 10.x.x.x) for host/UML
+ networking, rather than the public address space that your host is
+ connected to. UML does not enable IP Masquerading, so you will need
+ to create a static rule to enable it:
+
+
+ host%
+ iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
+
+
+
+
+ Replace eth0 with the interface that you use to talk to the rest of
+ the world.
+
+
+ Documentation on IP Masquerading, and SNAT, can be found at
+ www.netfilter.org <http://www.netfilter.org> .
+
+
+ If you can reach the local net, but not the outside Internet, then
+ that is usually a routing problem. The UML needs a default route:
+
+
+ UML#
+ route add default gw gateway IP
+
+
+
+
+ The gateway IP can be any machine on the local net that knows how to
+ reach the outside world. Usually, this is the host or the local net-
+ work's gateway.
+
+
+ Occasionally, we hear from someone who can reach some machines, but
+ not others on the same net, or who can reach some ports on other
+ machines, but not others. These are usually caused by strange
+ firewalling somewhere between the UML and the other box. You track
+ this down by running tcpdump on every interface the packets travel
+ over and see where they disappear. When you find a machine that takes
+ the packets in, but does not send them onward, that's the culprit.
+
+
+
+ 1133..1100.. II hhaavvee nnoo rroooott aanndd II wwaanntt ttoo ssccrreeaamm
+
+ Thanks to Birgit Wahlich for telling me about this strange one. It
+ turns out that there's a limit of six environment variables on the
+ kernel command line. When that limit is reached or exceeded, argument
+ processing stops, which means that the 'root=' argument that UML
+ usually adds is not seen. So, the filesystem has no idea what the
+ root device is, so it panics.
+
+
+ The fix is to put less stuff on the command line. Glomming all your
+ setup variables into one is probably the best way to go.
+
+
+
+ 1133..1111.. UUMMLL bbuuiilldd ccoonnfflliicctt bbeettwweeeenn ppttrraaccee..hh aanndd uuccoonntteexxtt..hh
+
+ On some older systems, /usr/include/asm/ptrace.h and
+ /usr/include/sys/ucontext.h define the same names. So, when they're
+ included together, the defines from one completely mess up the parsing
+ of the other, producing errors like:
+ /usr/include/sys/ucontext.h:47: parse error before
+ `10'
+
+
+
+
+ plus a pile of warnings.
+
+
+ This is a libc botch, which has since been fixed, and I don't see any
+ way around it besides upgrading.
+
+
+
+ 1133..1122.. TThhee UUMMLL BBooggooMMiippss iiss eexxaaccttllyy hhaallff tthhee hhoosstt''ss BBooggooMMiippss
+
+ On i386 kernels, there are two ways of running the loop that is used
+ to calculate the BogoMips rating, using the TSC if it's there or using
+ a one-instruction loop. The TSC produces twice the BogoMips as the
+ loop. UML uses the loop, since it has nothing resembling a TSC, and
+ will get almost exactly the same BogoMips as a host using the loop.
+ However, on a host with a TSC, its BogoMips will be double the loop
+ BogoMips, and therefore double the UML BogoMips.
+
+
+
+ 1133..1133.. WWhheenn yyoouu rruunn UUMMLL,, iitt iimmmmeeddiiaatteellyy sseeggffaauullttss
+
+ If the host is configured with the 2G/2G address space split, that's
+ why. See ``UML on 2G/2G hosts'' for the details on getting UML to
+ run on your host.
+
+
+
+ 1133..1144.. xxtteerrmmss aappppeeaarr,, tthheenn iimmmmeeddiiaatteellyy ddiissaappppeeaarr
+
+ If you're running an up to date kernel with an old release of
+ uml_utilities, the port-helper program will not work properly, so
+ xterms will exit straight after they appear. The solution is to
+ upgrade to the latest release of uml_utilities. Usually this problem
+ occurs when you have installed a packaged release of UML then compiled
+ your own development kernel without upgrading the uml_utilities from
+ the source distribution.
+
+
+
+ 1133..1155.. AAnnyy ootthheerr ppaanniicc,, hhaanngg,, oorr ssttrraannggee bbeehhaavviioorr
+
+ If you're seeing truly strange behavior, such as hangs or panics that
+ happen in random places, or you try running the debugger to see what's
+ happening and it acts strangely, then it could be a problem in the
+ host kernel. If you're not running a stock Linus or -ac kernel, then
+ try that. An early version of the preemption patch and a 2.4.10 SuSE
+ kernel have caused very strange problems in UML.
+
+
+ Otherwise, let me know about it. Send a message to one of the UML
+ mailing lists - either the developer list - user-mode-linux-devel at
+ lists dot sourceforge dot net (subscription info) or the user list -
+ user-mode-linux-user at lists dot sourceforge do net (subscription
+ info), whichever you prefer. Don't assume that everyone knows about
+ it and that a fix is imminent.
+
+
+ If you want to be super-helpful, read ``Diagnosing Problems'' and
+ follow the instructions contained therein.
+ 1144.. DDiiaaggnnoossiinngg PPrroobblleemmss
+
+
+ If you get UML to crash, hang, or otherwise misbehave, you should
+ report this on one of the project mailing lists, either the developer
+ list - user-mode-linux-devel at lists dot sourceforge dot net
+ (subscription info) or the user list - user-mode-linux-user at lists
+ dot sourceforge dot net (subscription info). When you do, it is
+ likely that I will want more information. So, it would be helpful to
+ read the stuff below, do whatever is applicable in your case, and
+ report the results to the list.
+
+
+ For any diagnosis, you're going to need to build a debugging kernel.
+ The binaries from this site aren't debuggable. If you haven't done
+ this before, read about ``Compiling the kernel and modules'' and
+ ``Kernel debugging'' UML first.
+
+
+ 1144..11.. CCaassee 11 :: NNoorrmmaall kkeerrnneell ppaanniiccss
+
+ The most common case is for a normal thread to panic. To debug this,
+ you will need to run it under the debugger (add 'debug' to the command
+ line). An xterm will start up with gdb running inside it. Continue
+ it when it stops in start_kernel and make it crash. Now ^C gdb and
+
+
+ If the panic was a "Kernel mode fault", then there will be a segv
+ frame on the stack and I'm going to want some more information. The
+ stack might look something like this:
+
+
+ (UML gdb) backtrace
+ #0 0x1009bf76 in __sigprocmask (how=1, set=0x5f347940, oset=0x0)
+ at ../sysdeps/unix/sysv/linux/sigprocmask.c:49
+ #1 0x10091411 in change_sig (signal=10, on=1) at process.c:218
+ #2 0x10094785 in timer_handler (sig=26) at time_kern.c:32
+ #3 0x1009bf38 in __restore ()
+ at ../sysdeps/unix/sysv/linux/i386/sigaction.c:125
+ #4 0x1009534c in segv (address=8, ip=268849158, is_write=2, is_user=0)
+ at trap_kern.c:66
+ #5 0x10095c04 in segv_handler (sig=11) at trap_user.c:285
+ #6 0x1009bf38 in __restore ()
+
+
+
+
+ I'm going to want to see the symbol and line information for the value
+ of ip in the segv frame. In this case, you would do the following:
+
+
+ (UML gdb) i sym 268849158
+
+
+
+
+ and
+
+
+ (UML gdb) i line *268849158
+
+
+
+
+ The reason for this is the __restore frame right above the segv_han-
+ dler frame is hiding the frame that actually segfaulted. So, I have
+ to get that information from the faulting ip.
+
+
+ 1144..22.. CCaassee 22 :: TTrraacciinngg tthhrreeaadd ppaanniiccss
+
+ The less common and more painful case is when the tracing thread
+ panics. In this case, the kernel debugger will be useless because it
+ needs a healthy tracing thread in order to work. The first thing to
+ do is get a backtrace from the tracing thread. This is done by
+ figuring out what its pid is, firing up gdb, and attaching it to that
+ pid. You can figure out the tracing thread pid by looking at the
+ first line of the console output, which will look like this:
+
+
+ tracing thread pid = 15851
+
+
+
+
+ or by running ps on the host and finding the line that looks like
+ this:
+
+
+ jdike 15851 4.5 0.4 132568 1104 pts/0 S 21:34 0:05 ./linux [(tracing thread)]
+
+
+
+
+ If the panic was 'segfault in signals', then follow the instructions
+ above for collecting information about the location of the seg fault.
+
+
+ If the tracing thread flaked out all by itself, then send that
+ backtrace in and wait for our crack debugging team to fix the problem.
+
+
+ 1144..33.. CCaassee 33 :: TTrraacciinngg tthhrreeaadd ppaanniiccss ccaauusseedd bbyy ootthheerr tthhrreeaaddss
+
+ However, there are cases where the misbehavior of another thread
+ caused the problem. The most common panic of this type is:
+
+
+ wait_for_stop failed to wait for <pid> to stop with <signal number>
+
+
+
+
+ In this case, you'll need to get a backtrace from the process men-
+ tioned in the panic, which is complicated by the fact that the kernel
+ debugger is defunct and without some fancy footwork, another gdb can't
+ attach to it. So, this is how the fancy footwork goes:
+
+ In a shell:
+
+
+ host% kill -STOP pid
+
+
+
+
+ Run gdb on the tracing thread as described in case 2 and do:
+
+
+ (host gdb) call detach(pid)
+
+
+ If you get a segfault, do it again. It always works the second time.
+
+ Detach from the tracing thread and attach to that other thread:
+
+
+ (host gdb) detach
+
+
+
+
+
+
+ (host gdb) attach pid
+
+
+
+
+ If gdb hangs when attaching to that process, go back to a shell and
+ do:
+
+
+ host%
+ kill -CONT pid
+
+
+
+
+ And then get the backtrace:
+
+
+ (host gdb) backtrace
+
+
+
+
+
+ 1144..44.. CCaassee 44 :: HHaannggss
+
+ Hangs seem to be fairly rare, but they sometimes happen. When a hang
+ happens, we need a backtrace from the offending process. Run the
+ kernel debugger as described in case 1 and get a backtrace. If the
+ current process is not the idle thread, then send in the backtrace.
+ You can tell that it's the idle thread if the stack looks like this:
+
+
+ #0 0x100b1401 in __libc_nanosleep ()
+ #1 0x100a2885 in idle_sleep (secs=10) at time.c:122
+ #2 0x100a546f in do_idle () at process_kern.c:445
+ #3 0x100a5508 in cpu_idle () at process_kern.c:471
+ #4 0x100ec18f in start_kernel () at init/main.c:592
+ #5 0x100a3e10 in start_kernel_proc (unused=0x0) at um_arch.c:71
+ #6 0x100a383f in signal_tramp (arg=0x100a3dd8) at trap_user.c:50
+
+
+
+
+ If this is the case, then some other process is at fault, and went to
+ sleep when it shouldn't have. Run ps on the host and figure out which
+ process should not have gone to sleep and stayed asleep. Then attach
+ to it with gdb and get a backtrace as described in case 3.
+
+
+
+
+
+
+ 1155.. TThhaannkkss
+
+
+ A number of people have helped this project in various ways, and this
+ page gives recognition where recognition is due.
+
+
+ If you're listed here and you would prefer a real link on your name,
+ or no link at all, instead of the despammed email address pseudo-link,
+ let me know.
+
+
+ If you're not listed here and you think maybe you should be, please
+ let me know that as well. I try to get everyone, but sometimes my
+ bookkeeping lapses and I forget about contributions.
+
+
+ 1155..11.. CCooddee aanndd DDooccuummeennttaattiioonn
+
+ Rusty Russell <rusty at linuxcare.com.au> -
+
+ +o wrote the HOWTO <http://user-mode-
+ linux.sourceforge.net/UserModeLinux-HOWTO.html>
+
+ +o prodded me into making this project official and putting it on
+ SourceForge
+
+ +o came up with the way cool UML logo <http://user-mode-
+ linux.sourceforge.net/uml-small.png>
+
+ +o redid the config process
+
+
+ Peter Moulder <reiter at netspace.net.au> - Fixed my config and build
+ processes, and added some useful code to the block driver
+
+
+ Bill Stearns <wstearns at pobox.com> -
+
+ +o HOWTO updates
+
+ +o lots of bug reports
+
+ +o lots of testing
+
+ +o dedicated a box (uml.ists.dartmouth.edu) to support UML development
+
+ +o wrote the mkrootfs script, which allows bootable filesystems of
+ RPM-based distributions to be cranked out
+
+ +o cranked out a large number of filesystems with said script
+
+
+ Jim Leu <jleu at mindspring.com> - Wrote the virtual ethernet driver
+ and associated usermode tools
+
+ Lars Brinkhoff <http://lars.nocrew.org/> - Contributed the ptrace
+ proxy from his own project <http://a386.nocrew.org/> to allow easier
+ kernel debugging
+
+
+ Andrea Arcangeli <andrea at suse.de> - Redid some of the early boot
+ code so that it would work on machines with Large File Support
+
+
+ Chris Emerson <http://www.chiark.greenend.org.uk/~cemerson/> - Did
+ the first UML port to Linux/ppc
+
+
+ Harald Welte <laforge at gnumonks.org> - Wrote the multicast
+ transport for the network driver
+
+
+ Jorgen Cederlof - Added special file support to hostfs
+
+
+ Greg Lonnon <glonnon at ridgerun dot com> - Changed the ubd driver
+ to allow it to layer a COW file on a shared read-only filesystem and
+ wrote the iomem emulation support
+
+
+ Henrik Nordstrom <http://hem.passagen.se/hno/> - Provided a variety
+ of patches, fixes, and clues
+
+
+ Lennert Buytenhek - Contributed various patches, a rewrite of the
+ network driver, the first implementation of the mconsole driver, and
+ did the bulk of the work needed to get SMP working again.
+
+
+ Yon Uriarte - Fixed the TUN/TAP network backend while I slept.
+
+
+ Adam Heath - Made a bunch of nice cleanups to the initialization code,
+ plus various other small patches.
+
+
+ Matt Zimmerman - Matt volunteered to be the UML Debian maintainer and
+ is doing a real nice job of it. He also noticed and fixed a number of
+ actually and potentially exploitable security holes in uml_net. Plus
+ the occasional patch. I like patches.
+
+
+ James McMechan - James seems to have taken over maintenance of the ubd
+ driver and is doing a nice job of it.
+
+
+ Chandan Kudige - wrote the umlgdb script which automates the reloading
+ of module symbols.
+
+
+ Steve Schmidtke - wrote the UML slirp transport and hostaudio drivers,
+ enabling UML processes to access audio devices on the host. He also
+ submitted patches for the slip transport and lots of other things.
+
+
+ David Coulson <http://davidcoulson.net> -
+
+ +o Set up the usermodelinux.org <http://usermodelinux.org> site,
+ which is a great way of keeping the UML user community on top of
+ UML goings-on.
+
+ +o Site documentation and updates
+
+ +o Nifty little UML management daemon UMLd
+ <http://uml.openconsultancy.com/umld/>
+
+ +o Lots of testing and bug reports
+
+
+
+
+ 1155..22.. FFlluusshhiinngg oouutt bbuuggss
+
+
+
+ +o Yuri Pudgorodsky
+
+ +o Gerald Britton
+
+ +o Ian Wehrman
+
+ +o Gord Lamb
+
+ +o Eugene Koontz
+
+ +o John H. Hartman
+
+ +o Anders Karlsson
+
+ +o Daniel Phillips
+
+ +o John Fremlin
+
+ +o Rainer Burgstaller
+
+ +o James Stevenson
+
+ +o Matt Clay
+
+ +o Cliff Jefferies
+
+ +o Geoff Hoff
+
+ +o Lennert Buytenhek
+
+ +o Al Viro
+
+ +o Frank Klingenhoefer
+
+ +o Livio Baldini Soares
+
+ +o Jon Burgess
+
+ +o Petru Paler
+
+ +o Paul
+
+ +o Chris Reahard
+
+ +o Sverker Nilsson
+
+ +o Gong Su
+
+ +o johan verrept
+
+ +o Bjorn Eriksson
+
+ +o Lorenzo Allegrucci
+
+ +o Muli Ben-Yehuda
+
+ +o David Mansfield
+
+ +o Howard Goff
+
+ +o Mike Anderson
+
+ +o John Byrne
+
+ +o Sapan J. Batia
+
+ +o Iris Huang
+
+ +o Jan Hudec
+
+ +o Voluspa
+
+
+
+
+ 1155..33.. BBuugglleettss aanndd cclleeaann--uuppss
+
+
+
+ +o Dave Zarzycki
+
+ +o Adam Lazur
+
+ +o Boria Feigin
+
+ +o Brian J. Murrell
+
+ +o JS
+
+ +o Roman Zippel
+
+ +o Wil Cooley
+
+ +o Ayelet Shemesh
+
+ +o Will Dyson
+
+ +o Sverker Nilsson
+
+ +o dvorak
+
+ +o v.naga srinivas
+
+ +o Shlomi Fish
+
+ +o Roger Binns
+
+ +o johan verrept
+
+ +o MrChuoi
+
+ +o Peter Cleve
+
+ +o Vincent Guffens
+
+ +o Nathan Scott
+
+ +o Patrick Caulfield
+
+ +o jbearce
+
+ +o Catalin Marinas
+
+ +o Shane Spencer
+
+ +o Zou Min
+
+
+ +o Ryan Boder
+
+ +o Lorenzo Colitti
+
+ +o Gwendal Grignou
+
+ +o Andre' Breiler
+
+ +o Tsutomu Yasuda
+
+
+
+ 1155..44.. CCaassee SSttuuddiieess
+
+
+ +o Jon Wright
+
+ +o William McEwan
+
+ +o Michael Richardson
+
+
+
+ 1155..55.. OOtthheerr ccoonnttrriibbuuttiioonnss
+
+
+ Bill Carr <Bill.Carr at compaq.com> made the Red Hat mkrootfs script
+ work with RH 6.2.
+
+ Michael Jennings <mikejen at hevanet.com> sent in some material which
+ is now gracing the top of the index page <http://user-mode-
+ linux.sourceforge.net/index.html> of this site.
+
+ SGI <http://www.sgi.com> (and more specifically Ralf Baechle <ralf at
+ uni-koblenz.de> ) gave me an account on oss.sgi.com
+ <http://www.oss.sgi.com> . The bandwidth there made it possible to
+ produce most of the filesystems available on the project download
+ page.
+
+ Laurent Bonnaud <Laurent.Bonnaud at inpg.fr> took the old grotty
+ Debian filesystem that I've been distributing and updated it to 2.2.
+ It is now available by itself here.
+
+ Rik van Riel gave me some ftp space on ftp.nl.linux.org so I can make
+ releases even when Sourceforge is broken.
+
+ Rodrigo de Castro looked at my broken pte code and told me what was
+ wrong with it, letting me fix a long-standing (several weeks) and
+ serious set of bugs.
+
+ Chris Reahard built a specialized root filesystem for running a DNS
+ server jailed inside UML. It's available from the download
+ <http://user-mode-linux.sourceforge.net/dl-sf.html> page in the Jail
+ Filesysems section.
+
+
+
+
+
+
+
+
+
+
+
+