Configuring User Mode Linux [UML] and
an environment to Linux kernel
development and debugging


Copyright (c) 2007 Rodrigo Ferreira Baroni .
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License".

A Virtual Machine is a term that has different meanings in Computer Science. This page refers to how to configure and build a virtual machine with the meaning of a fake computer created by software, being executed in the form of application in the user space, inside of a real GNU/Linux system. Such software that simulates the fake computer described in this document uses operating system level virtualization; its made by 2 components:

1) An operating system executing in the user space - an appropriate kernel (and User-Mode-Linux is such project that develops this kernel); and
2) Programs in user space - available in a file system.

This document describes the steps to build such components and how to integrate them, getting at the end a virtual machine GNU/Linux inside of the main one running in the computer. We used in the building of the file system a Debian tool name 'debbootstrap', but such tool can be installed in others systems as well - described at the end of this page, allowing this doc be used by any GNU/Linux distro. After the creation of the virtual machine, is described how used it to develop and debug the Linux kernel, and the develop environment with the steps to integrate the kernel changes with the tests and the debug phase.

This document is organized in 4 parts:


The last update of this document dates from April 14, 2007 - attention to dates, because configurations changes with time and the documentations becomes inapplicable (and there is already a lot of documentations that in the Internet about configuring a virtual machine using UML that are inapplicable). Contributions are always welcome, if you find bugs/suggestions and still time and disposal, send it through baroni@ime.usp.br

Thanks to everybody that helped, the professor Roberto Hirata from IME-USP that in the Operating System course at the institute allowed and stimulated the writing of this document while I was his student-assistant in the course, to IME-USP and CAPES by the opportunities that allowed the relation with this study, and all developers that helped to build the GNU/Linux platform, base of serious computing and with warranty with the preservation of the free access to the knowledge of science and technology, including the ones related with the User-Mode-Linux project!


Part I - Creating and configuring an UML kernel

The first step is create the UML kernel. An UML kernel is a kernel that every communication that would be done with the physic devices to control and manage them are modified, in such a way that the physic resources be available through a traditional kernel being executed in the computer. In the beginning was a project available in the form of patches, actually (since the version 2.6.9) is available together with the main kernel (patches no more needed - exceptions to the blaisorblade patches that add some extra improvements). Following below the proceedings needed to generate an UML kernel.


  • 1) Downloading the Linus's kernel tree with git
  • Install the git-core package (debian users: apt-get install git-core). Go to the desired directory to put the kernel sources (/usr/src/ traditionally):

    $ cd /usr/src/ 
    Download the tree:
    git-clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git linux-2.6-linus-gittree 
    This will create the linux-2.6-linus-git/ tree (in /usr/src/). This tree has the full historic of the kernel development since 2.6.11, and is possible to see what changed in each file executing: '$ gitk kernel/fork.c' (for example, to see the changes made in such file, or just '$ gitk' to see the full historic of all files). To keep the sources of the tree updated with its development, run the command: '$ git-pull' inside of such directory regularly.


  • 2) Extract a stable version of the tree
  • Inside of the directory .git/refs/tags/ there are files that point to marks in the tree, corresponding to the state of the tree when in the release of the versions: Go the the directory:

    $ cd /usr/src/linux-2.6-linus-gittree
    See the existent marks:
    $ ls .git/refs/tags 
    Pick a desired tree more recent that have UML support - to see witch one is this, look at the address of the project relative its releases: http://user-mode-linux.sourceforge.net/work/current/2.6/ and look for the last version (at the date that this doc was made - April 02 2007 - the last version with available patches is 2.6.21-rc1), and so we'll get such one.
    $ git-archive --format=tar --prefix=v2.6.21-rc1.uml/ v2.6.21-rc1 | (cd ../ && tar xf -) 
    This will extract the last candidate tree to the stable release (at the moment of this writing) and top of development - generating the directory v2.6.21-rc1.uml at the directory below (/usr/src/v2.6.21-rc1.uml).


  • 3) Compile the UML kernel:
  • Clean the old configurations (when exist - in the case you want 'clean' the tree after make different configurations and compile it several times, taking the tree back to its original state):

    $ make mrproper 
    $ make mrproper ARCH=um 
    To generate an appropriate .config to the UML (the .config file is the one that defines the configurations of the kernel):
    $ make defconfig ARCH=um 
    To configure the kernel, run its graphic interface with the command below. Install the ncurses-dev (apt-get libncurses5-dev if apt is available). Important: don't change the configuration about the kind of the processor (leave the default one), and also enable the option to compile the kernel with frame pointers.
    $ make menuconfig ARCH=um 
    And so try to compile the kernel:
    $ make linux ARCH=um 
    If you got some error, see how to fix errors trying to compile the kernel at the end of this doc

    Compile the modules:

    $ make modules ARCH=um 
    This will generate the executable kernel files "linux" and "vmlinux" at the root of kernel sources directory. You can try to run the kernel now, waiting for it to stop its execution with error when it try to mount the file system with the command below:
    $ ./vmlinux
    If you get some error saying about file system missing, everything is ok, there is no more thing to configure to the kernel than its file system - next part. Despite that, if you got the following error below:
    PROT_EXEC mmap in /dev/shm/...failed: Operation not permitted
    It's because the security restrictions of the system that do not allow to be executed in shared memory. Such restriction is resolved configuring another temporary file to be used: first, create such directory:
    $ mkdir /tmp/uml
    After that, change the permissions making the user owner of the kernel (run '$ man chmod' to know more) the owner of such directory created (change 'user' by the user owner of the kernel)
    $ chown user.user /tmp/uml
    Give full permissions to such directory:
    $ chmod 777 /tmp/uml
    And finally export the system variable that says what is the temporary directory:
    $ export TMPDIR=/tmp/uml


Part II - Creating and configuring a file system

Now build a file system to be used with the UML kernel. Following below the steps needed.

  • 1) Installing an Debian system in a directory
  • Create a directory (here, named "root_fs") and, to keep related things together, create such system inside of the uml kernel sources:

    $ cd /usr/src/v2.6.21-rc1.uml/
    $ mkdir root_fs
    With the debootstrap install a Debian system in such directory with the command below (if using a Debian system at the main machine, install it via 'apt-get install debootstrap', or see how to download and configure the deboostrap at the end of this document). This will create a Debian system testing version in the directory "root_fs/" (of 167Mb)(at the moment, etch is the testing release, but see at www.debian.org/releases/ if it changes or if you want another one):
    $ debootstrap etch root_fs/ http://ftp.debian.org/debian/

    If using an AMD-64 processor in your computer, substitute the above command by the below one:
    $ debootstrap --arch amd64 sarge root_fs/ http://amd64.debian.net/debian-amd64/


  • 2) Creating a file system in a file and setting its configurations to its use by the UML kernel
  • The kernel needs a __file__ of a file system, from where it will access. To make this one, first create a file with 300MB (leaving ~100MB of free space) in the directory of the kernel:

     $ cd /usr/src/v2.6.21-rc1.uml/ 
     $ dd if=/dev/zero of=linux_fs.ext3 bs=1M count=200
    Associate a loopback device with the file system's file. See what devices are available with the command '$ losetup -f' (running as root). The use of the loopback device is necessary to the layout of the file system (its meta data) be correctly inserted in the file system's file:
     $ losetup /dev/loop1 linux_fs.ext3 
    Create a file system in the device (that was associated with the file):
     $ mkfs.ext3 -j /dev/loop1 
    Mount the file system of the file in a directory to access it:
     $ mkdir /mnt/vmfs
     $ mount -t ext3 -o loop linux_fs.ext3 /mnt/vmfs 
    Copy the files of the Debian distro to your file system created in the file:
     $ cp -dpR root_fs/* /mnt/vmfs/ 
    Now install the modules of the UML kernel to the appropriated directories (/lib/modules) in the file system to be used by it (still inside of the UML kernel directory):
     $ make modules_install INSTALL_MOD_PATH=/mnt/vmfs/ ARCH=um 
    Now configure the system starting from some of its configuration files. First, edit the /mnt/vmfs/etc/initab file with a text editor, leaving only the first line:
     1:2345:respawn:/sbin/getty 38400 tty1
    Now edit the file /mnt/vmfs/etc/fstab:
    #<file system> <mount point>   <type>  <options>     <dump>   <pass>
    /dev/ubd0        /		ext3    defaults        0	0
    /dev/ubd1	none		swap	sw		0	0
    proc            /proc		proc    defaults        0	0 
    This says that the files /dev/ubd{0,1} of the main system will be used by the UML kernel as base to mount its file system. Such files can be created automatically at the moment of its use by devfs - in the main computer system, configure the udev inteface, setting the ubdX devices editing the file /etc/udev/links.conf (in the Debian system - in others see the udev manual of your GNU/Linux distro), as above:
    M ubd0 b 98 0
    M ubd1 b 98 16
    M ubd2 b 98 32
    M ubd3 b 98 48
    M ubd4 b 98 64
    M ubd5 b 98 80
    M ubd6 b 98 96
    M ubd7 b 98 112 
    or can also be created manually if desired - go the directory where the files associated with devices are located ("/dev/") and create the files with the command below:
     $ cd /dev 
     $ for i in 0 1 2 3 4 5 6 7; do mknod ubd$i b 98 $[ $i * 16 ]; done 
    Create a file named 'swap_file' to be used as swap device by the UML (of 200M here):
     $ dd if=/dev/zero of=swap_file count=200 bs=1M
     $ mkswap swap_file 
    Finally umount the file system designated to the virtual machine:
     $ umount /mnt/vmfs 
    Disassociate it from the loopback device:
     $ losetup -d /dev/loop1 
    Make the owner of the file system's file the same one of the UML kernel ('user' may be the owner of the kernel):
     $ chown user.user linux_fs.ext3 
    This way, the linux_fs.ext3 file will be the file system.


  • 3) Configuring the network of the virtual machine
  • To configure a network connection in the virtual machine we'll use the tun/tap method. Tun/tap is a connection that associates an interface with a IP or ETHERNET address to a device (/dev/net/tun), in such a way that when a process writes a package to such interface address this will be write at the /dev/net/tun device, that will send the package to the kernel network management layer, that will manage it as a normal network packet coming from that address associated. This way, tun/tap presents as a useful resource to communicate processes in the user space - see Documentation/networking/tuntap.txt in the kernel tree to know more. It's needed to enable the support to tun/tap at the main system kernel (see that checking if "CONFIG_TUN=y" in its .config file, otherwise you should recompile your kernel with such option).

    A program is needed to be used, to configure the device /dev/net/tun at the programming level, using IOCTL calls. We'll use an available program from the cvs of the UML project - 'tunctl'. Get it:

    $ mkdir /usr/src/uml-project/
    $ cd /usr/src/uml-project/
    $ cvs -d:pserver:anonymous@user-mode-linux.cvs.sourceforge.net:/cvsroot/user-mode-linux login (no password)
    $ cvs -d:pserver:anonymous@user-mode-linux.cvs.sourceforge.net:/cvsroot/user-mode-linux checkout tools
    $ cd tools/tunctl
    $ make
    

    There will be 3 distinct interfaces:
    1) eth0 interface in the main system: the internal network of the main system. To illustration this, we'll use the address 192.168.0.1 at the network 192.168.0.[1-7], netmask 255.255.255.248, using as gateway some another interface (e.g. eth1, eth0:1, ...) having some valid Internet IP.
    2) eth0 interface in the virtual machine with address 192.168.1.2 at network 192.168.1.[1-7], netmask 255.255.255.248, using as gateway the interface that follows (3):
    3) tap0 interface in the main system with address 192.168.1.1, at the network 192.168.1.[1-7], netmask 255.255.255.248, having as gateway the above (2) interface.

    The network connection between the virtual machine and the main system happens through the tap0 interface: when some packet is write at the eth0 of the virtual machine, it is send to the tun/tap interface at the main system, and vice-versa.

    The network configurations in a Debian system are defined by the default in the file /etc/network/interface, that is read by the /etc/init.d/networking script executed by the command '$ /etc/init.d/networking <start | restart | stop>' or by the 'ifup' and 'ifdown' programs. The interfaces (1) and (3) of the main system are defined editing such file as follows: (change the 'user' by some restrict user in the system):

    #/etc/network/interfaces
    
    auto lo  eth0
    
    iface lo inet loopback
    
    iface eth0 inet static
    	address 192.168.0.1
    	network 192.168.0.248
    	netmask 255.255.255.248
    	broadcast 192.168.0.248
    	gateway 192.168.0.1
    
    iface tap0 inet static
    	address 192.168.1.1 
    	netmask 255.255.255.248
    	pre-up /usr/src/uml-project/tools/tunctl/tunctl -u $( cat /etc/passwd | grep user | cut -f 3 -d :)
    	down /usr/src/uml-project/tools/tunctl/tunctl -d tap /dev/net/tun
    	gateway 192.168.1.2
    
    This way, when you desire to enable the tap0 interface in the main system (that must be done before to start the virtual machine execution), run 'ifup tap0', that so such file will be read and the definitions will be made and the commands preceding by 'up/down/pre-up' will be executed properly.

    Now to the virtual machine system, mount your file system and configure the interface (2) editing your configuration file (/mnt/vmfs/etc/network/interfaces) defined as follows:

     $ mount -t ext3 -o loop linux_fs.ext3 /mnt/vmfs 
    #/etc/network/interfaces - (uml virtual machine "vm001")
    auto lo  eth0
    
    iface lo inet loopback
    
    iface eth0 inet static
            address 192.168.1.2
            network 192.168.1.248
            netmask 255.255.255.248
            broadcast 192.168.1.248
            gateway 192.168.1.1
    
    Edit the /etc/hosts file of the virtual machine (in /mnt/vmfs/etc/hosts):
    #/etc/hosts - network configuration (uml virtual machine "vm001")
    127.0.0.1	localhost
    192.168.1.2	vm001
    192.168.0.1     motherhost
    
    And setting the hostname (named "vm001" here):
    $ echo "vm001" > /mnt/vmfs/etc/hostname
    
    Umount the file system destined to the virtual machine:
     $ umount /mnt/vmfs 

    If desire to open the access of the virtual machine (192.168.1.2) to the Internet through the network interface of the main system with the Internet IP ($EXTERN_IP here), use the iptables to create a network addresses table (NAT):

     $ echo 1 > /proc/sys/net/ipv4/ip_forward
     $ iptables -t nat -A POSTROUTING -s 192.168.1.2 -d ! 192.168.0.0/29 -o eth0 -j MASQUERADE
     $ iptables -I FORWARD -s 192.168.1.2 -m state --state ! INVALID -j ACCEPT
    
    And if desired to access it through the Internet (e.g. ssh service running at the virtual machine port 22), open a port in the main system (e.g. 1020 here) and redirect the connections going to this port coming from Internet to the virtual machine at port 22 (set EXTERN_IP properly):
     $ iptables -t nat -I PREROUTING -d ${EXTERN_IP} -p tcp --dport 1020 -j DNAT --to 192.168.1.2:22
     $ iptables -I FORWARD -p tcp -d 192.168.1.2 --dport 22 -j ACCEPT
    


Part III - The virtual machine: running it, and the environment to develop and debug the Linux kernel

This part describes the proceedings to use the virtual machine made with the UML kernel and the file system created.
Such virtual machine has been used a lot by developers of the Linux kernel to test their changes, what in another way (when using as the kernel of the main system) each bug mostly breaks the whole system, bringing the needed to reboot the machine every time. Besides that, it's also possible to debug the kernel, and follow the execution flow of the Linux operating system :)! Let's see how:

  • 1) Using the UML virtual machine:
  • Now that the UML kernel and the file system were created, enable the tap0 interface of the main system to be ready to the connection with the virtual machine:

     $ ifup tap0 
    Finally execute the kernel and start the virtual machine with the command:
     $ ./linux  ubd0=linux_fs.ext3 ubd1=swap_file mem=128M con0=fd:0,fd:1 con=xterm ssl=xterm eth0=tuntap,tap0 umid=vm001 
    If you had created the files /dev/ubdX in the main system manually, say to disable the use of the devfs inserting the 'devfs=none' string in the command line above.

    And so the virtual machine will start its execution, displaying the same screen that normal boot of a Linux kernel in a computer, and a xterm window will be opened with the login prompt. Access it as root (no password needed - set it with 'passwd' command after login)

    The Fedora Core 5 has presented a bug when running the UML kernel - freezing this one, right after it mounted its file system. If that is your case, create a new kernel to the main system (get the sources, compile and install it).

    You must have a kernel in the main system newer than 2.6.16-rc6 - if not, the following error message at startup time is got: "Kernel panic - not syncing: handle_trap - failed to wait at end of syscall, errno = 0, status = 2943".

    To shutdown the virtual machine, run:

     $ shutdown -h now 


  • 2) The Linux kernel develop environment
  • There are 3 main ways to make changes in the Linux kernel:

    • A) Creating/changing code in the Linux kernel sources
    • Start the development, change or apply patchs to test in the UML kernel sources. After the changes, compile your UML kernel again:

      $ make linux ARCH=um 

    • B) Creating/changing code of modules in the Linux kernel sources
    • If you are modifying some module, will be needed to compile the module and copy it to the file system of the virtual machine:

       $ make modules ARCH=um
       $ mount -t ext3 -o loop linux_fs.ext3 /mnt/vmfs/
       $ make modules_install INSTALL_MOD_PATH=/mnt/vmfs/ ARCH=um
       $ umount /mnt/vmfs/ 

    • C) Creating code to build an extern module, outside of the Linux kernel sources
    • Create a directory somewhere (e.g.: ~/devel/my_modules), and so start creating an source file to write the module. As example will be used the file "procinfo.c" to build an extern module that prints information of the processes running in the system.

      To this case, you must have in mind to what kernel version this module will be used, and have available the sources of such kernel (named 'target' here). To compile the source code of the module and improve the process, we create a Makefile as follows (change the KERNELDIR value to point to the path of the 'target' kernel sources)

      # Makefile
      
      obj-m := procinfo.o
      module-objs := procinfo.o
      
      KERNELDIR = /usr/src/v2.6.21-rc1.uml/
      
      default:
              make -C $(KERNELDIR) M=$(shell pwd) modules ARCH=um
      
      clean:
              rm -f  rm -f procinfo.o procinfo.ko procinfo.mod.c procinfo.mod.o Module.symvers
      And so, compile it:
       $ make 
      Mount the file system used by the UML kernel to copy the module created
      $ mount -t ext3 -o loop linux_fs.ext3 /mnt/vmfs/
      $ cp /path/to/the/module/created/procinfo.ko /mnt/vmfs/usr/src/
      $ umount /mnt/vmfs/ 
      After start the virtual machine, insert the module with 'insmod /usr/src/procinfo.ko' and remove it with 'rmmod procinfo.ko' (as root). If your virtual machine freezes, see if there isn't instances of the virtual machine hanging in the system as zombie processes, what can gives error message about resource unavailable (errno 11). Kill the processes after restart the virtual machine with any possible crash:
       $ pkill -9 linux 


  • 3) The Linux kernel debug environment
  • If you wanna debug errors, or the execution flow of the Linux kernel, you can use the GNU gdb. To debug modules additional steps are needed, and so, described in separated sections:

    • A) Kernel debug
    • Start the UML kernel at gdb:

       $ gdb ./linux 
      It's needed to inform the debugger to ignore the signals SIGUSR1 and SIGSEGV received:
       (gdb) handle SIGSEGV pass nostop noprint 
       (gdb) handle SIGUSR1 pass nostop noprint 
      Now insert a breakpoint at 'start_kernel' kernel function:
       (gdb) b start_kernel 
      And start the execution of the kernel in the GNU debugger:
       (gdb) r ubd0=linux_fs.ext3 ubd1=swap_file mem=128M con0=fd:0,fd:1 con=xterm ssl=xterm eth0=tuntap,tap0 umid=vm001
      If you desire to stop the execution of the kernel in the gdb, a lt;CTRL-C> irá will kill the own gdb because the terminal mode is RAW, so, the method to be used is to open another terminal (xterm) and send a SIGINT signal (asking for an INTerruption of its execution) to the main thread of the UML kernel (change '$(whoami)' by the user running the kernel):
       $  kill -INT $(cat /home/$(whoami)/.uml/vm001/pid) 
      And now we can continue the debug in the gdb (see below some examples of commands). Type 'c' (continue) to continue the normal execution of the kernel. Again, kill the previous pending processes always as needed by some crash:
       $ pkill -9 linux gdb port-helper 


    • B) Modules debug
    • To debug modules as soon as they are loaded and follow its execution we need to add its symbols in the gdb symbol table right in the moment that it is loaded and pointed by the global 'modules' pointer, that represents a list of modules loaded in the kernel, and point the location that it was loaded (memory address). To make these proceedings automatically, 3 files were created (you must change them properly with respect to the paths of the files used):

      1) umlgdb_dispatcher: will open an xterm to start a gdb running the kernel, and send a SIGINT to the kernel to stop it and bring the control back to the gdb when typed <ENTER>. <CTRL-C> will kill gdb and the kernel, exiting.
      2) umlgdb_wrapper: load the module symbols automatically as soon as it is inserted with the 'insmod' command in the virtual machine and stops the kernel in the gdb at the point of the call to the module init function. Such file was based in the original Chandan Kudige 'umlgdb' tool, available in the 'tools' directory of the UML project.
      3) .gdbinit: to be put in the UML kernel directory (/usr/src/v2.6.21-rc1.uml if following the part I); it will be read automatically by the gdb always that this one be executed in this directory. To start the UML kernel under gdb, gives execution permission to the umlgdb_dispatcher file, and run it:

       $ chmod +x umlgdb_dispatcher 
       $ ./umlgdb_dispatcher 
      What happens is that when some user insert a module with 'insmod' command inside of the virtual machine, the gdb will break, load the module symbol table at the memory address that the the module was loaded, and stop with a 's'(tep) command in the gdb prompt at the line that calls 'mod->init()', stepping to the first line of code of the module in it's init function when the user type <ENTER> in the gdb prompt - a screenshot displaying the execution of the umlgdb_dispatcher..


Some commands of the GNU gdb:

  Insert a breakpoint in a line of some file ...............................[b]reak x.c:105
  Insert a breakpoint in a function.........................................[b]reak some_function
  Insert a temporary breakpoint in a function  .............................tbreak another_function
  List the breakpoints made ................................................[i]nfo [b]reakpoints
  Disable a breakpoint (the 2nd listed) ....................................disable 2
  Don't stop at the 2nd breakpoint at the next 8 times .....................ignore 2 8
  Delete the 2nd breakpoint ................................................[d]elete 2
  Execute just the actual line of the program  .............................[s]tep
  If the next line is a function, run it, and stop at next line ............[n]ext
  Run 'next' to all functions calls until the end of the actual function....finish
  Continue the execution (up to the next breakpoint)........................[c]continue
  List 10 or more line of codes around the actual line .....................[l]ist
  Exit from GDB ............................................................[q]uit 

To get more information about debug and UML kernel under gdb, see:


Part IV - Solutions to eventual problems

  • Installing debootstrap in systems others than Debian

  • The debootstrap program is part of the Debian system, it's some shell scripts that use the glibc (and so, must be installed), and the method to install it in another systems is as follows:

    • 1) Copy the package to the directory /usr/local/src:
    •  $ cd /usr/local/src/ 
       $ wget http://ftp.debian.org/debian/pool/main/d/debootstrap/debootstrap-udeb_0.3.3_i386.udeb 
    • 2) Unpack the binary files from this one ('ar' is one of the binaries that completes the 'binutils' package).
    •  $ ar -x debootstrap-udeb_0.3.3_i386.udeb 
    • 3) Install coping the binaries to the default path of the system (/usr/bin), running as root:
    •  $ cd / 
       $ zcat /usr/local/src/work/data.tar.gz | tar xv 


  • Errors at UML kernel compile time

  • This section describes how to fix errors at the kernel compile time. For example, if you was using the 2.6.20-rc1 kernel, you may get the following error:

    [...]
    CC      mm/slab.o
    mm/slab.c:3557: error: conflicting types for ‘kmem_ptr_validate’
    include/linux/slab.h:58: error: previous declaration of ‘kmem_ptr_validate’ was here
    make[1]: ** [mm/slab.o] Erro 1
    make: ** [mm] Erro 2
    $
    
    So let's look if such error was already fixed, checking the patches - get the 'patches.tar' file available at:
    $ wget http://user-mode-linux.sourceforge.net/work/current/2.6/2.6.20-rc1/patches.tar 
    
    Extracting will create a 'patches/' directory with the patches inside:
    $ tar xv patches.tar 
    
    See if there is some patch that has something related to the same file in the error message - slab.c:
     $ cd patches/
     $ grep -n Index: *  | grep slab.c
    fastcall:4:Index: linux-2.6.17/mm/slab.c
    
    Checking the file at the line number pointed, and analyzing the error, wee see that this patch ('fastcall' file) refers to the fix of the error (mm/slab.c:3557: error: conflicting types for #kmem_ptr_validate#):
    $  cat patches/fastcall 
    # There's a mismatch between the definition and declaration of
    # kmem_ptr_validate.  This removes the "fastcall" from the definition.
    
    Index: linux-2.6.17/mm/slab.c
    ===================================================================
    --- linux-2.6.17.orig/mm/slab.c 2006-12-14 23:01:47.000000000 -0500
    +++ linux-2.6.17/mm/slab.c      2006-12-14 23:07:00.000000000 -0500
    @@ -3553,7 +3553,7 @@ EXPORT_SYMBOL(kmem_cache_zalloc);
      *
      * Currently only used for dentry validation.
      */
    -int fastcall kmem_ptr_validate(struct kmem_cache *cachep, const void *ptr)
    +int kmem_ptr_validate(struct kmem_cache *cachep, const void *ptr)
     {
            unsigned long addr = (unsigned long)ptr;
            unsigned long min_addr = PAGE_OFFSET;
    $
    
    So, apply such patch at the kernel sources directory:
    $ cd /usr/src/v2.6.20-rc1.uml
    
    $ cat patches/fastcall | patch -p1
    
    And try to recompile the kernel:
    $ make linux ARCH=um 
    
    If you get another error, do the same check in the 'patches.tar' file, and apply them as necessary, and continue to compile the kernel.