Compiling the kernel and modules
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:
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.
- Download the latest UML patch from
the download page
In this example, the file is uml-patch-2.4.0-prerelease.bz2.
- Download the matching kernel from your favourite kernel mirror,
- Make a directory and unpack the kernel into it.
tar -xjvf linux-2.4.0-prerelease.tar.bz2
- Apply the patch using
bzcat uml-patch-2.4.0-prerelease.bz2 | patch -p1
- 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
this page for the scoop on
running UML on your system.
- Finish with `make linux ARCH=um': the result is a file called `linux'
in the top directory of your source tree.
You may notice that the final binary is pretty large (many 10's of
megabytes for a debuggable UML). This is almost entirely symbol
information. The actual binary is comparable in size to a native
kernel. You can run that huge binary, and only the actual code and
data will be loaded into memory, so the symbols only consume disk
space unless you are running UML under gdb. You can strip UML:
host% strip linux
to see the true size of the UML kernel.
The sources are also available from cvs. You can browse the CVS pool
or access it anonymously via
If you get the CVS sources, you will have to check them out into an
empty directory. You will then have to copy each file into the corresponding
directory in the appropriate kernel pool.
If you don't have the latest kernel pool, you can get the corresponding
user-mode sources with
host% cvs co -r v_2_3_x linux
where 'x' is the version in your pool. Note that you will not get the bug
fixes and enhancements that have gone into subsequent releases.
If you build your own kernel, and want to boot it from one of the
filesystems distributed from this site, then, in nearly all cases,
devfs must be compiled into the kernel and mounted at boot time. The
exception is the tomsrtbt filesystem. For this,
devfs must either not be in the kernel at all, or "devfs=nomount" must
be on the kernel command line. Any disagreement between the kernel
and the filesystem being booted about whether devfs is being used will
result in the boot getting no further than single-user mode.
If you don't want to use devfs, you can remove the need for it from a
filesystem by copying /dev from someplace, making a bunch of
for i in 0 1 2 3 4 5 6 7; do mknod ubd$i b 98 $[ $i * 16 ]; done
and changing /etc/fstab and /etc/inittab to refer to the non-devfs devices.
Compiling and installing kernel modules
UML modules are built in the same way as the native kernel (with the
exception of the 'ARCH=um' that you always need for UML):
host% make modules ARCH=um
Any modules that you want to load into this kernel need to
be built in the user-mode pool. Modules from the native kernel won't
work. If you notice that the modules you get are much larger than
they are on the host, see the note above about the size of the final
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
If you can't mount the root filesystem on the host for some reason
(like it's a COW file), then an alternate approach is to mount the
UML kernel tree from the host into the UML with hostfs and run the modules_install inside
with the kernel not booted, mount the root filesystem in the top level
of the kernel pool:
host% mount root_fs mnt -o loop
make modules_install INSTALL_MOD_PATH=`pwd`/mnt ARCH=um
unmount the filesystem
host% umount mnt
boot the kernel on it
The depmod at the end may complain about unresolved symbols because
there is an incorrect or missing System.map installed in the UML
filesystem. This appears to be harmless. insmod or modprobe should
work fine at this point.
With UML booted, mount the host kernel tree inside UML at the same
location as on the host:
UML# mount none -t hostfs path to UML pool -o
path to UML pool
Run make modules_install:
UML# cd path to UML pool ; make modules_install
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 know, and they'll be "taken care of".
If you try building an external module against a UML tree, you will
find that it doesn't compile because of missing includes. There are
less obvious problems with the CFLAGS that the module Makefile or
script provides which would make it not run even if it did build. To
get around this, you need to provide the same CFLAGS that the UML
kernel build uses.
A reasonably slick way of getting the UML CFLAGS is
cd uml-tree ; make script 'SCRIPT=@echo $(CFLAGS)' ARCH=um
If the module build process has something that looks like
$(CC) $(CFLAGS) file
then you can define CFLAGS in a script like this
CFLAGS=`cd uml-tree ; make script 'SCRIPT=@echo $(CFLAGS)' ARCH=um`
and like this in a Makefile
CFLAGS=$(shell cd uml-tree ; make script 'SCRIPT=@echo
Compiling and installing uml_utilities
Many features of the UML kernel require a user-space helper program,
so a uml_utilities package is distributed separately from the kernel
patch which provides these helpers. Included within this is:
The uml_utilities tree is compiled with:
port-helper - Used by consoles which connect to xterms or ports
tunctl - Configuration tool to create and delete tap devices
uml_net - Setuid binary for automatic tap device configuration
uml_switch - User-space virtual switch required for daemon transport
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