linux [options ...]
This manual page briefly documents User-mode Linux.
This manual page was written for the Debian GNU/Linux distribution because the original program does not have a manual page. Check linux --help for an up-to-date synopsis.
This controls how much "physical" memory the kernel
allocates for the system. The size is specified as a
number followed by one of 'k', 'K', 'm', 'M', which have
the obvious meanings. This is not related to the amount
of memory in the physical machine. It can be more, and the
excess, if it's ever used, will just be swapped out.
Configure file as a named IO
memory region named name.
gdb-pid is used to attach an external debugger to UML.
This may be an already-running gdb or a debugger-like
process like strace.
Causes the tracing thread to pause until it is attached by
a debugger and continued. This is mostly for debugging
crashes early during boot, and should be pretty much
obsoleted by the debug switch.
This makes UML put process stacks in the same location as
they are on the host, allowing exploits such as stack
smashes to work against UML.
Starts up the kernel under the control of gdb. See the
kernel debugging tutorial and the debugging session pages
in the user-mode-linux-doc package for more information.
This is actually used by the generic kernel in exactly the
same way as in any other kernel. If you configure a number
of block devices and want to boot off something other than
ubd0, you would use something like "root=/dev/ubd5". Another
notation is the use of the major and the minor number of the
device, i.e. root=98:0, for ubd0.
To use hostfs for the root filesystem, use the syntax
"root=/dev/root rootflags=/ rootfstype=hostfs"
This is used to assign a unique identity to this UML
machine This is used for naming the pid file and
management console socket. I.e. to access to the management
console for an already running image (i.e. umid=test), just
launch uml_mconsole test from the host command line.
The location to place the pid and umid files.
This is used to boot UML from an initrd image. The
argument is the name of the file containing the image.
Enables the protection of kernel memory from processes
Attach a console to a host channel. Examples of channels
include ttys, ptys, pts terminals, xterms, and file
This will make UML attach the device to the
specified tty. 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.
This will cause UML to allocate a free host pty for
the device. The pty will be announced in the boot
log. You would attach to it via the corresponding
- pts terminal
This is similar to pty above, but the specified UML
device will be attached to a free pts device on the
host. Look at the boot log to see which one.
UML will run an xterm and the device will be
attached to it.
- file descriptors
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.
You can also specify different input and output
channels for a device by putting a comma between them:
up Serial Lines and Consoles (link to URL file:///usr/share/doc/user-mode-linux-doc/html/input.html) for a more information.
Attach a serial line to a host channel. The same options
are supported as for consoles, described above.
Create ide0 entries that map onto ubd devices.
This is used to associate a device with a file or in the
underlying filesystem. Usually, there is a filesystem in
the file, but that's not required. Swap devices
containing swap files can be specified like this. Also, a
file which doesn't contain a filesystem can have its
contents read in the virtual machine by running dd on the
device. Appending an 'r' will cause that device to be
mounted read-only. Appending an 's' tells UML to open the
file using O_SYNC (synchronous I/O).
The ubd option can no longer be used to specify a hostfs
root filesystem. See the "root" option for a more correct
This option is here solely to catch ubd -> udb typos, which can be
to impossible to catch visually unless you specifically look for
them. The only result of any option starting with 'udb' is an error
in the boot output.
Change the ubd device name to "hd", allowing programs
within UML to access UBD devices as if they were normal
Configure a network device. Formats and examples follow (one
for each configured transport).
When both CONFIG_MODE_TT and CONFIG_MODE_SKAS are enabled,
this option forces UML to run in tt (tracing thread) mode.
It is not the default because it's slower and less secure
than skas mode.
Disables SKAS3 usage, so that SKAS0 is used, unless you
specify mode=tt. Note that this was recently added - on
older kernels you must use simply "skas0".
Disables SKAS3 usage, so that SKAS0 is used, unless
you specify mode=tt.
Requests that the mconsole driver send a message to the named Unix
socket containing the name of the mconsole socket. This also
serves to notify outside processes when UML has booted far enough
to respond to mconsole requests.
This is used to force UML to use 2.4-style AIO even when 2.6 AIO is
available. 2.4 AIO is a single thread that handles one request at
a time, synchronously. 2.6 AIO is a thread which uses the 2.6 AIO
interface to handle an arbitrary number of pending requests. 2.6
AIO is not available in tt mode, on 2.4 hosts, or when UML is built
with /usr/include/linux/aio_abi.h not available. Many distributions
don't include aio_abi.h, so you will need to copy it from a kernel
tree to your /usr/include/linux in order to build an AIO-capable
This is used to set hostfs parameters. The root directory argument
is used to confine all hostfs mounts to within the specified
directory tree on the host. If this isn't specified, then a user
inside UML can mount anything on the host that's accessible to the
user that's running it.
The only flag currently supported is 'append', which specifies that
all files opened by hostfs will be opened in append mode.
- xterm=terminal emulator,title switch,exec switch
Specifies an alternate terminal emulator to use for the debugger,
consoles, and serial lines when they are attached to the xterm
channel. The values are the terminal emulator binary, the switch
it uses to set its title, and the switch it uses to execute a
subprocess, respectively. The title switch must have the form
exec switch must have the form '<switch> command arg1 arg2
The default values are 'xterm=xterm,-T,-e'. Values for
gnome-terminal are 'xterm=gnome-terminal,-t,-x'.
Turns off usage of PTRACE_LDT, even if host supports it.
To support PTRACE_LDT, the host needs to be patched using the
current skas3 patch.
Turns off usage of PTRACE_FAULTINFO, even if host supports it. To
support PTRACE_FAULTINFO, the host needs to be patched using the
current skas3 patch.
Turns off usage of /proc/mm, even if host supports it.
To support /proc/mm, the host needs to be patched using the current
Turns off syscall emulation patch for ptrace (SYSEMU) on.
SYSEMU is a performance-patch introduced by Laurent Vivier. It
changes behaviour of ptrace() and helps reducing host context
switch rate. To make it working, you need a kernel patch for your
See http://perso.wanadoo.fr/laurent.vivier/UML/ for further
Prints syntax information.
Prints the version number of the kernel.
Prints the configuration used to build the kernel.
To print all the options used to build the "linux" uml kernel
and save them in "config_file" you can use
linux --showconfig > config_file
User-mode Linux was written by Jeff Dike and others.
This manual page was written by Matt Zimmerman [email protected] for
the Debian GNU/Linux system, based on linux --help and the user-mode-linux website.