LINUX.UML(1)

📦
version 6.12um1+b21 · 2 versions
, in debian trixie

💖 UML HOWTO 💖

okay bestie, welcome to the UML HOWTO page 💅 this is the manual for User Mode Linux and it is, no cap, a whole journey. grab a snack, we're going in 🍓

✨ NAME ✨

UML HowTo — and that's the whole name, it's giving "mysterious" but the real tea is the table of contents below, which is basically the map of everything this doc is about 🗺️✨

  • Introduction
    • How is UML Different from a VM using Virtualization package X?
    • Why Would I Want User Mode Linux?
    • Why not to run UML
  • Building a UML instance
    • Creating an image
    • Edit key system files
  • Setting Up UML Networking
    • Network configuration privileges
    • Configuring vector transports
      • Common options
      • Shared Options
      • tap transport
      • hybrid transport
      • raw socket transport
      • GRE socket transport
      • l2tpv3 socket transport
      • BESS socket transport
      • VDE vector transport
    • Configuring Legacy transports
  • Running UML
    • Arguments
      • Mandatory Arguments:
      • Important Optional Arguments
    • Starting UML
    • Logging in
    • The UML Management Console
      • version
      • help
      • halt and reboot
      • config
      • remove
      • sysrq
      • cad
      • stop
      • go
      • proc
      • stack
  • Advanced UML Topics
    • Sharing Filesystems between Virtual Machines
      • Using layered block devices
      • Disk Usage
      • COW validity.
      • Cows can moo - uml_moo : Merging a COW file with its backing file
    • Host file access
      • Using hostfs
      • hostfs as the root filesystem
      • Hostfs Caveats
    • Tuning UML
  • Contributing to UML and Developing with UML
    • Tracing UML
    • Kernel debugging
    • Developing Device Drivers
    • Using UML as a Test Platform
      • Security Considerations

🌈 INTRODUCTION 🌈

Welcome to User Mode Linux 🫶

User Mode Linux is the first Open Source virtualization platform (first release date 1991) and the second virtualization platform for an x86 PC. iconic behavior, honestly 👑

🦋 How is UML Different from a VM using Virtualization package X? 🦋

okay so we've all been trained to assume that "virtualization" automatically means "some level of hardware emulation." bestie, it does not 💀 As long as a virtualization package gives the OS devices that the OS can recognize and already has a driver for, those devices do not need to emulate real hardware. Most OSes these days ship with built-in support for a bunch of "fake" devices that only ever show up under virtualization.

User Mode Linux takes this concept to the ultimate extreme — there is not a single real device in sight. It's 100% artificial, or if we're being correct about it, 100% paravirtual ✨ Every single UML device is an abstract concept that maps onto something the host already provides — files, sockets, pipes, etc. it's giving "we made it up and it works" 💅

The other big difference between UML and various virtualization packages is that UML draws a distinct line between the way the UML kernel and the UML programs operate.

  • The UML kernel is just a process running on Linux — same as any other program. It can be run by an unprivileged user and it does not require anything in terms of special CPU features 🎀
  • The UML userspace, though, is a bit different. The Linux kernel on the host machine assists UML in intercepting everything the program running on a UML instance is trying to do, and makes the UML kernel handle all of its requests 👀

This is different from other virtualization packages, which don't make any distinction between the guest kernel and guest programs. And that difference is exactly why UML has a whole list of advantages and disadvantages over, say, QEMU — which we will cover later in this document, promise 🫡

💜 Why Would I Want User Mode Linux? 💜

  • If the User Mode Linux kernel crashes, your host kernel is still fine ✨ It is not accelerated in any way (no vhost, no kvm, etc) and it is not trying to access any devices directly. It is, in fact, a process like any other 🫶
  • You can run a usermode kernel as a non-root user (you may need to arrange appropriate permissions for some devices).
  • You can run a very small VM with a minimal footprint for a specific task (for example 32M or less).
  • You can get extremely high performance for anything that's a "kernel specific task" — forwarding, firewalling, etc — while still being isolated from the host kernel 🔥
  • You can play with kernel concepts without breaking things.
  • You are not bound by "emulating" hardware, so you can try weird and wonderful concepts that are very difficult to support when emulating real hardware — like time travel and making your system clock depend on what UML does (very useful for things like tests) ⏰✨
  • It's fun 🎉

💀 Why not to run UML 💀

  • The syscall interception technique used by UML makes it inherently slower for any userspace applications. While it can do kernel tasks on par with most other virtualization packages, its userspace is slow. The root cause is that UML has a very high cost of creating new processes and threads (something most Unix/Linux applications take for granted) 😭
  • UML is strictly uniprocessor at present. If you want to run an application that needs many CPUs to function, it is clearly the wrong choice 🙈

🎀 BUILDING A UML INSTANCE 🎀

okay so here's the thing — there is no UML installer in any distribution 💀 With a normal virtualization package you can just use off-the-shelf install media to install into a blank VM, but there is no UML equivalent. You have to use appropriate tools on your host to build a viable filesystem image yourself, bestie 🫡

This is extremely easy on Debian — you can do it using debootstrap. It's also easy on OpenWRT — the build process can build UML images. All other distros — YMMV 🌸

🌸 Creating an image 🌸

Create a sparse raw disk image:

# dd if=/dev/zero of=disk_image_name bs=1 count=1 seek=16G

This will create a 16G disk image. The OS will initially allocate only one block and will allocate more as they are written by UML ✨ As of kernel version 4.19, UML fully supports TRIM (as usually used by flash drives). Using TRIM inside the UML image by specifying discard as a mount option, or by running tune2fs -o discard /dev/ubdXX, will request UML to return any unused blocks to the OS 💅

Create a filesystem on the disk image and mount it:

# mkfs.ext4 ./disk_image_name && mount ./disk_image_name /mnt

This example uses ext4, but any other filesystem — such as ext3, btrfs, xfs, jfs, etc — will work too 🫶

Create a minimal OS installation on the mounted filesystem:

# debootstrap buster /mnt http://deb.debian.org/debian

heads up: debootstrap does not set up the root password, fstab, hostname, or anything related to networking. It is up to the user to do that 🙈

Set the root password — the easiest way to do that is to chroot into the mounted image:

# chroot /mnt
# passwd
# exit

🧚 Edit key system files 🧚

UML block devices are called ubds 💅 The fstab created by debootstrap will be empty, and it needs an entry for the root file system:

/dev/ubd0   ext4    discard,errors=remount-ro  0       1

The image hostname will be set to the same as the host on which you are creating its image. It is a good idea to change that, so you avoid the whole "Oh, bummer, I rebooted the wrong machine" situation 😭

UML supports two classes of network devices:

  • the older uml_net ones, which are scheduled for obsoletion. These are called ethX.
  • the newer vector IO devices, which are significantly faster and have support for some standard virtual network encapsulations like Ethernet over GRE and Ethernet over L2TPv3. These are called vec0 ✨

Depending on which one is in use, /etc/network/interfaces will need entries like:

# legacy UML network devices
auto eth0
iface eth0 inet dhcp

# vector UML network devices
auto vec0
iface vec0 inet dhcp

At this point we have a UML image that's nearly ready to run — all we need now is a UML kernel and modules for it 🫶

Most distributions have a UML package. Even if you intend to use your own kernel, testing the image with a stock one is always a good start. These packages come with a set of modules that should be copied to the target filesystem. The location is distribution dependent. For Debian these reside under /usr/lib/uml/modules. Copy recursively the content of this directory to the mounted UML filesystem:

# cp -rax /usr/lib/uml/modules /mnt/lib/modules

If you have compiled your own kernel, you need to use the usual "install modules to a location" procedure by running:

# make INSTALL_MOD_PATH=/mnt/lib/modules modules_install

This will install modules into /mnt/lib/modules/$(KERNELRELEASE) ✨ To specify the full module installation path, use:

# make MODLIB=/mnt/lib/modules modules_install

At this point the image is ready to be brought up 🎉👑

🌸 SETTING UP UML NETWORKING 🌸

okay bestie, this is the part where your UML instance gets to talk to things 💬✨ UML networking is built to emulate an Ethernet connection. And that connection can be either:

  • point-to-point — like two machines wired up with a back-to-back cable, cute and intimate 🫶
  • a connection to a switch — social butterfly mode 🦋

UML supports a wide variety of ways to build these connections out to all of: your local machine, remote machine(s), local and remote UML instances, and other VM instances. versatile queen 👑

Here's the whole transport lineup, vibes included (the original ships this as a big table, so imma give you the same tea, one row at a time):

  • tap — type: vector — capabilities: checksum, tso — throughput: > 8Gbit
  • hybrid — type: vector — capabilities: checksum, tso, multipacket rx — throughput: > 6GBit
  • raw — type: vector — capabilities: checksum, tso, multipacket rx, tx" — throughput: > 6GBit
  • EoGRE — type: vector — capabilities: multipacket rx, tx — throughput: > 3Gbit
  • Eol2tpv3 — type: vector — capabilities: multipacket rx, tx — throughput: > 3Gbit
  • bess — type: vector — capabilities: multipacket rx, tx — throughput: > 3Gbit
  • fd — type: vector — capabilities: dependent on fd type — throughput: varies
  • vde — type: vector — capabilities: dep. on VDE VPN: Virt.Net Locator — throughput: varies
  • tuntap — type: legacy — capabilities: none — throughput: ~ 500Mbit
  • daemon — type: legacy — capabilities: none — throughput: ~ 450Mbit
  • socket — type: legacy — capabilities: none — throughput: ~ 450Mbit
  • ethertap — type: legacy — capabilities: obsolete — throughput: ~ 500Mbit
  • vde — type: legacy — capabilities: obsolete — throughput: ~ 500Mbit

and some notes that apply across the whole board 📋:

  • All transports which have tso and checksum offloads can deliver speeds approaching 10G on TCP streams. fast fr fr 🔥
  • All transports which have multi-packet rx and/or tx can deliver pps rates of up to 1Mps or more.
  • All legacy transports are generally limited to ~600-700MBit and 0.05Mps. it's giving vintage 🎀
  • GRE and L2TPv3 allow connections to all of: local machine, remote machines, remote network devices and remote UML instances.
  • Socket allows connections only between UML instances. no outsiders, exclusive 💅
  • Daemon and bess require running a local switch. This switch may be connected to the host as well.

🔐 Network configuration privileges

so here's the thing, bestie — the majority of the supported networking modes need root privileges. For example, in the legacy tuntap networking mode, users were required to be part of the group associated with the tunnel device.

For newer network drivers like the vector transports, root privilege is required to fire an ioctl to setup the tun interface and/or use raw sockets where needed.

But wait — you don't have to just run UML as root like a menace 😭 This can be achieved by granting the user a particular capability instead. In case of vector transport, a user can add the capability CAP_NET_ADMIN or CAP_NET_RAW to the uml binary. Thenceforth, UML can be run with normal user privilges, along with full networking. (yes "privilges", the original said it like that, we keep the receipts 📝)

For example:

# sudo setcap cap_net_raw,cap_net_admin+ep linux

⚙️ Configuring vector transports

All vector transports support a similar syntax:

If X is the interface number as in vec0, vec1, vec2, etc, the general syntax for options is:

vecX:transport="Transport Name",option=value,option=value,...,option=value

🧁 Common options

These options are common for all transports:

  • depth=int — sets the queue depth for vector IO. This is the amount of packets UML will attempt to read or write in a single system call. The default number is 64 and is generally sufficient for most applications that need throughput in the 2-4 Gbit range. Higher speeds may require larger values.
  • mac=XX:XX:XX:XX:XX — sets the interface MAC address value.
  • gro=[0,1] — sets GRO off or on. Enables receive/transmit offloads. The effect of this option depends on the host side support in the transport which is being configured. In most cases it will enable TCP segmentation and RX/TX checksumming offloads. The setting must be identical on the host side and the UML side. The UML kernel will produce warnings if it is not. For example, GRO is enabled by default on local machine interfaces (e.g. veth pairs, bridge, etc), so it should be enabled in UML in the corresponding UML transports (raw, tap, hybrid) in order for networking to operate correctly.
  • mtu=int — sets the interface MTU
  • headroom=int — adjusts the default headroom (32 bytes) reserved if a packet will need to be re-encapsulated into for instance VXLAN.
  • vec=0 — disable multipacket IO and fall back to packet at a time mode

🤝 Shared Options

  • ifname=str — Transports which bind to a local network interface have a shared option - the name of the interface to bind to.
  • src, dst, src_port, dst_port — all transports which use sockets which have the notion of source and destination and/or source port and destination port use these to specify them.
  • v6=[0,1] — to specify if a v6 connection is desired for all transports which operate over IP. Additionally, for transports that have some differences in the way they operate over v4 and v6 (for example EoL2TPv3), sets the correct mode of operation. In the absence of this option, the socket type is determined based on what do the src and dst arguments resolve/parse to.

🎀 tap transport

Example:

vecX:transport=tap,ifname=tap0,depth=128,gro=1

This will connect vec0 to tap0 on the host. Tap0 must already exist (for example created using tunctl) and UP.

tap0 can be configured as a point-to-point interface and given an IP address so that UML can talk to the host. Alternatively, it is possible to connect UML to a tap interface which is connected to a bridge. pick your adventure 🌈

While tap relies on the vector infrastructure, it is not a true vector transport at this point, because Linux does not support multi-packet IO on tap file descriptors for normal userspace apps like UML. This is a privilege which is offered only to something which can hook up to it at kernel level via specialized interfaces like vhost-net. A vhost-net like helper for UML is planned at some point in the future. manifesting it for them 🕯️✨

Privileges required: tap transport requires either:

  • tap interface to exist and be created persistent and owned by the UML user using tunctl. Example tunctl -u uml-user -t tap0
  • binary to have CAP_NET_ADMIN privilege

🦋 hybrid transport

Example:

vecX:transport=hybrid,ifname=tap0,depth=128,gro=1

This is an experimental/demo transport which couples tap for transmit and a raw socket for receive. The raw socket allows multi-packet receive resulting in significantly higher packet rates than normal tap.

Privileges required: hybrid requires CAP_NET_RAW capability by the UML user as well as the requirements for the tap transport.

🔥 raw socket transport

Example:

vecX:transport=raw,ifname=p-veth0,depth=128,gro=1

This transport uses vector IO on raw sockets. While you can bind to any interface including a physical one, the most common use it to bind to the "peer" side of a veth pair with the other side configured on the host.

Example host configuration for Debian:

/etc/network/interfaces:

auto veth0
iface veth0 inet static
     address 192.168.4.1
     netmask 255.255.255.252
     broadcast 192.168.4.3
     pre-up ip link add veth0 type veth peer name p-veth0 && \
       ifconfig p-veth0 up

UML can now bind to p-veth0 like this:

vec0:transport=raw,ifname=p-veth0,depth=128,gro=1

If the UML guest is configured with 192.168.4.2 and netmask 255.255.255.0 it can talk to the host on 192.168.4.1

The raw transport also provides some support for offloading some of the filtering to the host. The two options to control it are:

  • bpffile=str — filename of raw bpf code to be loaded as a socket filter
  • bpfflash=int — 0/1 allow loading of bpf from inside User Mode Linux. This option allows the use of the ethtool load firmware command to load bpf code.

and that's where this part wraps, bestie — more coming 💖

okay so in either case the bpf code gets loaded straight into the host kernel 😭 and like, rn it's limited to legacy bpf syntax (not ebpf), but bestie… it's still a security risk, fr fr. Not recommended to allow this unless the User Mode Linux instance is considered trusted. periodt. 🚫

Privileges required: raw socket transport requires CAP_NET_RAW capability.

🌈 GRE socket transport

Example:

vecX:transport=gre,src=$src_host,dst=$dst_host

This sets up an Ethernet over GRE tunnel (aka GRETAP or GREIRB, same vibe, different name 💅) that hooks your UML instance up to a GRE endpoint at host dst_host. GRE comes with these extra little options:

  • rx_key=int — GRE 32-bit integer key for rx packets, and listen: if you set this, you must set txkey too
  • tx_key=int — GRE 32-bit integer key for tx packets, and if you set this one, rx_key must be set too
  • sequence=[0,1] — turn on GRE sequence
  • pin_sequence=[0,1] — pretend the sequence is always reset on every packet (you need this to interoperate with some really broken implementations 😭)
  • v6=[0,1] — force IPv4 or IPv6 sockets respectively
  • GRE checksum is not presently supported

GRE has a couple of caveats you need to know, bestie:

  • You can use only one GRE connection per IP address. There is literally no way to multiplex connections, because each GRE tunnel is terminated directly on the UML instance.
  • The key is not really a security feature. It was intended as one, but its "security" is laughable 💀. It is however a useful feature to make sure the tunnel isn't misconfigured.

Here's an example config for a Linux host with a local address of 192.168.128.1 connecting to a UML instance at 192.168.129.1:

/etc/network/interfaces:

auto gt0
iface gt0 inet static
 address 10.0.0.1
 netmask 255.255.255.0
 broadcast 10.0.0.255
 mtu 1500
 pre-up ip link add gt0 type gretap local 192.168.128.1 \
        remote 192.168.129.1 || true
 down ip link del gt0 || true

Also, GRE has been tested against a whole variety of network equipment. she's got range 💅✨

Privileges required: GRE requires CAP_NET_RAW

🦋 l2tpv3 socket transport

Warning. L2TPv3 has a "bug". It's the "bug" known as "has more options than GNU ls". 😭 While it does have some advantages, there are usually easier (and way less verbose) ways to connect a UML instance to something. For example, most devices that support L2TPv3 also support GRE. iykyk.

Example:

vec0:transport=l2tpv3,udp=1,src=$src_host,dst=$dst_host,srcport=$src_port,dstport=$dst_port,depth=128,rx_session=0xffffffff,tx_session=0xffff

This configures an Ethernet over L2TPv3 fixed tunnel that connects the UML instance to a L2TPv3 endpoint at host $dst_host, using the L2TPv3 UDP flavour and UDP destination port $dst_port.

L2TPv3 always requires these additional options:

  • rx_session=int — l2tpv3 32-bit integer session for rx packets
  • tx_session=int — l2tpv3 32-bit integer session for tx packets

Since the tunnel is fixed, these aren't negotiated — they're preconfigured on both ends. no surprises, bestie 🫶

On top of that, L2TPv3 supports these optional parameters:

  • rx_cookie=int — l2tpv3 32-bit integer cookie for rx packets — same functionality as the GRE key, more about preventing misconfiguration than providing actual security
  • tx_cookie=int — l2tpv3 32-bit integer cookie for tx packets
  • cookie64=[0,1] — use 64-bit cookies instead of 32-bit
  • counter=[0,1] — enable l2tpv3 counter
  • pin_counter=[0,1] — pretend the counter is always reset on every packet (needed to interoperate with some really broken implementations 💀)
  • v6=[0,1] — force v6 sockets
  • udp=[0,1] — use raw sockets (0) or the UDP (1) version of the protocol

L2TPv3 has a number of caveats:

  • you can use only one connection per IP address in raw mode. There's no way to multiplex connections, since each L2TPv3 tunnel is terminated directly on the UML instance. UDP mode can use different ports for this purpose, so that's your loophole ✨

Here's an example of how to configure a Linux host to connect to UML via L2TPv3:

/etc/network/interfaces:

auto l2tp1
iface l2tp1 inet static
 address 192.168.126.1
 netmask 255.255.255.0
 broadcast 192.168.126.255
 mtu 1500
 pre-up ip l2tp add tunnel remote 127.0.0.1 \
        local 127.0.0.1 encap udp tunnel_id 2 \
        peer_tunnel_id 2 udp_sport 1706 udp_dport 1707 && \
        ip l2tp add session name l2tp1 tunnel_id 2 \
        session_id 0xffffffff peer_session_id 0xffffffff
 down ip l2tp del session tunnel_id 2 session_id 0xffffffff && \
        ip l2tp del tunnel tunnel_id 2

Privileges required: L2TPv3 requires CAP_NET_RAW for raw IP mode, and no special privileges for the UDP mode. slay.

👑 BESS socket transport

BESS is a high performance modular network switch. iconic behavior tbh.

<https://github.com/NetSys/bess>

It has support for a simple sequential packet socket mode, which in the more recent versions uses vector IO for high performance. 🔥

Example:

vecX:transport=bess,src=$unix_src,dst=$unix_dst

This configures a BESS transport using the unix_src Unix domain socket address as source and the unix_dst socket address as destination.

For BESS configuration and how to allocate a BESS Unix domain socket port, please see the BESS documentation. she'll tell you everything 💅

<https://github.com/NetSys/bess/wiki/Built-In-Modules-and-Ports>

BESS transport does not require any special privileges. 🎀

🌸 VDE vector transport

Virtual Distributed Ethernet (VDE) is a project whose main goal is to provide a highly flexible support for virtual networking.

<http://wiki.virtualsquare.org/#/tutorials/vdebasics>

Common usages of VDE include fast prototyping and teaching. cute AND useful 🦋

Examples:

vecX:transport=vde,vnl=tap://tap0

use tap0

vecX:transport=vde,vnl=slirp://

use slirp

vec0:transport=vde,vnl=vde:///tmp/switch

connect to a vde switch

vecX:transport=\"vde,vnl=cmd://ssh remote.host //tmp/sshlirp\"

connect to a remote slirp (instant VPN: convert ssh to VPN, it uses sshlirp)

<https://github.com/virtualsquare/sshlirp>

vec0:transport=vde,vnl=vxvde://234.0.0.1

connect to a local area cloud (all the UML nodes using the same multicast address running on hosts in the same multicast domain (LAN) will be automagically connected together to a virtual LAN). automagically. main character energy fr 💜

⭐ Configuring Legacy transports

Legacy transports are now considered obsolete. Please use the vector versions instead, bestie, trust me on this one. 🙈

🏃‍♀️ RUNNING UML 💨

okay so this section assumes that either the user-mode-linux package from the distribution or a custom built kernel has been installed on the host. 💅

These add an executable called linux to the system. This is the UML kernel. It can be run just like any other executable. It will take most normal linux kernel arguments as command line arguments. Additionally, it will need some UML-specific arguments in order to do something useful. ✨

📝 Arguments

🔒 Mandatory Arguments 💖

  • mem=int[K,M,G] — amount of memory. By default in bytes. It will also accept K, M or G qualifiers.
  • ubdX[s,d,c,t]= — virtual disk specification. This is not really mandatory, but it is likely to be needed in nearly all cases so we can specify a root file system. The simplest possible image specification is the name of the image file for the filesystem (created using one of the methods described in Creating an image).
    • UBD devices support copy on write (COW). The changes are kept in a separate file which can be discarded allowing a rollback to the original pristine image. If COW is desired, the UBD image is specified as: cow_file,master_image. Example: ubd0=Filesystem.cow,Filesystem.img
    • UBD devices can be set to use synchronous IO. Any writes are immediately flushed to disk. This is done by adding s after the ubdX specification.
    • UBD performs some heuristics on devices specified as a single filename to make sure that a COW file has not been specified as the image. To turn them off, use the d flag after ubdX.
    • UBD supports TRIM - asking the Host OS to reclaim any unused blocks in the image. To turn it off, specify the t flag after ubdX.
  • root= — root device - most likely /dev/ubd0 (this is a Linux filesystem image)

✨ Important Optional Arguments ✨

If UML is run as "linux" with no extra arguments, it will try to start an xterm for every console configured inside the image (up to 6 in most Linux distributions). Each console is started inside an xterm. This makes it nice and easy to use UML on a host with a GUI. It is, however, the wrong approach if UML is to be used as a testing harness or run in a text-only environment. 😭

In order to change this behaviour we need to specify an alternative console and wire it to one of the supported "line" channels. For this we need to map a console to use something different from the default xterm.

Example which will divert console number 1 to stdin/stdout:

con1=fd:0,fd:1

UML supports a wide variety of serial line channels which are specified using the following syntax

conX=channel_type:options[,channel_type:options]

If the channel specification contains two parts separated by comma, the first one is input, the second one output.

  • The null channel - Discard all input or output. Example con=null will set all consoles to null by default.
  • The fd channel - use file descriptor numbers for input/output. Example: con1=fd:0,fd:1.
  • The port channel - start a telnet server on TCP port number. Example: con1=port:4321. The host must have /usr/sbin/in.telnetd (usually part of a telnetd package) and the port-helper from the UML utilities (see the information for the xterm channel below). UML will not boot until a client connects.
  • The pty and pts channels - use system pty/pts.
  • The tty channel - bind to an existing system tty. Example: con1=/dev/tty8 will make UML use the host 8th console (usually unused).
  • The xterm channel - this is the default - bring up an xterm on this channel and direct IO to it. Note that in order for xterm to work, the host must have the UML distribution package installed. This usually contains the port-helper and other utilities needed for UML to communicate with the xterm. Alternatively, these need to be compiled and installed from source. All options applicable to consoles also apply to UML serial lines which are presented as ttyS inside UML.

🚀 Starting UML

We can now run UML. periodt 💫

# linux mem=2048M umid=TEST \
 ubd0=Filesystem.img \
 vec0:transport=tap,ifname=tap0,depth=128,gro=1 \
 root=/dev/ubda con=null con0=null,fd:2 con1=fd:0,fd:1

This will run an instance with 2048M RAM and try to use the image file called Filesystem.img as root. It will connect to the host using tap0. All consoles except con1 will be disabled and console 1 will use standard input/output making it appear in the same terminal it was started. iconic 🫶

🔑 Logging in

If you have not set up a password when generating the image, you will have to shut down the UML instance, mount the image, chroot into it and set it - as described in the Generating an Image section. If the password is already set, you can just log in. no cap 🌸

👑 The UML Management Console

In addition to managing the image from "the inside" using normal sysadmin tools, it is possible to perform a number of low-level operations using the UML management console. The UML management console is a low-level interface to the kernel on a running UML instance, 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. the way that this works is honestly giving main character energy. 💅

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

  • get the kernel version
  • add and remove devices
  • halt or reboot the machine
  • Send SysRq commands
  • Pause and resume the UML
  • Inspect processes running inside UML
  • Inspect UML internal /proc state

You need the mconsole client (uml_mconsole) which is a part of the UML tools package available in most Linux distributions. ✨

You also need CONFIG_MCONSOLE (under 'General Setup') enabled in the UML kernel. 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 on 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:

# uml_mconsole debian

or

# uml_mconsole /home/jdike/.uml/debian/mconsole

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

  • version
  • help
  • halt
  • reboot
  • config
  • remove
  • sysrq
  • help
  • cad
  • stop
  • go
  • proc
  • stack

🏷️ version

This command takes no arguments. It prints the UML version:

(mconsole)  version
OK Linux OpenWrt 4.14.106 #0 Tue Mar 19 08:19:41 2019 x86_64

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 a device interrupt to the UML. UML mconsole is treated internally as a UML device. slay 💖

🆘 help

This command takes no arguments. It prints a short help screen with the supported mconsole commands.

🛑 halt and reboot

These commands 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
bestie, this is not a graceful shutdown. no syncing, no clean userspace exit. it's giving chaos. 💀

⚙️ config

"config" adds a new device to the virtual machine. This is supported by most UML device drivers. 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

🗑️ remove

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

(mconsole)  remove ubd3

📨 sysrq

This command 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/admin-guide/sysrq.rst in your favorite kernel tree to see what letters are valid and what they do.

🎛️ cad

This invokes the Ctl-Alt-Del action in the running image. What exactly this ends up doing is up to init, systemd, etc. Normally, it reboots the machine.

⏸️ stop

This puts the UML in a loop reading mconsole requests until a 'go' mconsole command is received. This is very useful as a debugging/snapshotting tool. obsessed 🦋

▶️ go

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

📄 proc

This takes one argument - the name of a file in /proc which is printed to the mconsole standard output

🧵 stack

This takes one argument - the pid number of a process. Its stack is printed to a standard output. iykyk ⭐

🔥 ADVANCED UML TOPICS 🔥

okay bestie, this is where it gets spicy. sit down, hydrate, we're going deep 💅

Sharing Filesystems between Virtual Machines

girl. do NOT do the obvious thing. Do not try to share filesystems by just booting two UMLs from the same file. That is literally the same thing as booting two physical machines off a shared disk. It will result in filesystem corruption. No cap. Don't.

Using layered block devices

The ✨correct✨ way to share a filesystem between two virtual machines is to use the copy-on-write (COW) layering capability of the ubd block driver. Here's the vibe: any changed blocks get stored in the private COW file, while reads come from either device — the private one if the requested block is valid in it, and the shared one if it's not. So the majority of data, the stuff that's unchanged, is shared between an arbitrary number of virtual machines, and each one only has a much smaller file containing the changes it made. Boot a large number of UMLs from a large root filesystem and you get a huge disk space saving. Iconic.

Sharing file system data also helps performance, because the host can cache the shared data using a much smaller amount of memory, so UML disk requests get served from the host's memory instead of its disks. 🧚

major caveat alert 🚨 On multisocket NUMA machines, running many UML instances with a shared master image and COW changes may cause issues like NMIs from excess of inter-socket traffic.

If you're running UML on high-end hardware like this, make sure to bind UML to a set of logical CPUs residing on the same socket using the taskset command, or have a look at the "tuning" section. iykyk.

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. And here's the cute part: the COW file need not exist. If it doesn't, the driver will create and initialize it. 🎀

Disk Usage

UML has TRIM support which will release any unused space in its disk image files to the underlying OS. So important, bestie: use either ls -ls or du to verify the actual file size. 👑

COW validity.

Any changes to the master image will invalidate all COW files. If this happens, UML will NOT automatically delete any of the COW files and will refuse to boot. In this case the only solution is to either restore the old image (including its last modified timestamp) or remove all COW files — which will result in their recreation. Any changes in the COW files will be lost. 😭

Cows can moo - uml_moo : Merging a COW file with its backing file

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

The utility that does this is uml_moo. Its usage is:

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 noticeably faster than a non-destructive merge. 🔥

uml_moo is installed with the UML distribution packages and is available as a part of UML utilities.

Host file access

If you want to access files on the host machine from inside UML, you can treat it as a separate machine and either nfs mount directories from the host or copy files into the virtual machine with scp. But here's the thing — since UML is running on the host, it can access those files just like any other process and make them available inside the virtual machine without the need to use the network. 🌈 This is 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.

SECURITY WARNING Hostfs without any parameters to the UML Image will allow the image to mount any part of the host filesystem and write to it. Always confine hostfs to a specific "harmless" directory (for example /var/tmp) if running UML. This is especially important if UML is being run as root. 💀

Using hostfs

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

# 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:

# 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:

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

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

hostfs as the root filesystem

It's possible to boot from a directory hierarchy on the host using hostfs rather than using the standard filesystem in a file. To start, you need that hierarchy. The easiest way is to loop mount an existing root_fs file:

#  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:

#  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. ⭐

Hostfs Caveats

Hostfs does not support keeping track of host filesystem changes on the host (outside UML). As a result, if a file is changed without UML's knowledge, UML will not know about it and its own in-memory cache of the file may be corrupt. While it is possible to fix this, it is not something which is being worked on at present. 🙈

Tuning UML

UML at present is strictly uniprocessor. It will, however, spin up a number of threads to handle various functions.

The UBD driver, SIGIO and the MMU emulation do that. If the system is idle, these threads will be migrated to other processors on a SMP host. This, unfortunately, will usually result in LOWER performance because of all of the cache/memory synchronization traffic between cores. As a result, UML will usually benefit from being pinned on a single CPU, especially on a large system. This can result in performance differences of 5 times or higher on some benchmarks. 😳

Similarly, on large multi-node NUMA systems UML will benefit if all of its memory is allocated from the same NUMA node it will run on. The OS will NOT do that by default. In order to do that, the sysadmin needs to create a suitable tmpfs ramdisk bound to a particular node and use that as the source for UML RAM allocation by specifying it in the TMP or TEMP environment variables. UML will look at the values of TMPDIR, TMP or TEMP for that. If that fails, it will look for shmfs mounted under /dev/shm. If everything else fails use /tmp/ regardless of the filesystem type used for it:

mount -t tmpfs -ompol=bind:X none /mnt/tmpfs-nodeX
TEMP=/mnt/tmpfs-nodeX taskset -cX linux options options options..

💖 CONTRIBUTING TO UML AND DEVELOPING WITH UML 💖

okay bestie, if you're reading this section it's because you wanna build stuff with UML, not just run it. and honestly? iconic choice. ✨

UML is an excellent platform to develop new Linux kernel concepts — filesystems, devices, virtualization, etc. it gives you unrivalled opportunities to create and test them without being constrained to emulating specific hardware. 🦋

Example — want to try how Linux will work with 4096 "proper" network devices?

Not an issue with UML. 💅 at the same time, this is something which is difficult with other virtualization packages — they are constrained by the number of devices allowed on the hardware bus they are trying to emulate (for example 16 on a PCI bus in qemu).

If you have something to contribute such as a patch, a bugfix, a new feature, please send it to linux-um@lists.infradead.org. 🫶

Please follow all standard Linux patch guidelines such as cc-ing relevant maintainers and run ./scripts/checkpatch.pl on your patch. For more details see Documentation/process/submitting-patches.rst

⚠️ Note — the list does not accept HTML or attachments, all emails must be formatted as plain text.

Developing always goes hand in hand with debugging. First of all, you can always run UML under gdb and there will be a whole section later on on how to do that. That, however, is not the only way to debug a Linux kernel. Quite often adding tracing statements and/or using UML specific approaches such as ptracing the UML kernel process are significantly more informative.

🔍 Tracing UML 🔍

When running, UML consists of a main kernel thread and a number of helper threads. The ones of interest for tracing are NOT the ones that are already ptraced by UML as a part of its MMU emulation. 🙅‍♀️

These are usually the first three threads visible in a ps display. The one with the lowest PID number and using most CPU is usually the kernel thread. The other threads are the disk (ubd) device helper thread and the SIGIO helper thread. Running ptrace on this thread usually results in the following picture:

host$ strace -p 16566
--- SIGIO {si_signo=SIGIO, si_code=POLL_IN, si_band=65} ---
epoll_wait(4, [{EPOLLIN, {u32=3721159424, u64=3721159424}}], 64, 0) = 1
epoll_wait(4, [], 64, 0)                = 0
rt_sigreturn({mask=[PIPE]})             = 16967
ptrace(PTRACE_GETREGS, 16967, NULL, 0xd5f34f38) = 0
ptrace(PTRACE_GETREGSET, 16967, NT_X86_XSTATE, [{iov_base=0xd5f35010, iov_len=832}]) = 0
ptrace(PTRACE_GETSIGINFO, 16967, NULL, {si_signo=SIGTRAP, si_code=0x85, si_pid=16967, si_uid=0}) = 0
ptrace(PTRACE_SETREGS, 16967, NULL, 0xd5f34f38) = 0
ptrace(PTRACE_SETREGSET, 16967, NT_X86_XSTATE, [{iov_base=0xd5f35010, iov_len=2696}]) = 0
ptrace(PTRACE_SYSEMU, 16967, NULL, 0)   = 0
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_TRAPPED, si_pid=16967, si_uid=0, si_status=SIGTRAP, si_utime=65, si_stime=89} ---
wait4(16967, [{WIFSTOPPED(s) && WSTOPSIG(s) == SIGTRAP | 0x80}], WSTOPPED|__WALL, NULL) = 16967
ptrace(PTRACE_GETREGS, 16967, NULL, 0xd5f34f38) = 0
ptrace(PTRACE_GETREGSET, 16967, NT_X86_XSTATE, [{iov_base=0xd5f35010, iov_len=832}]) = 0
ptrace(PTRACE_GETSIGINFO, 16967, NULL, {si_signo=SIGTRAP, si_code=0x85, si_pid=16967, si_uid=0}) = 0
timer_settime(0, 0, {it_interval={tv_sec=0, tv_nsec=0}, it_value={tv_sec=0, tv_nsec=2830912}}, NULL) = 0
getpid()                                = 16566
clock_nanosleep(CLOCK_MONOTONIC, 0, {tv_sec=1, tv_nsec=0}, NULL) = ? ERESTART_RESTARTBLOCK (Interrupted by signal)
--- SIGALRM {si_signo=SIGALRM, si_code=SI_TIMER, si_timerid=0, si_overrun=0, si_value={int=1631716592, ptr=0x614204f0}} ---
rt_sigreturn({mask=[PIPE]})             = -1 EINTR (Interrupted system call)

This is a typical picture from a mostly idle UML instance. 😌

  • UML interrupt controller uses epoll — this is UML waiting for IO interrupts:

epoll_wait(4, [{EPOLLIN, {u32=3721159424, u64=3721159424}}], 64, 0) = 1

  • The sequence of ptrace calls is part of MMU emulation and running the UML userspace.
  • timer_settime is part of the UML high res timer subsystem mapping timer requests from inside UML onto the host high resolution timers.
  • clock_nanosleep is UML going into idle (similar to the way a PC will execute an ACPI idle).

As you can see UML will generate quite a bit of output even in idle. The output can be very informative when observing IO. It shows the actual IO calls, their arguments and returns values. 🔥

🐛 Kernel debugging 🐛

You can run UML under gdb now, though it will not necessarily agree to be started under it. If you are trying to track a runtime bug, it is much better to attach gdb to a running UML instance and let UML run.

Assuming the same PID number as in the previous example, this would be:

# gdb -p 16566

This will STOP the UML instance, so you must enter cont at the GDB command line to request it to continue. It may be a good idea to make this into a gdb script and pass it to gdb as an argument. 💀

🛠️ Developing Device Drivers 🛠️

Nearly all UML drivers are monolithic. While it is possible to build a UML driver as a kernel module, that limits the possible functionality to in-kernel only and non-UML specific. The reason for this is that in order to really leverage UML, one needs to write a piece of userspace code which maps driver concepts onto actual userspace host calls.

This forms the so-called "user" portion of the driver. While it can reuse a lot of kernel concepts, it is generally just another piece of userspace code. This portion needs some matching "kernel" code which resides inside the UML image and which implements the Linux kernel part.

Note: There are very few limitations in the way "kernel" and "user" interact.

UML does not have a strictly defined kernel-to-host API. It does not try to emulate a specific architecture or bus. UML's "kernel" and "user" can share memory, code and interact as needed to implement whatever design the software developer has in mind. The only limitations are purely technical. Due to a lot of functions and variables having the same names, the developer should be careful which includes and libraries they are trying to refer to. 🙈

As a result a lot of userspace code consists of simple wrappers. E.g. os_close_file() is just a wrapper around close() which ensures that the userspace function close does not clash with similarly named function(s) in the kernel part.

🧪 Using UML as a Test Platform 🧪

UML is an excellent test platform for device driver development. As with most things UML, "some user assembly may be required". It is up to the user to build their emulation environment. UML at present provides only the kernel infrastructure. 🧚

Part of this infrastructure is the ability to load and parse fdt device tree blobs as used in Arm or Open Firmware platforms. These are supplied as an optional extra argument to the kernel command line:

dtb=filename

The device tree is loaded and parsed at boottime and is accessible by drivers which query it. At this moment in time this facility is intended solely for development purposes. UML's own devices do not query the device tree.

🔒 Security Considerations 🔒

Drivers or any new functionality should default to not accepting arbitrary filename, bpf code or other parameters which can affect the host from inside the UML instance. For example, specifying the socket used for IPC communication between a driver and the host at the UML command line is OK security-wise. Allowing it as a loadable module parameter isn't. 🚨

If such functionality is desirable for a particular application (e.g. loading BPF "firmware" for raw socket network transports), it should be off by default and should be explicitly turned on as a command line parameter at startup.

⚠️ Even with this in mind, the level of isolation between UML and the host is relatively weak. If the UML userspace is allowed to load arbitrary kernel drivers, an attacker can use this to break out of UML. Thus, if UML is used in a production application, it is recommended that all modules are loaded at boot and kernel module loading is disabled afterwards.

and that's a wrap, bestie!! 🎀 you just read the whole thing — every last corner of the UML HOWTO. this page signs off as UML HOWTO(), so give it up for the source and version tag and the date that goes with it, because she ate and left no crumbs. go build something unhinged. 👑💜

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