liminix/doc/development.rst

Development
###########

As a developer working on Liminix, or implementing a service or
module, you probably want to test your changes more conveniently
than by building and flashing a new image every time. This section
documents various affordances for iteration and experiments.

In general, packages and tools that run on the "build" machine are
available in the ``buildEnv`` derivation and can most easily
be added to your environment by running :command:`nix-shell`.



Emulated devices
****************

Liminix has a ``qemu`` device, which generates images suitable for
running on your build machine using the free `QEMU machine emulator <http://www.qemu.org>`_.
This is useful for developing userland without needing to keep
flashing or messing with U-Boot: it also enables testing against
emulated network peers using `QEMU socket networking <https://wiki.qemu.org/Documentation/Networking#Socket>`_,
which may be preferable to letting Liminix loose on your actual LAN.
To build it,

.. code-block:: console

    nix-build -I liminix-config=path/to/your/configuration.nix --arg device "import ./devices/qemu" -A outputs.default

This creates a :file:`result/` directory containing a :file:`vmlinux`
and a :file:`rootfs`, and also a shell script :file:`run.sh` which
invokes QEMU to run that kernel with that filesystem. It connects the Liminix
serial console and the `QEMU monitor  <https://www.qemu.org/docs/master/system/monitor.html>`_ to stdin/stdout. Use ^P (not ^A) to switch to the monitor.

If you run with ``--background /path/to/some/directory`` as the first
parameter, it will fork into the background and open Unix sockets in
that directory for console and monitor.  Use :command:`nix-shell --run
connect-vm` to connect to either of these sockets, and ^O to
disconnect.

.. _qemu-networking:

Networking
==========

VMs can network with each other using QEMU
socket networking.  We observe these conventions, so that we can run
multiple emulated instances and have them wired up to each other in
the right way:

* multicast 230.0.0.1:1234  : access (interconnect between router and "isp")
* multicast 230.0.0.1:1235  : lan
* multicast 230.0.0.1:1236  : world (the internet)

Any VM started by a :command:`run.sh` script is connected to "lan" and
"access", and the emulated border network gateway (see below) runs
PPPoE and is connected to "access" and "world".

.. _border-network-gateway:

Border Network Gateway
----------------------

In pkgs/routeros there is a derivation to install and configure
`Mikrotik RouterOS <https://mikrotik.com/software>`_ as a PPPoE access
concentrator connected to the ``access`` and ``world`` networks, so that
Liminix PPPoE client support can be tested without actual hardware.

This is made available as the :command:`routeros` command in
``buildEnv``, so you can do something like::

    mkdir ros-sockets
    nix-shell
    nix-shell$ routeros ros-sockets
    nix-shell$ connect-vm ./ros-sockets/console

to start it and connect to it. Note that by default it runs in the
background. It is connected to "access" and "world" virtual networks
and runs a PPPoE service on "access" - so a Liminix VM with a
PPPOE client can connect to it and thus reach the virtual internet.
[ check, but pretty sure this is not the actual internet ]

`Liminix does not provide RouterOS licences and it is your own
responsibility if you use this to ensure you're compliant with the
terms of Mikrotik's licencing. It may be supplemented or replaced in
time with configurations for RP-PPPoE and/or Accel PPP.`

Hardware devices
****************

.. _serial:

U-Boot and serial shenanigans
=============================

Every device that we have so far encountered in Liminix uses `U-Boot,
the "Universal Boot Loader" <https://docs.u-boot.org/en/latest/>`_ so
it's worth knowing a bit about it. "Universal" is in this context a
bit of a misnomer, though: encountering *mainline* U-Boot is very rare
and often you'll find it is a fork from some version last updated
in 2008. Upgrading U-Boot is more or less complicated depending on the
device and is outside scope for Liminix.

To speak to U-Boot on your device you'll usually need a serial
connection to it.  This is device-specific. Usually it involves
opening the box, locating the serial header pins (TX, RX and GND) and
connecting a USB TTL converter to them.

The Rolls Royce of USB/UART cables is the `FTDI cable
<https://cpc.farnell.com/ftdi/ttl-232r-rpi/cable-debug-ttl-232-usb-rpi/dp/SC12825?st=usb%20to%20uart%20cable>`_,
but there are cheaper alternatives based on the PL2303 and CP2102 chipsets.  Or
get creative and use the `UART GPIO pins <https://pinout.xyz/>`_ on a Raspberry Pi. Whatever you do, make sure
that the voltages are compatible: if your device is 3.3V (this is
typical but not universal), you don't want to be sending it 5v or
(even worse) 12v.

Run a terminal emulator such as Minicom on the computer at other end
of the link. 115200 8N1 is the typical speed.

.. NOTE::

   TTL serial connections typically have no form of flow control and
   so don't always like having massive chunks of text pasted into
   them - and U-Boot may drop characters while it's busy. So don't
   necessarily expect to copy-paste large chunks of text into the
   terminal emulator and have it work just like that.

   If using Minicom, you may find it helps to bring up the "Termimal
   settings" dialog (C^A T), then configure "Newline tx delay" to
   some small but non-zero value.

When you turn the router on you should be greeted with some messages
from U-Boot, followed by the instruction to hit some key to stop
autoboot. Do this and you will get to the prompt. If you didn't see
anything, the strong likelihood is that TX and RX are the wrong way
around. If you see garbage, try a different speed.

Interesting commands to try first in U-Boot are :command:`help` and
:command:`printenv`.

To do anything useful with U-Boot you will probably need a way to get
large binary files onto the device, and the usual way to do this is by
adding a network connection and using TFTP to download them. It's
quite common that the device's U-Boot doesn't speak DHCP so it will
need a static LAN address. You might also want to keep it away from
your "real" LAN: see :ref:`bng` for some potentially useful tooling
to use it on an isolated network.


TFTP
====

.. _tftp server:

How you get your image onto hardware will vary according to the
device, but is likely to involve taking it apart to add wires to
serial console pads/headers, then using U-Boot to fetch images over
TFTP.  The OpenWrt documentation has a `good explanation <https://openwrt.org/docs/techref/hardware/port.serial>`_ of what you may expect to find on
the device.

There is a rudimentary TFTP server bundled with the system which runs
from the command line, has an allowlist for client connections, and
follows symlinks, so you can have your device download images direct
from the :file:`./result` directory without exposing :file:`/nix/store/` to the
internet or mucking about copying files to :file:`/tftproot`. If the
permitted device is to be given the IP address 192.168.8.251 you might
do something like this:

.. code-block:: console

    nix-shell --run "tufted -a 192.168.8.251 result"

Now add the device and server IP addresses to your configuration:

.. code-block:: nix

  boot.tftp = {
    serverip = "192.168.8.111";
    ipaddr = "192.168.8.251";
  };

and then build the derivation for ``outputs.default`` or
``outputs.mtdimage`` (for which it will be an alias on any device
where this is applicable). You should find it has created

* :file:`result/firmware.bin` which is the file you are going to flash
* :file:`result/flash.scr` which is a set of instructions to U-Boot to
  download the image and write it to flash after erasing the appropriate
  flash partition.

.. NOTE::

   TTL serial connections typically have no form of flow control and
   so don't always like having massive chunks of text pasted into
   them - and U-Boot may drop characters while it's busy. So don't
   necessarily expect to copy-paste the whole of :file:`boot.scr` into
   a terminal emulator and have it work just like that. You may need
   to paste each line one at a time, or even retype it.


For a faster edit-compile-test cycle, you can build a TFTP-bootable
image instead of flashing. In your device configuration add

.. code-block:: nix

  imports = [
    ./modules/tftpboot.nix
  ];

and then build ``outputs.tftpboot``. This creates a file in
``result/`` called ``boot.scr``, which you can copy and paste into
U-Boot to transfer the kernel and filesystem over TFTP and boot the
kernel from RAM.


.. _bng:

Networking
==========

You probably don't want to be testing a device that might serve DHCP,
DNS and routing protocols on the same LAN as you (or your colleagues,
employees, or family) are using for anything else, because it will
interfere. You also might want to test the device against an
"upstream" connection without having to unplug your regular home
router from the internet so you can borrow the cable/fibre/DSL.

``bordervm`` is included for this purpose. You will need

* a Linux machine with a spare (PCI or USB) ethernet device which you can dedicate to Liminix

* an L2TP service such as https://www.aa.net.uk/broadband/l2tp-service/

You need to "hide" the Ethernet device from the host - for PCI this
means configuring it for VFIO passthru; for USB you need to unload the
module(s) it uses. I have this segment in configuration.nix which you
may be able to adapt:

.. code-block:: nix

  boot = {
    kernelParams = [ "intel_iommu=on" ];
    kernelModules = [
      "kvm-intel" "vfio_virqfd" "vfio_pci" "vfio_iommu_type1" "vfio"
    ];

    postBootCommands = ''
      # modprobe -i vfio-pci
      # echo vfio-pci > /sys/bus/pci/devices/0000:01:00.0/driver_override
    '';
    blacklistedKernelModules = [
      "r8153_ecm" "cdc_ether"
    ];
  };
  services.udev.extraRules = ''
    SUBSYSTEM=="usb", ATTRS{idVendor}=="0bda", ATTRS{idProduct}=="8153", OWNER="dan"
  '';

Then
you can execute :command:`run-border-vm` in a ``buildEnv`` shell,
which starts up QEMU using the NixOS configuration in
:file:`bordervm-configuration.nix`.

In this VM

* your Liminix checkout is mounted under :file:`/home/liminix/liminix`

* TFTP is listening on the ethernet device and serving
  :file:`/home/liminix/liminix`.  The server IP address is 10.0.0.1

* a PPPOE-L2TP relay is running on the same ethernet card.  When the
  connected Liminix device makes PPPoE requests, the relay spawns
  L2TPv2 Access Concentrator sessions to your specified L2TP LNS.
  Note that authentication is expected at the PPP layer not the L2TP
  layer, so the PAP/CHAP credentials provided by your L2TP service can
  be configured into your test device - bordervm doesn't need to know
  about them.

To configure bordervm, you need a file called :file:`bordervm.conf.nix`
which you can create by copying and appropriately editing  :file:`bordervm.conf-example.nix`

.. note::

    If you make changes to the bordervm configuration after executing
    :command:`run-border-vm`, you need to remove the :file:`border.qcow2` disk
    image file otherwise the changes won't get picked up.


Running tests
*************

You can run all of the tests by evaluating :file:`ci.nix`, which is the
input I use in Hydra. Note that it expects Nixpkgs stable `and` unstable
as inputs, because it builds the qemu device against both.

.. code-block:: console

    nix-build --argstr liminix `pwd`  --arg  nixpkgs "<nixpkgs>" \
     --argstr unstable `pwd`/../unstable-nixpkgs/ ci.nix

To run a single named test, use the ``-A`` flag. For example, ``-A pppoe``




Troubleshooting
***************

Diagnosing unexpectedly large images
====================================

Sometimes you can add a package and it causes the image size to balloon
because it has dependencies on other things you didn't know about. Build the
``outputs.manifest`` attribute, which is a JSON representation of the
filesystem, and you can run :command:`nix-store --query` on it.

.. code-block:: console

    nix-build -I liminix-config=path/to/your/configuration.nix \
      --arg device "import ./devices/qemu" -A outputs.manifest \
      -o manifest
    nix-store -q --tree manifest


Contributing
************

Contributions are welcome, though in these early days there may be a
bit of back and forth involved before patches are merged:
Please get in touch somehow `before` you invest a lot of time into a
code contribution I haven't asked for.  Just so I know it's expected
and you're not wasting time doing something I won't accept or have
already started on.


Nix language style
==================

This section describes some Nix language style points that we
attempt to adhere to in this repo.

* favour ``callPackage`` over raw ``import`` for calling derivations
  or any function that may generate one - any code that might need
  ``pkgs`` or parts of it.

* prefer ``let inherit (quark) up down strange charm`` over
  ``with quark``, in any context where the scope is more than a single
  expression or there is more than one reference to ``up``, ``down``
  etc.  ``with pkgs; [ foo bar baz]`` is OK,
  ``with lib; stdenv.mkDerivation { ... }`` is usually not.

* ``<liminix>`` is defined only when running tests, so don't refer to it
  in "application" code

* the parameters to a derivation are sorted alphabetically, except for
  ``lib``, ``stdenv`` and maybe other non-package "special cases"

* indentation is whatever emacs nix-mode says it is.

* where a ``let`` form defines multiple names, put a newline after the
  token ``let``, and indent each name two characters

* to decide whether some code should be a package or a module?
  Packages are self-contained - they live in ``/nix/store/eeeeeee-name``
  and don't directly change system behaviour by their presence or
  absense. modules can add to
  ``/etc`` or ``/bin`` or other global state, create services, all that
  side-effecty stuff.  Generally it should be a package unless it
  can't be.



Copyright
=========

The Nix code in Liminix is MIT-licenced (same as Nixpkgs), but the
code it combines from other places (e.g. Linux, OpenWrt) may have a
variety of licences.  I have no intention of asking for copyright
assignment: just like when submitting to the Linux kernel you retain
the copyright on the code you contribute.


Code of Conduct
===============

Please govern yourself in Liminix project venues according to the
`Code of Conduct <https://gti.telent.net/dan/liminix/src/commit/7bcf6b15c3fdddafeda13f65b3cd4a422dc52cd3/CODE-OF-CONDUCT.md>`_


Where to send patches
=====================


Liminix' primary repo is https://gti.telent.net/dan/liminix but you
can't send code there directly  because it doesn't have open registrations.

* There's a `mirror on Github <https://github.com/telent/liminix>`_ for
  convenience and visibility: you can open PRs against that

* or, you can send me your patch by email using `git send-email <https://git-send-email.io/>`_

* or in the future, some day, we will have federated Gitea using
  ActivityPub.