# nerves_runtime
[](https://circleci.com/gh/nerves-project/nerves_runtime)
[](https://hex.pm/packages/nerves_runtime)
`nerves_runtime` is a core component of Nerves. It contains applications and
libraries that are expected to be useful on all Nerves devices.
Here are its features:
* Generic system and filesystem initialization (suitable for use with
[`shoehorn`](https://github.com/nerves-project/shoehorn))
* Introspection of Nerves system, firmware, and deployment metadata
* Device reboot and shutdown
* A small Linux kernel `uevent` application for capturing hardware change events
and more
The following sections describe the features in more detail. For more
information, see the [hex docs](https://hexdocs.pm/nerves_runtime).
## System Initialization
`nerves_runtime` provides an OTP application (`nerves_runtime`) that can
initialize the system when it is started. For this to be useful,
`nerves_runtime` must be started before other OTP applications, since most will
assume that the system is already initialized before they start. To set up
`nerves_runtime` to work with `shoehorn`, you will need to do the following:
1. Include `shoehorn` in `mix.exs`
2. Include `shoehorn` in your `rel/config.exs`
3. Ensure that `:nerves_runtime` is at the beginning of the `init:` list in
your `config/config.exs`:
```elixir
config :shoehorn,
overlay_path: "",
init: [:nerves_runtime, :other_app1, :other_app2],
app: :your_app
```
### Kernel Modules
`nerves_runtime` will attempt to auto-load kernel modules by calling `modprobe`
using the `modalias` supplied by the device's `uevent` message. You can disable
this feature by configuring `autoload: false` in your application configuration:
```elixir
config :nerves_runtime, :kernel,
autoload_modules: false
```
`nerves_runtime` can optionally report device insertions and removals through
[SystemRegistry](https://github.com/nerves-project/system_registry). This is
currently the default, but you can disable it via configuration:
```elixir
config :nerves_runtime, :kernel,
use_system_registry: false
```
## Filesystem Initialization
Nerves systems generally ship with one or more application filesystem
partitions. These are used for persisting data that is expected to live between
firmware updates. The root filesystem cannot be used since it is mounted as
read-only by default.
`nerves_runtime` takes an unforgiving approach to managing the application
partition: if it can't be mounted as read-write, it gets re-formatted. While
filesystem corruption should be a rare event, even with unexpected loss of
power, Nerves devices may not always be accessible for manual recovery. This
default behavior provides a basic recoverability guarantee.
To verify that this recovery works, Nerves systems usually leave the application
filesystems uninitialized so that the format operation happens on the first
boot. This means that the first boot takes slightly longer than subsequent
boots.
Note that a common implementation of "reset to factory defaults" is to purposely
corrupt the application partition and reboot.
`nerves_runtime` uses firmware metadata to determine how to mount and initialize
the application partition. The following variables are important:
* `[partition].nerves_fw_application_part0_devpath` - the path to the
application partition (e.g. `/dev/mmcblk0p3`)
* `[partition].nerves_fw_application_part0_fstype` - the type of filesystem
(e.g. `ext4`)
* `[partition].nerves_fw_application_part0_target` - where the partition should
be mounted (e.g. `/root` or `/mnt/appdata`)
## Nerves System and Firmware Metadata
All official Nerves systems maintain a list of key-value pairs for tracking
various information about the system. This information is not intended to be
written frequently. To get this information, you can call one of the following:
* `Nerves.Runtime.KV.get_all_active/0` - return all key-value pairs associated
with the active firmware.
* `Nerves.Runtime.KV.get_all/0` - return all key-value pairs, including those
from the inactive firmware, if any.
* `Nerves.Runtime.KV.get_active/1` - look up the value of a key associated with
the active firmware.
* `Nerves.Runtime.KV.get/1` - look up the value of a key, including those from
the inactive firmware, if any.
Global Nerves metadata includes the following:
Key | Build Environment Variable | Example Value | Description
------------------- | ---------------------------- | ---------------- | -----------
`nerves_fw_active` | N/A | `"a"` | This key holds the prefix that identifies the active firmware metadata. In this example, all keys starting with `"a."` hold information about the running firmware.
`nerves_fw_devpath` | `NERVES_FW_DEVPATH` | `"/dev/mmcblk0"` | This is the primary storage device for the firmware.
`nerves_serial_number` | N/A | `"12345abc"` | This is a text serial number. See [Serial numbers](#serial_numbers) for details.
`nerves_fw_validated` | N/A | `0` | Set to "1" to indicate that the currently running firmware is valid. (Only supported on some platforms)
`nerves_fw_autovalidate` | N/A | `1` | Set to "1" to indicate that firmware updates are valid without any additional checks. (Only supported on some platforms)
Firmware-specific Nerves metadata includes the following:
Key | Example Value | Description
:------------------------------------ | :---------------- | :----------
`nerves_fw_application_part0_devpath` | `"/dev/mmcblkp3"` | The block device that contains the application partition
`nerves_fw_application_part0_fstype` | `"ext4"` | The application partition's filesystem type
`nerves_fw_application_part0_target` | `"/root"` | Where to mount the application partition
`nerves_fw_architecture` | `"arm"` | The processor architecture (Not currently used)
`nerves_fw_author` | `"John Doe"` | The person or company that created this firmware
`nerves_fw_description` | `"Stuff"` | A description of the project
`nerves_fw_platform` | `"rpi3"` | A name to identify the board that this runs on. It can be checked in the `fwup.conf` before performing an upgrade.
`nerves_fw_product` | `"My Product"` | A product name that may show up in a firmware selection list, for example
`nerves_fw_version` | `"1.0.0"` | The project's version
`nerves_fw_vcs_identifier` | `"bdeead38..."` | A `git` SHA or other identifier (optional)
`nerves_fw_misc` | `"anything..."` | Any application info that doesn't fit in another field (optional)
Note that the keys are stored in the environment block prefixed by the firmware
slot for which they pertain. For example, `a.nerves_fw_description` is the
description for the firmware in the "A" slot.
Several of the keys can be set in the `mix.exs` file of your main Nerves
project. This is the preferred way to set them because it requires the least
amount of effort.
Assuming that your `fwup.conf` respects the `fwup` variable names listed in the
table, the keys can also be overridden by setting environment variables at build
time. Depending on your project, you may prefer to set them using a customized
`fwup.conf` configuration file instead.
The `fwup -m` value shows the key that you'll see if you run `fwup -m -i
<project.fw>` to extract the firmware metadata from the `.fw` file.
Key in `Nerves.Runtime` | Key in `mix.exs` | Build Environment Variable | Key in `fwup -m`
------------------------------------- | --------------------------- | ------------------------------------- | ----------------
`nerves_fw_application_part0_devpath` | N/A | `NERVES_FW_APPLICATION_PART0_DEVPATH` | N/A
`nerves_fw_application_part0_fstype` | N/A | `NERVES_FW_APPLICATION_PART0_FSTYPE` | N/A
`nerves_fw_application_part0_target` | N/A | `NERVES_FW_APPLICATION_PART0_TARGET` | N/A
`nerves_fw_architecture` | N/A | `NERVES_FW_ARCHITECTURE` | `meta-architecture`
`nerves_fw_author` | `:author` | `NERVES_FW_AUTHOR` | `meta-author`
`nerves_fw_description` | `:description` | `NERVES_FW_DESCRIPTION` | `meta-description`
`nerves_fw_platform` | N/A | `NERVES_FW_PLATFORM` | `meta-platform`
`nerves_fw_product` | `:name` | `NERVES_FW_PRODUCT` | `meta-product`
`nerves_fw_version` | `:version` | `NERVES_FW_VERSION` | `meta-version`
`nerves_fw_vcs_identifier` | N/A | `NERVES_FW_VCS_IDENTIFIER` | `meta-vcs-identifier`
`nerves_fw_misc` | N/A | `NERVES_FW_MISC` | `meta-misc`
## Device Reboot and Shutdown
Rebooting, powering-off, and halting a device work by signaling to
[`erlinit`](https://github.com/nerves-project/erlinit) an intention to shutdown
and then exiting the Erlang VM by calling `:init.stop/0`. The
`Nerves.Runtime.reboot/0` and related utilities are helper methods for this.
Once they return, the Erlang VM will likely only be available momentarily before
shutdown. If the OTP applications cannot be stopped within a timeout as
specified in the `erlinit.config`, `erlinit` will ungracefully terminate the
Erlang VM.
## Reverting firmware
If you'd like to go back to the previous version of firmware running on a
device, you can do that if the Nerves system supports it. At the IEx prompt,
run:
```elixir
iex> Nerves.Runtime.revert
```
Running this command manually is useful in development. Production use requires
more work to protect against faulty upgrades.
### Assisted firmware validation and automatic revert
Nerves firmware updates protect against update corruption and powerloss
midway into the update procedure. However, what happens if the firmware update
contains bad code that hangs the device or makes breaks something important like
networking? Some Nerves systems support tentative runs of new firmware and if
something goes wrong, they'll revert back.
At a high level, this involves some additional code from the developer that
knows what constitutes "working". This could be "is it possible to connect to
the firmware update server within 5 minutes of boot?"
Here's the process:
1. New firmware is installed in the normal manner. The `Nerves.Runtime.KV`
variable, `nerves_validated` is set to 0.
2. The system reboots like normal.
3. The device starts a five minute reboot timer (your code needs to do this)
4. The application attempts to make a connection to the firmware update server.
5. On a good connection, the application sets `nerves_validated` to 1 and
cancels the reboot timer.
6. On error, the reboot timer failing, or a hardware watchdog timeout, the
system reboots. The bootloader reverts to the previous firmware.
To use this feature, the `nerves_autovalidate` variable must be set to 0. This
can be done at device provisioning time (like when the serial number is set) or
inside a custom `fwup.conf`.
### Best effort automatic revert
Unfortunately, the bootloader for platforms like the Raspberry Pi makes it
difficult to implement the above mechanism. The following strategy cannot
protect against kernel and early boot issues, but it can still provide value:
1. Upgrade firmware the normal way. Record that the next boot will be the first
one in the application data partition.
2. On the reboot, if this is the first one, record that the boot happened and
revert the firmware with `reboot: false`. If this is not the first boot,
carry on.
3. When you're happy with the new firmware, revert the firmware again with
`reboot: false`. I.e., revert the revert. It is critical that `revert` is
only called once.
To make this handle hangs, you'll want to enable a hardware watchdog.
## Operating system log collection
Operating system-level messages from `/dev/log` and `/proc/kmsg`, forwarding
them to `Logger` with an appropriate level to match the syslog priority parsed
out of the message.
## Serial numbers
Nerves systems support several methods for assigning serial numbers to devices.
By default serial numbers are derived using board-specific identifiers.
Currently, no one place exists that can be queried for the serial number.
However, many people are using the `nerves_serial_number` key in the U-Boot environment
block to store a serial number for their device. This location is not "secure"
against a determined person who wants to clone a device. However, it is good
enough for many use cases and is available on all platforms supported by Nerves.
See
[embedded-elixir](https://embedded-elixir.com/post/2018-06-15-serial_number/)
for how to assign serial numbers to devices.
## Using nerves_runtime in tests
Applications that depend on `nerves_runtime` for accessing provisioning
information from the `Nerves.Runtime.KV` can mock the contents through the
Application config.
```elixir
config :nerves_runtime, :modules, [
{Nerves.Runtime.KV.Mock, %{"key" => "value"}}
]
```
