Approach to Software Update Management

Each vertical configuration describes a sub-system of the product.

Update management

The software update components are responsible for platform level software update. This sub-system is critical in an automotive context since it allows to deploy new functionalities, fix important bugs and security issues or update assets such as driving data. A well designed update sub-system can save a lot of money to car manufacturers by avoiding products recalls and to end-users by not having to go to a mechanic to fix software problems.

Use cases

A platform developer should be able to deploy rootfs, kernel, bootloader, ECU's flash memory, configuration and/or system design modifications to a fleet of devices currently running the platform.

End-users should be able to apply those modifications using their Internet connectivity: for example with LTE or WiFi, using Over The Air (OTA) mechanisms. They (or their mechanics) should also have the possibility to install updates from physical devices such as USB keys or SD cards.

Requirements

In order to meet safety concerns, an update mechanism has to be fault resilient. For instance, if a power failure happens during an update, the system should not end up in an inconsistent state. This requirement implies that updates have to be atomic/transactional. (i.e: never partially applied)

In order to modify the rootfs without affecting the system execution, the update solution can proceed with one of those two mechanisms:

  • A/B symmetric partitioning where two different versions of the system are deployed on two distinct partitions (that could even be located on two different memory chips). This has the benefit of always having one working partition. When the partition A is upgrading, it modifies the partition B and, when done, switches the default bootloader entry to B. This solution only requires a downtime when rebooting, however it is expensive both in terms of components and disk space.

  • Normal+recovery asymmetric partitioning where a large partition includes the normal system and a secondary small partition can boot into a minimal working system able to update the main partition. This solution is cheaper but introduces a longer downtime than a symmetric partitioning.

  • In-place upgrading where a single partition is capable of modifying itself in an atomic way. This is notably used by OSTree. This requires less space than the previous methods and a short downtime.

If an update introduces regressions, the system should be able to revert automatically to a previous working state. This can be achieved using different mechanisms. Typical solutions include:

  • With A/B symmetric partitioning, if one partition fails booting, the bootloader can detect the error and boot the other partition.

  • With a normal+recovery partitioning, if an error is detected, the bootloader can reboot into recovery mode and fix the problem.

  • With an in-place upgrading solution, specifically OSTree, different boot entries can deploy different versions of the rootfs.

The above requirement underlines the fact that an update mechanism can sometimes constrain the design of the rest of the system. An ideal update solution should make few assumptions on the rest of the system's architecture. (for instance: bootloader dependencies, number of partitions, read-only or read-write partitions etc...) This would limit the complexity of integration of the solution. Some update solutions also support several update mechanism schemes and give more flexibility to the platform developer.

An update mechanism should limit the resources (i.e: Disk space, RAM usage, Bandwidth consumption etc...) usage on end-devices but also on the update servers. Typical solutions are:

  • Differential updates: avoid too large downloads and processing. However, this can cause problems if a local data block is corrupted.

  • Full downloads: alternatively, downloading complete images solves the issue of local corrupted data but is much more resource intensive.

On a different side, updates can also be file or block based which affect the portability of update data across devices and also the size of downloaded information.

An OTA server should propose various fleet and deployment management scenarios. For example, a car manufacturer should be able to deploy updates on a certain range of devices. (for example by car models and/or geographically and/or following a schedule)

An update mechanism has to guarantee fundamental security capabilities. The two mainly expected features are:

  • Integrity: this guarantees that data haven't been modified from the server to the client. (for example, by man-in-the-middle attacks)

  • Authentication: this guarantees that update have been created by an authorized entity. (for example, by a Tier-1 or OEM vendor) This can be achieved using digital signatures, CMAC or HMAC.

In order to implement those checks correctly, it would be interesting to rely on a deep root of trust with TPM or TEE.

An automotive update mechanism should strive to minimize downtimes when updating and should not run while the car is driving.

An update solution should easily be integrated to a given Graphical User Interface. This can be achieved with APIs such as D-Bus interfaces or C++ libraries.

A plus for an update solution in the context of an automotive Linux platform would also be to have an integration with Yocto.

Case studies

This appendix summarizes the researches that led to the above choice. The following paragraphs analyzes various update solutions in the specific context of PELUX.

Sysup

This is a very simple tool to achieve A/B partition switching. It is actually just an initramdisk script that runs pivot_root on the wanted partition. It is very simple and straightforward but actually, it does not even contain an upgrade solution. Also, it does not allow fallback if the kernel or bootloader fails. This solution can not be enough for the needs of the automotive industry and will not be retained for PELUX.

Smart2

This tool has not been updated for a while, contains lots of legacy code and pending issues. It is also just a package manager which can not guarantee atomic updates. Atomic updates being essential in the context of car systems, smart2 can not be used for PELUX.

Swupd

This solution offers a variety of disk layouts possibilities. It can also download source from remote or local media which is a good point. However, the approach of Swupd is to favor speed over failure resilience which means that the system can end up in an inconsistent state and can not rollback. Also, this tool is only able to update the rootfs. Overall, this is not an acceptable solution for the automotive use case and it has not been kept for PELUX.

Fwup

This self-contained tool offers a variety of functions useful in the context of critical embedded systems. It supports atomic updates (with A/B and recovery schemes) and rollback, digital signature, local and remote updates, potentially MCUs upgrade thanks to "file-resources" and it integrates well with Yocto. Unfortunately, it does not support fleet management in itself and needs to be combined with something else.

Resin

This is a containerized update tool that relies on two Docker containers: a resin supervisor and an application container both running on top of a stateless OS. This offers a very interesting approach to zero-downtime upgrading and A/B partitioning thanks to a "hand over" mechanism between two application containers. Unfortunately, this tool relies on a commercial offer with very complex pricing when it comes to large fleet of devices. Moreover, this tool does not updates the host OS (bootloader, kernel, rootfs) and it requires applications designed to be ran in a container environment which is not the case in PELUX. Because of those two reasons, this solution has not been retained for PELUX.

Adaptive AUTOSAR UCM

The Adaptive Autosar Update and Configuration Management functional cluster that is in charge of distributing updates across the vehicle could potentially be developed in the future and become a standard for the industry. However, as of today, it is purely speculative, it would require a high stage of integration into an actual vehicle and it would still require some sort of component in the PELUX Linux platform side to apply the updates. While this is useful to keep in mind for the future, this can not be retained for PELUX.

Mender

This is a block based update solution that supports rollback and atomic updates. It guarantees integrity and authentication security requirements, has a fully-featured deployment panel and a handy Yocto layer. Mender is easy to integrate to an embedded Linux system but at the cost of its lack of flexibility. Mender imposes an A/B scheme with two additional partitions for bootloader and data. The kernels also have to be located in the A and B partitions as files. The goal of PELUX being to serve as a baseline for various projects, we will prefer a more flexible solution such as one of those detailed below.

OSTree

OSTree is an elegant file-based update mechanism that uses hard links to achieve in-place(no A/B partitioning) atomic updates. It is often described as a "git for operating systems". It currently benefits from a very large and active community. It has support for rollback. It integrates with Yocto easily. It was chosen by AGL for all of those reasons. However, OSTree suffers from some limitations if the rootfs to be upgraded is corrupted and since OSTree is only able to update file systems, it can not always upgrade kernels and can not flash other types of firmwares such as Bootloaders or MCUs. Hence, this solution may not be enough on its own depending on the needs of the project.

QtOTA

This solution contains a set of scripts and QML APIs to easily integrate OSTree in a Yocto and Qt/QML system. QtOTA seems preferable over OSTree alone if the final system is tightly linked to a Qt architecture. However, it suffers from the same limitations as OSTree such as the incapacity to update Bootloaders or MCUs.

GENIVI SOTA (Aktualizr)

GENIVI defined a modular architecture for Software over-the-air update deployment split into a SOTA Server, SOTA Client and installer. The SOTA server offers various deployment scenarios based a on VIN (vehicle identifiers) registry. The client side, whose current reference implementation is Aktualizr, can download any kind of data from the server and relay that to an installer. It is also worth noting that this implementation supports complex security mechanisms using Uptane and RVI. Aktualizr is not enough on its own, it needs to be integrated with an installer to provide a fully featured update solution.

SWUpdate

This tool is extremely flexible, it is even described by its developers as an update framework. It is fault resilient, supports atomic updates, fallback(with both A/B and Normal+recovery). it makes few assumptions regarding the base system, flashes entire compressed images, it can interface with complex fleet management systems such as Hawkbit, it guarantees integrity and authentication, offers APIs for GUI integration, is easily integrated to Yocto and can be extended with handlers to upgrade FPGAs, MCUs or other components and is well documented. SWUpdate meets the requirements of PELUX.

RAUC

This solution is failsafe, atomic, can revert to a previous state, is flexible enough when it comes to partition layout, uses a bundle of images that can be downloaded from the network or from local media, interfaces with Hawkbit, has authentication and integrity mechanisms, offers a DBus API, integrates well with Yocto and can be extended to flash other components. RAUC is very similar to SWUpdate and also qualifies for the needs of PELUX.

Conclusions

If RVI (as opposed to just HTTPS) or Uptane (as opposed to just TLS) or the Vehicle fleet management of GENIVI SOTA (as opposed to Hawkbit) is considered useful, we advise to combine Aktualizr with the upgrade solution chosen below:

If you want to be able to download and flash full images we advise to use SWUpdate or RAUC (those two solutions offer pretty much the same functionalities). However, if you decide to use differential updates, we advise to use OSTree instead.

For PELUX, we decided that Aktualizr was not needed for our use cases. We also decided to start with full images flashing and maybe explore OSTree later on. We then chose to start with SWUpdate alone and then combine it with OSTree.