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Building and using MCUboot with Zephyr

MCUboot began its life as the bootloader for Mynewt. It has since acquired the ability to be used as a bootloader for Zephyr as well. There are some pretty significant differences in how apps are built for Zephyr, and these are documented here.

Please see the design document for documentation on the design and operation of the bootloader itself. This functionality should be the same on all supported RTOSs.

The first step required for Zephyr is making sure your board has flash partitions defined in its device tree. These partitions are:

  • boot_partition: for MCUboot itself
  • slot0_partition: the primary image slot
  • slot1_partition: the secondary image slot
  • scratch_partition: the scratch slot

Currently, the two image slots must be contiguous. If you are running MCUboot as your stage 1 bootloader, boot_partition must be configured so your SoC runs it out of reset.

The flash partitions are typically defined in the Zephyr boards folder, in a file named boards/<arch>/<board>/<board>.dts. An example .dts file with flash partitions defined is the frdm_k64f's in boards/arm/frdm_k64f/frdm_k64f.dts. Make sure the labels in your board's .dts file match the ones used there.

Building the bootloader itself

The bootloader is an ordinary Zephyr application, at least from Zephyr's point of view. There is a bit of configuration that needs to be made before building it. Most of this can be done as documented in the CMakeLists.txt file in boot/zephyr. There are comments there for guidance. It is important to select a signature algorithm, and decide if slot0 should be validated on every boot.

To build MCUboot, create a build directory in boot/zephyr, and build it as usual:

  cd boot/zephyr
  mkdir build && cd build
  cmake -GNinja -DBOARD=<board> ..
  ninja

In addition to the partitions defined in DTS, some additional information about the flash layout is currently required to build MCUboot itself. All the needed configuration is collected in boot/zephyr/include/target.h. Depending on the board, this information may come from board-specific headers, Device Tree, or be configured by MCUboot on a per-SoC family basis.

After building the bootloader, the binaries should reside in build/zephyr/zephyr.{bin,hex,elf}, where build is the build directory you chose when running cmake. Use the Zephyr build system flash target to flash these binaries, usually by running make flash (or ninja flash, etc.) from the build directory.

Building Applications for the bootloader

In addition to flash partitions in DTS, some additional configuration is required to build applications for MCUboot.

The directory samples/zephyr/hello-world in the MCUboot tree contains a simple application with everything you need. You can try it on your board and then just make a copy of it to get started on your own application; see samples/zephyr/README.md for a tutorial.

More details:

  • CONFIG_TEXT_SECTION_OFFSET must be set to allow room for the boot image header. Typically this is set in the app's prj.conf. It must also be aligned to a boundary that the particular MCU requires the vector table to be aligned on.

  • Your board must provide a DTS zephyr,code-partition chosen node which ensures it is built and linked into the DTS slot0_partition. This is typically achieved by creating a dts.overlay file that contains the chose node description:

         chosen {
                zephyr,code-partition = &slot0_partition;
         };

With this, build the application as your normally would.

Signing the application

In order to upgrade to an image (or even boot it, if MCUBOOT_VALIDATE_SLOT0 is enabled), the images must be signed. To make development easier, MCUboot is distributed with some example keys. It is important to stress that these should never be used for production, since the private key is publicly available in this repository. See below on how to make your own signatures.

There is a sign.sh script that gives some examples of how to make these signatures.

Flashing the application

The application itself can flashed with regular flash tools, but will need to be loaded at the offset of SLOT-0 for this particular target. These images can also be marked for upgrade, and loaded into SLOT-1, at which point the bootloader should perform an upgrade. It is up to the image to mark slot-0 as "image ok" before the next reboot, otherwise the bootloader will revert the application.

Managing signing keys

The signing keys used by MCUboot are represented in standard formats, and can be generated and processed using conventional tools. However, the Mynewt project has developed some tools to make this easier, and the imgtool directory contains a small program to use these tools, as well as some additional tools for generating and extracting public keys. If you will be using your own keys, it is recommended to build this tool following the directions within the directory.

Generating a new keypair

Generating a keypair with imgtool is a matter of running the keygen subcommand:

    $ imgtool keygen -k mykey.pem -t rsa-2048

The argument to -t should be the desired key type. See the imgtool README.rst for more details on the possible key types.

Extracting the public key

The generated keypair above contains both the public and the private key. It is necessary to extract the public key and insert it into the bootloader. The keys live in boot/zephyr/keys.c, and can be extracted using imgtool:

    $ imgtool getpub -k mykey.pem

This will output the public key as a C array that can be dropped directly into the keys.c file.

Once this is done, this new keypair file (mykey.pem in this example) can be used to sign images.