Recently I had to work with OSTree which I think is a very promising and interesting project.

The OSTree site advertises it as

a tool for managing bootable, immutable, versioned filesystem trees

and it uses a combination of chroot and hard links to provide atomic upgrades and rollback of filesystem trees. You can refer to the project’s online manual and README for information about the motivation behind OSTree and its implementation.

When updating the kernel and initial RAM disk images OSTree creates Boot Loader Specification entries. These are drop-in snippets that define a kernel and init ramdisk that have to be used and boot arguments that the bootloader has to pass to the kernel command line on boot.

Currently only gummiboot and GRUB have support for these boot loader snippets.

We wanted to use OSTree on embedded devices and since most boards use U-Boot as their boot loader I needed to figure out what was the best approach to add a U-Boot support for OSTree.

Obviously this would have been to add boot loader spec entries support to U-Boot but there are two issues with this approach:

a) U-Boot does not currently support multi-boot

Since U-Boot is designed for embedded it just boots a single default operating system while the boot loader specification was designed for multi-boot. You can drop any number of snippets under /boot/loader/entries and the boot loader should populate its boot menu.

b) Not every vendor allows you to replace the boot loader

Some vendors do not allow to replace the boot loader binaries (i.e: store it on a read-only memory), implements DRM that requires binaries to be signed or are using a very old U-Boot version which would require to backport this support.

So, the solution was to make OSTree to generate boot information instead that could be used by any already deployed U-Boot. The same approach is used on OSTree to support syslinux.

U-Boot allows to modify boards default boot commands by reading and executing a bootscript file (boot.scr) or importing a plain text file (uEnv.txt) that contains environment variables that could parameterize its default boot command or a bootscript.

So, on deploy or upgrade OSTree reads the Boot Loader Specification snippets files and generates a uEnv.txt file that contains the booting information in a format that could be understood by U-Boot.

The uEnv.txt env var file is created in the path /boot/loader.${bootversion}/uEnv.txt. Also, a /boot/uEnv.txt symbolic link to loader/uEnv.txt is created so U-Boot can always import the file from a fixed path.

Since U-Boot does not support a menu to list a set of Operative Systems, the most recent boot loader entry from the list is used.

To boot an OSTree using the generated uEnv.txt file, a board has to parameterize its default boot command using the following variables defined by OSTree:

    ${kernel_image}:  path to the Linux kernel image
    ${ramdisk_image}: path to the initial ramdisk image
    ${bootargs}:      parameters passed to the kernel command line

Alternatively, for boards that don’t support this scheme, a bootscript that overrides the default boot command can be used.

An example of such a bootscript could be:

    setenv scriptaddr 0x40008000
    setenv kernel_addr 0x40007000
    setenv ramdisk_addr 0x42000000
    load mmc 0:1 ${scriptaddr} uEnv.txt
    env import -t ${scriptaddr} ${filesize}
    load mmc 0:1 ${kernel_addr} ${kernel_image}
    load mmc 0:1 ${ramdisk_addr} ${ramdisk_image}
    bootz ${kernel_addr} ${ramdisk_addr}

For more information you can refer to the commit that added U-Boot as backend and OStree test for U-Boot.

I would like to thank my employer Collabora for sponsoring this work.