ostree-prepare-root — Change the view of a mounted root filesystem to an ostree deployment
ostree prepare-root {TARGET}
At its core, ostree operates on an existing mounted filesystem. Tooling such
as ostree admin deploy will create a new directory that can be
used as a bootable target. This tool is designed to run in an initramfs and
set up "remapping" mounts as a view into that filesystem.
As of more recently, this tool also has optional support for composefs, which creates a distinct mount point layered on top of the underlying filesystem.
The most common pattern today is to use systemd in an initramfs. The systemd
unit shipped upstream is ordered in this way:
After=sysroot.mount and Before=initrd-root-fs.target
When it runs, the mounted filesystem at the provided TARGET (usually /sysroot)
will be changed such that what appears at /sysroot is actually the
"deployment root" - i.e. a particular versioned subdirectory. What was formerly the
"physical root" i.e. the real root of the filesystem will appear as /sysroot/sysroot.
For /var, by default a bind mount is created from the deployment root to /sysroot/var.
A read-only bind mount is created over /sysroot/usr. The immutable bit (see chattr(1)) is set on the deployment
root, so this provides basic protection for filesystem mutation. If the sysroot.readonly
option is enabled, then /sysroot/sysroot is mounted read-only to provide further protection and a writable bind mount for
/sysroot/etc is created.
Finally, when higher level tooling such as systemd performs a switch-root operation, what
was /sysroot becomes / and after the transition into
the real root, the system will be booted into the "deployment", which is a versioned immutable
filesystem tree. The ostree tooling running in the real root thereafter performs further changes
by operating on /sysroot which is now the "physical root".
The /usr/lib/ostree/prepare-root.conf (or /etc/ostree/prepare-root.conf) config file is parsed by ostree-prepare-root. This file must
be present in the initramfs. The default dracut module will copy it from the real root if present.
sysroot.readonlyA boolean value; the default is false unless composefs is enabled. If this is set to true, then the /sysroot mount point is mounted read-only.
etc.transientA boolean value; the default is false. If this is set to true, then the /etc mount point is mounted transiently i.e. a non-persistent location.
root.transientA boolean value; the default is false.
Setting this flag to true requires composefs (See composefs.enabled).
When enabled, the root mount point / will be an overlayfs whose contents will be stored
in a tmpfs, and hence discarded on OS upgrade or reboot.
This option is independent of etc.transient and sysroot.readonly;
it is supported for example to have root.transient=true but etc.transient=false in which case changes to /etc continue
to persist across updates, with the default OSTree 3-way merge applied.
Also related to persistence it is important to emphasize that /sysroot (the physical root filesystem) is still persistent
by default; in-place OS upgrades can be applied.
Enabling this option can make it significantly easier to adopt an image-based model in some circumstances. For example, if you have a configuration management system that is inspecting machine-specific state and e.g. dynamically installing packages or applying configuration, it can more easily be adapted to run on each boot, while still shifting a portion (or ideally most) image configuration to build time as part of the base image/commit.
composefs.enabledThis can be yes, no, maybe,
signed, or verity. The default is no.
If set to yes, signed, or verity,
then composefs is always used, and the boot fails if it is not available.
If set to signed or verity,
before the content of a file is read,
the integrity of its backing OSTree object is validated by the digest stored in the image.
Additionally, if set to signed, boot will fail if the image cannot be
validated by a public key.
Setting this to maybe will cause composefs to be used at runtime only
if the deployment has a composefs generated, which causes unpredicable and confusing semantics
and is not recommended. In practice with the current version of ostree,
in the case where composefs is enabled at build time for both the version that made the
deployment (often an older OS version), this will be equivalent to yes.
But in general one either wants composefs or not, so choose an explicit value for that.
composefs.keypathPath to a file with Ed25519 public keys in the initramfs, used if
composefs.enabled is set to signed. The default value for this is
/etc/ostree/initramfs-root-binding.key. For a valid signed boot the target OSTree
commit must be signed by at least one public key in this file, and the commitfs digest listed in the
commit must match the target composefs image.
The following kernel commandline parameters are also parsed:
ostree.prepare-root.composefsThis accepts the same values as composefs.enabled above, and overrides the config file (if present).
For example, specifying ostree.prepare-root.composefs=0 will disable composefs, even if it is enabled by default in the initrd config.