Chapter 6. Managing kernel arguments in bootc systems

The bootc tool uses generic operating system kernels. You can add support to inject kernel arguments by adding a custom configuration, in the TOML format, in /usr/lib/bootc/kargs.d. For example:

# /usr/lib/bootc/kargs.d/10-example.toml
kargs = ["mitigations=auto,nosmt"]

You can also make these kernel arguments architecture-specific by using the match-architectures key. For example:

# /usr/lib/bootc/kargs.d/00-console.toml
kargs = ["console=ttyS0,114800n8"]
match-architectures = ["x86_64"]

You can use bootc install to add kernel arguments during the install time in the following ways:

You can use the kernel arguments on Day 2 operations, by adding the arguments and applying them on a switch, upgrade, or edit. Adding kernel arguments and using it for Day 2 operations involves the following high-level steps:

To add kernel arguments into a container image, use a Containerfile. The following is an example:

FROM registry.redhat.io/rhel9/rhel-bootc:latest

RUN mkdir -p /usr/lib/bootc/kargs.d
RUN cat <<EOF >> /usr/lib/bootc/kargs.d/console.toml
kargs = ["console=ttyS0,114800n8"]
match-architectures = ["x86_64"]
EOF

RUN cat <<EOF >> /usr/lib/bootc/kargs.d/01-mitigations.toml
kargs = ["mitigations=on", "systemd.unified_cgroup_hierarchy=0"]
match-architectures = ["x86_64", "aarch64"]
EOF

You can use boot install with the --karg to inject kernel arguments during installation time. As a result, the kernel arguments become machine-local state.

For example, to inject kernel arguments, use the following command:

# bootc install to-filesystem --karg

The bootc-image-builder tool supports the customizations.kernel.append during install-time.

To add the kernel arguments with bootc-image-builder, use the following customization:

{
  "customizations": {
    "kernel": {
      "append": "mitigations=auto,nosmt"
    }
  }
}

The changes that you make to kargs.d files and include in a container build are applied after the installation, and the difference between the set of kernel arguments is applied to the current boot loader configuration. This preserves any machine-local kernel arguments. You can use any tool to edit the /boot/loader/entries files, which are in a standardized format. The /boot file has read-only access to limit the set of tools that can write to this filesystem.

To perform machine local changes, you also can edit kernel arguments on a bootc system or an`rpm-ostree` system, by using the rpm-ostree kargs command. The changes are made through the user/lib/bootc/kargs.d path, which also handles "Day 2" changes, besides the first boot changes.

The following are the options that you can use to add, modify or remove kernel arguments.

rpm-ostree kargs [option]

For more information, check the help:

# rpm-ostree kargs --help

The following is an example:

# rpm-ostree kargs --append debug
Staging deployment... done
Freed: 40.1 MB (pkgcache branches: 0)
Changes queued for next boot. Run "systemctl reboot" to start a reboot