Chapter 10. Managing kernel arguments in bootc systems You can use bootc to configure kernel arguments. By default, bootc uses the boot loader configuration files that are stored in /boot/loader/entries . This directory defines arguments provided to the Linux kernel. The set of kernel arguments is machine-specific state, but you can also manage the kernel arguments by using container updates. The boot loader menu entries are shared between multiple operating systems and boot loaders are installed on one device. Note Currently, the boot loader entries are written by an OSTree backend. 10.1. How to add support to inject kernel arguments with bootc 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: You can also make these kernel arguments architecture-specific by using the match-architectures key. For example: 10.2. How to modify kernel arguments by using bootc install configs You can use bootc install to add kernel arguments during the install time in the following ways: Adding kernel arguments into the container image. Adding kernel arguments by using the bootc install --karg command. 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: Create files within /usr/lib/bootc/kargs.d with kernel arguments. Fetch the container image to get the OSTree commit. Use the OSTree commit to return the file tree. Navigate to /usr/lib/bootc/kargs.d . Read each file within the directory. Push the contents of each kargs file into a file containing all the needed kargs . Pass the kargs to the stage() function. Apply these arguments to switch, upgrade, or edit. 10.3. How to inject kernel arguments in the Containerfile To add kernel arguments into a container image, use a Containerfile. The following is an example: 10.4. How to inject kernel arguments at installation time 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: Note Currently, bootc does not have an API to manipulate kernel arguments. This is only supported by rpm-ostree , by using the rpm-ostree kargs command. 10.5. How to add install-time kernel arguments with bootc-image-builder 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: 10.6. About changing kernel arguments post-install with kargs.d 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. 10.7. How to edit kernel arguments in bootc systems 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] --append=KEY=VALUE Appends a kernel argument. It is useful with, for example, console= that can be used multiple times. You can use an empty value for an argument. --replace=KEY=VALUE=NEWVALUE Replaces an existing kernel argument. You can replace an argument with KEY=VALUE only if one value already exists for that argument. --delete=KEY=VALUE Deletes a specific kernel key-value pair argument or an entire argument with a single key-value pair. --append-if-missing=KEY=VALUE Appends a kernel argument. Does nothing if the key is already present. --delete-if-present=KEY=VALUE Deletes a specific kernel key-value pair argument. Does nothing if the key is missing. --editor Uses an editor to modify the kernel arguments. For more information, check the help: The following is an example:

Commands:
/usr/lib/bootc/kargs.d/10-example.toml kargs = ["mitigations=auto,nosmt"]
/usr/lib/bootc/kargs.d/00-console.toml kargs = ["console=ttyS0,114800n8"] match-architectures = ["x86_64"]
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
bootc install to-filesystem --karg
{ "customizations": { "kernel": { "append": "mitigations=auto,nosmt" } } }
rpm-ostree kargs --help
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