Modify kernel boot options

On Ubuntu Core 20/22, when using the default GNU GRUB bootloader, the kernel command line options can be customised without breaking the integrity of the UEFI Secure Boot mechanism (requires snapd 2.50+).

The kernel command line is formed from mode arguments set by snapd, a static element declared in the bootloader configuration script, and optional extra arguments.

For example, the run mode kernel command line is:

snapd_recovery_mode=run console=ttyS0 console=tty1 panic=-1

The recovery mode of a system labeled 20210512 would look like this:

snapd_recovery_mode=recover snapd_recovery_system=20210512 console=ttyS0 console=tty1 panic=-1

In the above examples, the mode arguments are snapd_recovery_mode and snapd_recovery_system. The static command line content for the grub bootloader is console=ttyS0 console=tty1 panic=-1 which is defined in the snapd source code.

Kernel command line files

The kernel command line can be customised by adding one of two possible files to the top level of the filesystem in the Gadget snap:

1. Add a cmdline.extra file containing the extra kernel command line arguments, such as custom.option=1.

   These arguments are automatically appended to the command line:

   run mode:
   
   snapd_recovery_mode=run console=ttyS0 console=tty1 panic=-1 custom.option=1
   
   recovery mode of system 20210512:
   
   snapd_recovery_mode=recover snapd_recovery_system=20210512 console=ttyS0 console=tty1 panic=-1 custom.option=1

2. Add a cmdline.full file containing the full kernel command line to replace the built-in static command line entirely.

   For example, if cmdline.full file has the following contents:

   # my custom option
   custom.option=1
   # use only ttyS0
   console=ttyS0
   

   The kernel command line will contain the following:
   
   run mode:
   snapd_recovery_mode=run custom.option=1 console=ttyS0
   
   recovery mode of system 20210512:
   
   snapd_recovery_mode=recover snapd_recovery_system=20210512 custom.option=1 console=ttyS0

Both kernel command line extension methods also apply to install mode.

The gadget snap can contain only one of cmdline.full or cmdline.extra; the presence of both files at the same time is treated as an error.

Extending the kernel command line using drop-in files is also supported on systems using the full disk encryption. See Full disk encryption for more details.

Customising the kernel command line

There are two general approaches to incorporate either of the above custom kernel command line files into an Ubuntu Core image.

The first requires that the snapcraft.yaml is edited to add the kernel command line file to the root of the gadget filesystem, using the dump plugin, for example.

The second is to modify an existing gadget snap directly, and this procedure is outlined below.

Modify an existing gadget snap

When building your own custom Ubuntu Core image, the gadget snap that you include can be modified manually to include the kernel command line file.

To do this, first retrieve the gadget snap you wish to use in the image. The following command, for example, will download the PC gadget snap with a base of core20:

$ snap download pc --channel=20/stable
Fetching snap "pc"
Fetching assertions for "pc"
Install the snap with:
   snap ack pc_115.assert
   snap install pc_115.snap

In the above example, the downloaded gadget snap is called pc_115.snap, but this will change according to revisions and architectures. You may even source your own gadget snap locally.

All snaps are compressed with the Squashfs filesystem and the gadget snap next needs to be decompressed locally with the unsquashfs command:

$ unsquashfs pc_20-0.4_amd64.snap
Parallel unsquashfs: Using 8 processors
13 inodes (34 blocks) to write
[==========================|] 34/34 100%
created 12 files
created 6 directories
created 0 symlinks
created 0 devices
created 0 fifos

The above command will create a new directory called squashfs-root containing the files and folders of the gadget snap. We can now create the required cmdline.extra or cmdline.full file (see above) containing our kernel command line arguments:

$ echo "option1=foo option2=bar" > squashfs-root/cmdline.extra

With the cmdline.extra or cmdline.full file created, the gadget snap can be recompressed back into a snap using the snap pack <squashfs-root> command:

$ snap pack squashfs-root
built: pc_20-0.4_amd64.snap

The final step is to build a new Ubuntu Core image with the modified gadget snap. This requires a model assertion with grade: dangerous set and the ubuntu-image command to compile the image. See Custom images for more details.

You can then build the image with the new gadget snap using ubuntu-image:

$ ubuntu-image snap my-model.model --snap pc_20-0.4_amd64.snap
Fetching snapd
Fetching pc-kernel
Fetching core20
Fetching htop
WARNING: "pc" installed from local snaps disconnected from a store cannot be refreshed subsequently!
Copying "pc_20-0.4_amd64.snap" (pc)

The resultant image can now be installed on your device and will include your custom kernel command line options.