Creating an ubuntu core image with custom gadget snap

snap
#1

Hey guys,

I was hoping to get a little direction on my current situation. I am trying to use a model assertion along with the ubuntu-image snap to build a signed image for an amd64 device. Using the command “ubuntu-image snap example.model” I get the warning “WARNING: rootfs structure size 1.17 GiB smaller than actual rootfs contents 1.32 GiB”. Attempting to run the generated image results in an error similar to the warning above. After doing a little research, I think I need to modify the gadget snap I’m using to increase the system-seed/ubuntu-seed to 1400M. I used this guide and this gadget snap as my base. I was successful in compiling the gadget snap, but I am unable to figure out how to include it with my model assertion. Is there a command to generate a unsigned model assertion from the json representation and include the local compiled gadget snap?

#2

look at the --snap option of ubuntu-image that allows you to point to a local snap file …

#3

I tried that before posting. Is there a command to generate a model assertion from the json representation with a grade of dangerous? Would I still make an entry under that snaps property for the local gadget?

#4

as long as the gadget is named the same as the one from the store it will simply override the one from the store when --snap is used, you should not need to change anything with your model (if it is already set to dangerous)

#5

If you put "grade": "dangerous", in your model json, it’ll be included in the signed model definition. You can then use an unpublished gadget snap (i.e. no "id" or "default-channel" for its listing in "snaps").

If you just want to do a quick and dirty test, you could do the following:

  1. Use snap download to download a copy of the gadget you were using.
  2. Unpack it with unsquashfs.
  3. Edit squashfs-root/gadget.yaml to adjust the size of the ubuntu-seed partition as needed.
  4. Pack the gadget again with snap pack squashfs-root.
  5. When building your grade: dangerous image with ubuntu-image, point it at your modified snap with the --snap argument.
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