Valentin David 2af8390b32 snap-bootstrap: Mount snaps read only ...

While squashfs is always read-only, when mount creates the required
loop device, it does not know that squashfs is a read-only
filesystem. This causes the loop devices for snaps mounted in the
initramfs to be be writable. Since the snaps are stored on
`/run/mnt/data`, it means the `/run/mnt/data` cannot be remounted
read-only.

There is a mount cycle now in Ubuntu Core where `/run/mnt/data` cannot
be unmounted because it contains the snap for the Core snap which is
mounted as root. After switching to the shutdown ramfs, then systemd
tries again to umount the remaining file systems as well as the root
file system. At this point, it still cannot umount `/run/mnt/data`, so
`systemd-shutdown` tries to remount read-only. But since the loop
device for the Core snap that cannot be unmounted is still writable,
then `/run/mnt/data` cannnot be remounted read-only.

This results in Ubuntu Core not properly unmounting `/run/mnt/data`.
This can be seen in the screen or serial console right before powering
off or rebooting:

```
Failed to remount '/oldroot/run/mnt/data' read-only: Device or resource busy
```

With this fix, this error disappears. While other errors about
unmounting are still here, it is safer because everything is
read-only.

2021-12-03 14:05:31 +01:00

..

task.yaml

snap-bootstrap: Mount snaps read only

2021-12-03 14:05:31 +01:00