Also, ubuntu-core image has some required kernel flags in /proc/cmdline
$ cat /proc/cmdline BOOT_IMAGE=(loop)/kernel.img root=LABEL=writable snap_core=core_x1.snap snap_kernel=pc-kernel_x1.snap ro net.ifnames=0 init=/lib/systemd/systemd console=tty1 console=ttyS0 panic=-1
Specifically, 'snap_core=' and 'snap_kernel='
Also, ubuntu-core image has some required kernel flags in /proc/cmdline
$ cat /proc/cmdline (loop)/ kernel. img root=LABEL=writable snap_core= core_x1. snap snap_kernel= pc-kernel_ x1.snap ro net.ifnames=0 init=/lib/ systemd/ systemd console=tty1 console=ttyS0 panic=-1
BOOT_IMAGE=
Specifically, 'snap_core=' and 'snap_kernel='