To to see what file the loopback device is attached to, execute:
# losetup /dev/loop0
then to increase its size you can give, for example
# dd if=/dev/zero bs=1MiB of=/path/to/file conv=notrunc oflag=append count=xxx
where xxx is the number of MiB you want to add.
If the file system is not currently mounted on /, to make the new space available for use you can give:
# losetup -c /dev/loop0
# resize2fs /dev/loop0
If the image you are trying to resize is Ubuntu Touch/Ubports rootfs (which is found in /data/system.img) and you are trying to do this from the device, now reboot to recovery and then after entering in the device with adb shell, give:
# losetup /dev/block/loop0 /data/system.img
# e2fsck -f /dev/block/loop0
# resize2fs -f /dev/block/loop0 # if your fs is ext
# xfs_growfs /dev/block/loop0 # if your fs is xfs
xfstryxfs_growfsinstead ofresize2fs. To identify if the file system of the loop device useblkid– Augustus Francis Sep 04 '18 at 20:58