Read only root filesystem with overlay fs

nccaruso · January 9, 2024, 11:04pm

I am looking for feedback on modifying an existing system based on imx8 to support a read-only root file system with overlayfs to allow the system to run as if the root filesystem were not read-only. The scope of this is limited to modifying our current system, which does not use bitbake but starts with your minimal image and then adds packages to result in something which looks like ubuntu 20:

root@toradex-imx8:~# uname -a

Linux toradex-imx8 5.4.193-imx8-base-os #1 SMP PREEMPT Mon Dec 4 13:45:11 EST 2023 aarch64 aarch64 aarch64 GNU/Linux

root@toradex-imx8:~# cat /etc/os-release
NAME=“Ubuntu”
VERSION=“20.04.6 LTS (Focal Fossa)”
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME=“Ubuntu 20.04.6 LTS”
VERSION_ID=“20.04”
HOME_URL=“https://www.ubuntu.com/”
SUPPORT_URL=“https://help.ubuntu.com/”
BUG_REPORT_URL=“Bugs : Ubuntu”
PRIVACY_POLICY_URL=“Data privacy | Ubuntu”
VERSION_CODENAME=focal
UBUNTU_CODENAME=focal
root@toradex-imx8:~#

I am trying to determine if there is a way to make the rootfs read only and mount overlay filesystems to allow the system to run as if the root were writeable. I don’t understand how to modify the boot sequence to make this happen given the constraints above; using bitbake is not within the scope of this assignment.

Any advice would be appreciated.

cbrake · January 10, 2024, 1:56pm

We have an option for overlay fs in the Yoe Updater – the following may help if you want to use the overlayfs feature:

github.com

YoeDistro/yoe-distro/blob/master/sources/meta-yoe/recipes-support/updater/files/updater.installer#L325


      
                fsck -y $device
                if ! mount $device $mount; then
                  return 1
                fi
              fi
            else
              return 1
            fi
          }
          
          mount_overlay_data() {
            mkdir -p /mnt/data/upper /mnt/data/upper/data /mnt/data/work /mnt/data/persistent /mnt/overlay
            mount -t overlay -o ro,lowerdir=/mnt/data/persistent:/mnt/rootfs,upperdir=/mnt/data/upper,workdir=/mnt/data/work overlay /mnt/overlay
          }
          
          mount_overlay_tmpfs() {
            TMPUPPER=/mnt/tmpupper
            mkdir $TMPUPPER
            mount -t tmpfs tmpupper $TMPUPPER
            mkdir -p $TMPUPPER/upper $TMPUPPER/work /mnt/data/persistent /mnt/overlay
            mount -t overlay -o ro,lowerdir=/mnt/data/persistent:/mnt/rootfs,upperdir=$TMPUPPER/upper,workdir=$TMPUPPER/work overlay /mnt/overlay

drew.tx · January 16, 2024, 8:17pm

Hi @nccaruso

Without knowing the details of your Ubuntu-ization of our Yocto image, it’s hard to give concrete advice here. The scripts referenced by @cbrake show how it is done for the Yoe Distro and something similar will need to be done in your build scripts. Hopefully, those scripts will help you understand the required steps.

Drew