Added overlay support which mounts /minimal/rootfs if it is found on the boot media. Note that this doesn't work on vfat. Works fine on CD image and on separate ext2 partition.

src/02_build_kernel.sh

@@ -14,6 +14,9 @@ make defconfig
 # Changes the name of the system to 'minimal'.
 sed -i "s/.*CONFIG_DEFAULT_HOSTNAME.*/CONFIG_DEFAULT_HOSTNAME=\"minimal\"/" .config
 
+# Enable overlay support, e.g. merge ro and rw directories.
+sed -i "s/.*CONFIG_OVERLAY_FS.*/CONFIG_OVERLAY_FS=y/" .config
+
 # Compile the kernel with optimization for "parallel jobs" = "number of processors".
 # Good explanation of the different kernels:
 # http://unix.stackexchange.com/questions/5518/what-is-the-difference-between-the-following-kernel-makefile-terms-vmlinux-vmlinux

src/08_generate_rootfs.sh

@@ -54,8 +54,8 @@ EOF
 chmod +x prepare.sh
 
 # The script '/etc/switch.sh' is automatically executed as part of the '/init'
-# process. We copy all files/folders to new mountpoint and then execute the
-# command 'switch_root'.
+# process. We overlay the '/minimal/rootfs' folder from the boot media, copy all
+# files and folders to new mountpoint and then execute the command 'switch_root'.
 cat > switch.sh << EOF
 #!/bin/sh
 
@@ -63,28 +63,74 @@ cat > switch.sh << EOF
 mount -t tmpfs none /mnt
 
 # Create folders for all crytical file systems.
+echo "Create folders for all crytical file systems..."
 mkdir /mnt/dev
 mkdir /mnt/sys
 mkdir /mnt/proc
 mkdir /mnt/tmp
+echo "...done."
 
-# Move all crytical file systems in the new mountpoint.
+# Copy root folders in the new mountpoint.
+echo "Copying the root file system to /mnt..."
+cp -a bin etc lib lib64 root sbin src usr /mnt
+echo "...done."
+
+mkdir /tmp/mnt
+echo "Searching available devices for /minimal/rootfs folder..."
+for DEVICE in /dev/* ; do
+  DEV=\$(echo "\${DEVICE##*/}")
+  SYSDEV=\$(echo "/sys/class/block/\$DEV")
+
+  case \$DEV in
+    *loop*) continue ;;
+  esac
+
+  if [ ! -d "\$SYSDEV" ] ; then
+    continue
+  fi
+
+  mount \$DEVICE /tmp/mnt 2>/dev/null
+  if [ -d /tmp/mnt/minimal/rootfs ] ; then
+    echo "  Found /minimal/rootfs folder on device \$DEV."
+    
+    touch /tmp/mnt/minimal/rootfs/minimal.pid 2>/dev/null
+    if [ -f /tmp/mnt/minimal/rootfs/minimal.pid ] ; then
+      echo "  Trying to overlay in read/write mode..."
+      rm -f /tmp/mnt/minimal/rootfs/minimal.pid
+      mount -t overlay -o lowerdir=/mnt,upperdir=/tmp/mnt/minimal/rootfs,workdir=/tmp/mnt/minimal/work none /mnt
+      OUT=\$?
+      if [ ! "\$OUT" = "0" ] ; then
+        echo "  Mount failed (probably on vfat), moving on with other devices."
+        continue
+      fi
+    else
+      echo "  Trying to overlay in read only mode..."
+      mkdir -p /tmp/minimal/work
+      mkdir -p /tmp/minimal/rootfs
+      mount -t overlay -o lowerdir=/mnt:/tmp/mnt/minimal/rootfs,upperdir=/tmp/minimal/rootfs,workdir=/tmp/minimal/work none /mnt
+    fi
+    
+    echo "  ...done."
+    
+    break
+  else
+    umount /tmp/mnt 2>/dev/null
+  fi
+done
+echo "...done."
+
+# Move crytical file systems to the new mountpoint.
 echo "Remounting /dev, /sys, /tmp and /proc in /mnt..."
 mount --move /dev /mnt/dev
 mount --move /sys /mnt/sys
-mount --move /tmp /mnt/tmp
 mount --move /proc /mnt/proc
-echo "...done."
-
-# Copy all root folders in the new mountpoint.
-echo "Moving the rest of the root file system to /mnt..."
-cp -a bin etc lib lib64 root sbin src usr /mnt
+mount --move /tmp /mnt/tmp
 echo "...done."
 
 # The new mountpoint becomes file system root. All original root folders are
 # deleted automatically as part of the command execution. The '/sbin/init' 
 # process is invoked and it becomes the new PID 1 parent process.
-exec switch_root /mnt/ /sbin/init
+exec switch_root /mnt /sbin/init
 
 echo "You can never see this... unless there is a serious bug..."
 sleep 99999

src/11_generate_iso.sh

@@ -41,6 +41,14 @@ chmod +rx src/*.sh
 chmod +r src/.config
 chmod +r src/*.txt
 
+# Create the overlay directory '/minimal'. All files in this folder are merged
+# in the root folder and can be manipulated thanks to overlayfs.
+mkdir -p minimal/rootfs
+cd minimal/rootfs
+echo 'Sample file 1 from CD.' > file_from_cd_1.txt
+echo 'Sample file 2 from CD.' > file_from_cd_2.txt
+cd ../..
+
 # Create ISOLINUX configuration file.
 echo 'default kernel.bz  initrd=rootfs.gz' > ./isolinux.cfg
 

src/qemu32.sh

@@ -1,4 +1,4 @@
 #!/bin/sh
 
-qemu-system-i386 -m 64M -cdrom minimal_linux_live.iso
+qemu-system-i386 -m 64M -hda hdd.img -cdrom minimal_linux_live.iso -boot d
 

src/qemu64.sh

@@ -1,4 +1,4 @@
 #!/bin/sh
 
-qemu-system-x86_64 -m 64M -cdrom minimal_linux_live.iso
+qemu-system-x86_64 -m 64M -hda hdd.img -cdrom minimal_linux_live.iso -boot d