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Chrome OS Factory Shim


This folder contains the major scripts for the “Chrome OS factory shim”. The shim is used for installing a Chrome OS image (kernel, rootfs and firmware) to a device. It's also known as “(factory) install shim”, “RMA shim”, or “Reset shim”.

The factory shim is designed to allow operators removing USB stick once it's booted, so the boot process is slightly different. The shim relies on initramfs technology to bootstrap and load all contents into memory, then start an upstart service to display the menu.

Building a factory shim

Inside chroot, do:

build_packages --board $BOARD
build_image --board $BOARD factory_install

The output disk image is in ~/trunk/src/build/images/$BOARD/latest/factory_install_shim.bin.

If you have local changes in src/platform/factory_installer, please remember to do

cros_workon --board $BOARD start factory_installer
emerge-$BOARD factory_installer

If you have local changes in src/platform/initramfs, please remember to do

cros_workon --board $BOARD start chromeos-initramfs

There‘s no need to emerge chromeos-initramfs because it’s always re-built in build_image stage.

Using factory shim

Factory shims are signed in a special way for security reasons. It needs to boot with “developer switch turned on” and “boot in recovery mode”.

Boot from install shim (clamshells / convertibles)

  1. Enter recovery mode
  2. Press CTRL + D to turn on developer switch
  3. Press ENTER to confirm
  4. Enter recovery mode again (no need to wait for wiping)
  5. Insert and boot from USB stick with rma_image.bin

Boot from install shim (tablets / detachables)

  1. Enter recovery mode
  2. Press VOL_UP + VOL_DOWN to show recovery menu
  3. Press VOL_UP or VOL_DOWN to move the cursor to “Confirm Disabling OS Verification”, and press POWER to select it
  4. Enter recovery mode again (no need to wait for wiping)
  5. Insert and boot from USB stick with rma_image.bin

See here for instructions to enter recovery mode.

If you boot factory shim in developer mode (Ctrl-U), some functions won't work, such as recovering TPM.

Factory shim menu

If you boot into a factory shim successfully, you will see a shim menu, followed by a prompt to select an action.

Please select an action and press Enter.

  I Install              Performs a network or USB install
  R Reset                Performs a factory reset; finalized devices only
  S Shell                Opens bash; available only with developer firmware
  V View configuration   Shows crossystem, VPD, etc.
  D Debug info and logs  Shows useful debugging information and kernel/firmware logs
  Z Zero (wipe) storage  Makes device completely unusable
  C SeCure erase         Performs full storage erase, write a verification pattern
  Y VerifY erase         Verifies the storage has been erased with option C
  T Reset TPM            Call chromeos-tpm-recovery
  U Update TPM Firmware  Update TPM firmware
  E Perform RSU          Perform RSU (RMA Server Unlock)
  M Enable factory mode  Enable TPM factory mode
  F Perform factory FAI  Perform Factory FAI (First Article Inspection)


The install shim also checks /etc/lsb-factory for flags that decides the default action of the shim menu (listed from high priority to low priority).

  1. NETBOOT_RAMFS=1: This flag is automatically set when using netboot firmware. The install shim will set the default action to (I) Install.
  2. RMA_AUTORUN=true: This flag is set by image_tool when creating an RMA shim. Please see RMA shim README for the behavior of this parameter.
  3. DEFAULT_ACTION=<action>: This flag directly sets the default action to . For instance, DEFAULT_ACTION=i sets the default action to (I) Install.

Board-specific actions

With technique installer resource in factory-board, you can add some board-specific actions by defining variables and functions in in private overlay. For example:



menu_board() {
    menu_line X "Magic Command" "Run magic command"

action_x() {
    echo "Magic on board ${BOARD}."

Debugging a factory shim

Factory shims do not provide shells by default for security reason. If you can still see virtual terminal consoles, try VT0, VT1, VT2, VT3 - there are lots of debug messages there.

Getting a shell

If you do need a shell to debug, add cros_debug to kernel command line. You can do this in build_image:

build_image --board $BOARD --boot_args cros_debug factory_install

For an existing image, you can use to change kernel command line easily:

# inside chroot
cd ~/trunk/src/platform/vboot_reference/scripts/image_signing
   --partitions 2 --recovery --edit_config

This will bring an editor to allow editing command line.

Note is also available on all Chrome OS image (even factory shim) - try /usr/share/vboot/bin/

If you boot a factory shim with cros_debug, then you should have one shell in VT2 or VT3. Moreover, if you can enter the menu, ‘S’ will give you the full shell.

Fail to start frecon

The frecon (or frecon-lite) provides text-based console. If you can't see anything on screen, redirect the console to another device, for example Servo consoles so you can check why frecon failed. To do this, add console=ttyS0,115200n8 to kernel command line (use the or add --boot_args as explained in previous section). Some devices may need different TTY name for example ttyS1. Please check the care-and-feed doc of your device.

Debugging frecon issues

If the menu or frecon will die and adding cros_debug does not help, you probably want to attach serial console (for example SuzyQ) and get everything except factory shim UI (menu) there. To do that:

  1. Check if you need to build a new image.

Open the /usr/sbin/ and find the TTY_CONSOLE= line. If it already has valid serial console (for example ttyS0), move to step 3.

  1. Add TTY_CONSOLE and build image.

Edit the make.conf in board overlay, to find or add one setting (assume serial console is ttyS0):


Then,then re-build the factory_installer package and factory shim:

emerge-$BOARD factory_installer
build_image --board $BOARD factory_install
  1. Enable console service.

Mount the rootfs and rename /etc/init/console-ttyS0.conf to something that does not start as console:

# First enable RW for rootfs. Assume the USB is in /dev/sdX.
cd ~/trunk/src/platform/vboot_reference/scripts/image_signing
sudo ./ -i /dev/sdX --recovery \
   --remove_rootfs_verification --partitions 2
# Mount (assume your shim is in /dev/sdX)
sudo mount /dev/sdX /media
cd /media/etc/init
sudo mv console-ttyS0.confg debug-ttyS0.conf
cd - # To leave /media folder so we can unmount.
sudo umount /media
  1. Boot with the shim. You should have serial console now.