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@DennisLfromGA
Last active December 7, 2025 20:14
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[BROKEN] Script for ChromeOS Flex to add cros_debug / dev mode option in grub.cfg
##########################################################################
### THIS SCRIPT NO LONGER WORKS DUE TO RECENT CHANGES IN CHROMEOS Flex ###
##########################################################################
### Script to add cros_debug / dev mode option in grub.cfg
### ( Each Flex update removes cros_debug / dev mode option
### Be sure to run this script AFTER each update and BEFORE rebooting )
ANS=
ROOTDEVICE=
ROOTDEVICEPREFIX=
P12MOUNTPOINT=/var/p12
GRUBCFGPATH=$P12MOUNTPOINT/efi/boot
GRUBCFGFILE=grub.cfg
# Exits the script with return code $1, spitting out message $@ to stderr
error() {
local ecode="$1"
shift
echo "$*" 1>&2
return "$ecode"
}
# Find the root device
# Sets:
# - $ROOTDEVICE as the root device (e.g. /dev/sda or /dev/mmcblk0)
# - $ROOTDEVICEPREFIX as a prefix for partitions (/dev/sda, /dev/mmcblk0p)
findrootdevice() {
ROOTDEVICE="`rootdev -d -s`"
if [ -z "$ROOTDEVICE" ]; then
error 1 "Cannot find root device."
fi
if [ ! -b "$ROOTDEVICE" ]; then
error 1 "$ROOTDEVICE is not a block device."
fi
# If $ROOTDEVICE ends with a number (e.g. mmcblk0), partitions are named
# ${ROOTDEVICE}pX (e.g. mmcblk0p1). If not (e.g. sda), they are named
# ${ROOTDEVICE}X (e.g. sda1).
ROOTDEVICEPREFIX="$ROOTDEVICE"
if [ "${ROOTDEVICE%[0-9]}" != "$ROOTDEVICE" ]; then
ROOTDEVICEPREFIX="${ROOTDEVICE}p"
fi
}
findrootdevice
if [ ! -d $P12MOUNTPOINT ]; then
# Make mountpoint
echo "Making directory to mount partition 12"
sudo mkdir -p $P12MOUNTPOINT
elif [ ! -d $GRUBCFGPATH ]; then
# Mount EFI partition
echo "Mounting ${ROOTDEVICEPREFIX}12 on $P12MOUNTPOINT"
sudo mount ${ROOTDEVICEPREFIX}12 $P12MOUNTPOINT
fi
# Check for cros_debug option, need 5
DEBUGCOUNT="$(grep 'cros_efi cros_debug' $GRUBCFGPATH/$GRUBCFGFILE | wc -l)"
if [ "$DEBUGCOUNT" -eq 5 ]; then
echo "cros_debug has been added to $GRUBCFGFILE, no changes needed."
exit
fi
# grub.cfg needs editing
echo "Editing $GRUBCFGFILE to add 'cros_debug' option."
echo "
PLEASE NOTE:
I claim no responsibility for any damage or harm that may occur to your
device as a result of running this script. PROCEED AT YOR OWN RISK.
"
echo -n "[ press ENTER to continue - Ctrl-C to abort ] "; read ANS
cp $GRUBCFGPATH/$GRUBCFGFILE /tmp
# vi /tmp/$GRUBCFGFILE
sed -e 's/cros_efi /cros_efi cros_debug /' -e 's/cros_efi root/cros_efi cros_debug root/' /tmp/$GRUBCFGFILE
sudo cp $GRUBCFGPATH/$GRUBCFGFILE $GRUBCFGPATH/${GRUBCFGFILE}.bak
sudo cp /tmp/$GRUBCFGFILE $GRUBCFGPATH/
echo "Done, you can reboot now."
exit
@porteusconf
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porteusconf commented Jul 27, 2024

Please leave it up as it is a top hit in google search, and if nothing else will help spread the word that cros_debug option no longer seems to boot dev mode. If and when we find how to turn on developer mode on newer flex versions we can link it here. No answers in now-locked too-short discussion about 126 at worthless link: https://support.google.com/chromeosflex/thread/285244470/unable-to-enter-developer-mode-on-chromeos-flex-version-126

@porteusconf
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The cros_debug flag may be enabling shell at crosh prompt again ;-) At least for me, as of Feb 2025. I have dev-channel flex on old intel efi thinkpad, that today updated to Version 134.0.6998.5 (Official Build) dev (64-bit). I dual-boot to linux (using refind), and manually edited grub.cfg (typically in /dev/sda12 partition) adding in 5 places "cros_debug" (after cros_efi). Doing ctrl-alt-T at crosh prompt the shell command gave me a bash prompt. I was getting seg faults but fixed it per https://superuser.com/questions/1842739/chromeos-after-update-segmentation-fault-for-all-shell-commands. Chromebrew is only reason I want dev-mode, and so I am happy it works again :-)

CREW_FORCE_INSTALL=1 bash <(curl -Ls git.io/vddgY) && . ~/.bashrc
crosh> shell
Starting Chromebrew bash.
chronos@localhost / $ date
Mon Feb 10 04:32:36 -00 2025
chronos@localhost / $  crew install btop
Btop installed!

@porteusconf
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porteusconf commented Feb 14, 2025

Just tested again for new install of flex on a second laptop, and the "cros_debug" let me get shell at crosh prompt. But then flex updated itself to 236 and after update I no longer got shell at crosh prompt, as apparently the update removed cros_debug. But good news is after I added back cros_debug, I was able to get /bin/bash shell at crosh prompt in newest flex os on my old thinkpad. Also, I think the sed command to use might better be something like this:

# I think sed below does this: for each line: if the line already has "cros_debug" do nothing  (skip  to next line)
# But If line does not have "cros_debug" then change "cros_efi" to "cros_efi cros_debug"
sed "/cros_debug/! s/cros_efi/cros_efi cros_debug/g"
# Should prevent issues if script is run more than once, ensuring grub.cfg has only  one "grub_debug" per line.

I'll update with step-by-step howto and details when I have a chance. But it seems the "BROKEN" warning might be removed for now ;-)

@porteusconf
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porteusconf commented Mar 3, 2025

Even after recent 133 update I still can type "shell" at crosh prompt and get bash prompt. Around 2025-02-28, several 10-15 year old laptops updated to 133 version of chromeos-flex. for example: .

- OS variant: Chrome OS
- OS version: reven-release/R133-16151.53.0
- OS channel: stable-channel

BUT, after updates I did have to re-edit grub.cfg (by choosing to boot to Linux in refind). Sometimes, even after the edit, crosh still would not run the shell command. But I did a second edit, and after 2nd reboot, crosh gave me a shell prompt. So, seems like Google and/or the chromium-os folks have (unofficially) restored ability to get dev-mode (shell at crosh prompt) and this ability is no longer broken, at least for most of 2025 for all my 5 or so ChromeOS-flex laptops. And chromebrew can install and run any of the crew packages, including Firefox/brave, etc, on all of them. It is nice not to have to use chrome on flex (or even non-flex) Chromebooks ;-)

@yeahman45
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does cros_debug still work on the latest chromeos flex ? 137?

@DennisLfromGA
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Yes, it does but they've changed ownership on partition 12 / efi and the script can no longer mount it. I now use LinuxLoops to install Flex since it has an option to install it with dev mode enabled.

@yeahman45
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yeahman45 commented Jul 9, 2025 via email

@DennisLfromGA
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But i can just edit the grub.cfg with a live linux right?

Possibly, maybe just try it and let us know.

@yeahman45
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yeahman45 commented Jul 10, 2025 via email

@supechicken
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It looks like Google has removed all kernel blobs (and also GRUB) from the EFI partition and switched to crdyboot-based booting process. The kernel is now located in a separate partition (KERN-A/B) with its cmdline, just like what a normal Chromebook does.

As a result, booting Flex with Linux bootloaders like GRUB is no longer possible.

BTW, I got developer mode working on latest R142 by patching crdyboot itself, check here if anyone are interested :)

@DennisLfromGA
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Thanx @supechicken, they keep finding ways to make it harder but that just makes it more rewarding when it's defeated. ;-)

@porteusconf
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Worked for me too for chrome-os-flex 142.0.7444.220 on old Lenovo g700. Yes... much thanks. And surprised the 64-byte SIG files did not have to be changed. ;-)

crosh> uname -a
Linux localhost 6.6.99-08727-gaac38b365d2c #1 SMP PREEMPT_DYNAMIC Wed, 8 Oct 2025 17:51:29 -0700 x86_64 Intel(R) Pentium(R) CPU 2030M @ 2.50GHz GenuineIntel GNU/Linux
crosh> shell
chronos@reven / $ date
Sun Dec  7 15:10:11 EST 2025
chronos@reven / $ uname -a
Linux localhost 6.6.99-08727-gaac38b365d2c #1 SMP PREEMPT_DYNAMIC Wed, 8 Oct 2025 17:51:29 -0700 x86_64 Intel(R) Pentium(R) CPU 2030M @ 2.50GHz GenuineIntel GNU/Linux
chronos@reven / $ cat /etc/lsb-release 
CHROMEOS_ARC_ANDROID_SDK_VERSION=33
CHROMEOS_ARC_VERSION=14390219
CHROMEOS_AUSERVER=https://tools.google.com/service/update2
CHROMEOS_BOARD_APPID={C924E0C4-AF80-4B6B-A6F0-DD75EDBCC37C}
CHROMEOS_CANARY_APPID={90F229CE-83E2-4FAF-8479-E368A34938B1}
CHROMEOS_DEVSERVER=
CHROMEOS_RELEASE_APPID={C924E0C4-AF80-4B6B-A6F0-DD75EDBCC37C}
CHROMEOS_RELEASE_BOARD=reven-signed-mp-v8keys
CHROMEOS_RELEASE_BRANCH_NUMBER=57
CHROMEOS_RELEASE_BUILDER_PATH=reven-release/R142-16433.57.0
CHROMEOS_RELEASE_BUILD_NUMBER=16433
CHROMEOS_RELEASE_BUILD_TYPE=Official Build
CHROMEOS_RELEASE_CHROME_MILESTONE=142
CHROMEOS_RELEASE_DESCRIPTION=16433.57.0 (Official Build) stable-channel reven 
CHROMEOS_RELEASE_KEYSET=mp-v8
CHROMEOS_RELEASE_NAME=Chrome OS
CHROMEOS_RELEASE_PATCH_NUMBER=0
CHROMEOS_RELEASE_TRACK=stable-channel
CHROMEOS_RELEASE_UNIBUILD=1
CHROMEOS_RELEASE_VERSION=16433.57.0
DEVICETYPE=CHROMEBOOK
GOOGLE_RELEASE=16433.57.0
chronos@reven / $  

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