In this guide I will explain the procedure to activate ADB ROOT on a stock firmware on KaiOS and Android. This procedure carries a double risk: the loss of official updates (resolvable) or the rupture of the device (often irreversible).

Yes, replacing the original firmware with a custom one is a huge plus. In case of errors the device can incur in the best of cases in continuous reboot, but even there is the risk of not being able to turn on anymore. Proceed at your own risk!

"In computing, firmware is a specific class of software that provides the low-level control for the device's specific hardware. Firmware can either provide a standardized operating environment for the device's more complex software (allowing more hardware-independence), or, for less complex devices, act as the device's complete operating system, performing all control, monitoring and data manipulation functions.

Firmware is held in non-volatile memory devices such as ROM, EPROM, or flash memory. Changing the firmware of a device is nowadays a common procedure; some firmware memory devices are permanently installed and cannot be changed after manufacture. Common reasons for updating firmware include fixing bugs or adding features to the device." (source Wikipedia)

If you don't have a certified boot.img for your KaiOS / Android device on the Internet, take it using the shell. Use a temporary root access on your device and perform dd from ADB shell:

dd if=/dev/block/bootdevice/by-name/boot of=/sdcard/boot.img bs=2048

the boot.img file will be saved on the SD card.

1) Create a new folder and put your boot.img there, with this rooted ADBD binary file (this one comes from GerdaOS);

2) Open a terminal into the new folder and perform the following commands, we will do the operation in three blocks (just like the previous guide about Recovery Mode, but the "b" block changes):

a. in this way you extract the boot.img and the internal initrd.img that contains the essential files

abootimg -x boot.img

abootimg-unpack-initrd

b1. with these commands you remove the stock ADBD binary in /ramdisk/sbin/ and replace it with the new one

rm ramdisk/sbin/adbd

cp adbd ramdisk/sbin/adbd

b2. now you have to edit the default.prop file in this main folder

cp ramdisk/default.prop ./

use a text editor and change the following values:

• ro.debuggable must be 1 (in this way the device is able to use debug);

• ro.adb.secure must be 0 (needed to enable ADB).

now push again the default.prop file in its place:

rm ramdisk/default.prop

cp default.prop ramdisk/default.prop

c. with these commands you remove initrd.img from the main folder and repack the new one, then rewrite it in boot.img

rm initrd.img

abootimg-pack-initrd

abootimg -u boot.img -r initrd.img

WARNING! You should get an error like this:

"boot.img: updated is too big for the Boot Image (MaxValue vs MinValue bytes)"

then get MaxValue and launch this last command:

abootimg -u boot.img -r initrd.img -c bootsize=MaxValue

Now your custom firmware is finally ready to be flashed!

Thanks to Mus Tofa for this guide