Recovery Mode is a way of starting a device from which it is possible to manage various operations on the system partition as well as from root. When test keys are active, the user has endless possibilities between flashing partitions, restoring and installing various parts of the system or, more safely, direct intervention on the user's data partition.

To access it through ADB use the command:

adb reboot recovery

In many cases this partition is patched by the device manufacturer so as to prevent the end user from being able to perform operations such as de-branding and installing third-party software, or even installing a new operating system on the device.

The partition integrity check is managed in most cases by a file on the system partition which, through a script, reinstalls the original partition at the first boot: /system/bin/install-recovery.sh

By disabling the aforementioned file the warranty is lost, but in return you will be able to permanently get your customized Recovery.

However it is possible to use a personalized Recovery also temporarily, and then restore it automatically at the first start, just in time to perform the operations necessary to the user.

With a dev-key recovery we have no permissions to execute zips. Only signed updates are accepted on a stock recovery.

With a custom Recovery we can flashing every other zip we need on our devices or to be able to use ADB from Recovery Mode.

Use a temporary root access on your device and perform dd from ADB shell:

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

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

1) Create a new folder and put your recovery.img there, with the keys.v12 file;

2) Open a terminal into the new folder and perform the following commands, we will do the operation in three blocks:

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

abootimg -x recovery.img

abootimg-unpack-initrd

b. with these commands you remove the stock dev-keys in /ramdisk/res/ and replace them with the keys from the keys.v12 file

rm ramdisk/res/keys

cp keys.v12 ramdisk/res/keys

Normally only the adb sideload command can be used,but only packages signed by OEMs can be flashed. In case you want to use ADB as root in recovery mode, you can proceed in this way (OPTIONAL):

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, it is often enabled by default).

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, repack the new one end rewrite it in recovery.img

rm initrd.img

abootimg-pack-initrd

abootimg -u recovery.img -r initrd.img

Now your custom recoveri.img is ready to be flashed!

In this part of the guide we will follow two steps, everything depends on whether your custom recovery will be a temporary (from point 1 to point 5) or a permanent (point 6) replacement.

Now we made a backup of the stock Recovery and have replaced it with a custom one. This passage is important if you want temporary use the new recovery without loosing the OTA updates, because you can use the custom one just one time, it will be replaced with the stock one at the first boot because the file /system/bin/install-recovery.sh;

6a) Now we need to disable integrity checking from the system partition. This will block official updates, but you can always restore it if you first backup your system partition, in this way:

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

6b) Now that we have the system's backup we can disable the file that reinstalls the stock version of Recovery, /system/bin/install-recovery.sh

mount -o remount,rw /system

echo '#!/system/bin/sh' > /system/bin/install-recovery.sh

echo 'exit 0' >> /system/bin/install-recovery.sh

chown root:root /system/bin/install-recovery.sh

chmod 750 /system/bin/install-recovery.sh

sync

mount -o remount,ro /system

reboot