Here are 3 ways to allow debugging on boot by connecting to adb. This is often called "insecure adb" - while it is not as "insecure" as you might think 😉
Since A7 you have to authorize your adb server which either requires to add your adb pub key into the ramdisk (directly on / ) or you must put it in /data/misc/adb/adb_keys, e.g when in recovery. Details here. This method will not work on FDE encrypted devices though. for this you need to mod the boot.img (see "Option A: Modding an existing boot.img").
important is that it has to have proper label and permission set. e.g when in recovery:
FORMAT(!) DATA in recovery!
REBOOT from recovery into recovery afterwards!
then go on:
adb push ~/.android/adbkey.pub /tmp/
adb shell
mkdir -p /data/misc/adb/
mv /tmp/adbkey.pub /data/misc/adb/adb_keys
chcon u:object_r:adb_keys_file:s0 /data/misc/adb/adb_keys
chown system:shell /data/misc/adb/adb_keys
chmod 640 /data/misc/adb/adb_keys
Choose an option and follow
depending on the device you could unpack/repack your boot.img by one of
- AIK (recommended)
- abootimg - which is also part of mAid
- bootimgtool - which is also part of mAid
Whatever you use try first to: unpack -> repack -> flash (so without any modifications).
if that does not boot anymore use another tool. e.g. AIK does not work for the LG G4 sometimes so I use bootimgtool (-v qcom) instead.
default.prop (on newer android this might be named build.prop):
------------------------
ro.adb.secure=0
ro.secure=0
ro.debuggable=1
persist.service.adb.enable=1
persist.service.debuggable=1
persist.sys.usb.config=adb
# if that does not work try: persist.sys.usb.config=mtp,adb
!!! IMPORTANT: !!!
remove ANY other line which contains one of settings above
within your unpacked boot.img put your adb key directly into the root of the RAMDISK (details here) as follows:
cp ~/.android/adbkey.pub <PATH-TO-RAMDISK-ROOTDIR>/adb_keys
in case of AIK this would be:
cp ~/.android/adbkey.pub <path>/AIK-Linux/ramdisk/adb_keys
repack the boot.img and flash
when on a12 (and likely later) and you just wanna enable adb logcat during build (not truly insecure, its more like half-insecure): https://github.com/crdroidandroid/android_build/commit/a9f4cd5493ead4134aba6e17e80c81eb104cdf43
for a full insecure (i.e allowing adb root and then e.g. adb shell dmesg during boot) use one of the other options.
Within your android source directory (replace <ROM> with the vendor path of your ROM you building for. e.g vendor/lineage/config/common.mk)
sed -i "s/secure=./secure=0/g" build/core/main.mk
sed -i "s/secure=./secure=0/g" vendor/<ROM>/config/common.mk