Phone stuck in fastboot mode

Question

I've rooted my OnePlus 6 yesterday. To do so, I followed this guide:

Root Your OnePlus 6 with Magisk — A Beginner's Guide

TWRP version: 3.5.1_9-0
Magisk version: 22.0

I also installed the Magisk app on my phone

Now I've edited my hosts-file (needed to change some permissions to be able to modify this file ) to block some sites. I've restarted the phone once successfully, but now the phone will not boot anymore. When I power on, I see the OnePlus logo very briefly and then it gets stuck on the "bootloader is unlocked" warning for a long time. It then goes on into fastboot mode. If i press start from there, the process repeats. If I press recovery mode I still get into TWRP, but rebooting from there also repeats the process and does not boot up my phone successfully.

I'm not sure, but I don't think editing the hosts file is what caused this? Shouldn't I be able to edit this file without bricking my device?

Any help on what happened and how I can fix this bootloop without wiping/reinstalling my entire device would be greatly appreciated; I don't want to lose all my settings =).

Answer 1

The problem was that I edited the system hosts file. This causes the system to be unable to boot; the reason is dm-verity which denies boot (chain of trust) when any modified system file is detected.

To fix the issue, I followed https://www.droidwin.com/restore-oneplus-6-stock-via-fastboot-commands/

Basically, it explains how to download the stock ROM, extract the .bin file into multiple .img files, and how to flash them subsequently.

Since only the boot and system partitions were modified, and I did not want to lose my data, I flashed them using

fastboot flash boot_a boot.img 
fastboot flash boot_b boot.img 
fastboot flash system_a system.img 
fastboot flash system_b system.img

This fixed the issue =).