[OnePlus 5T  Questions & Answers] znarg:
Hi,

TL;DR: Bootloader unlocked. "fastboot boot twrp.img" worked fine until I upgraded to OxygenOS 5.1.4 (from 5.0.4). Now "fastboot boot twrp.img" just does nothing.

I had a OxygenOS system (Android 8.0, I believe it was OOS 5.0.4), rooted with Magisk. Everything was fine. Stock recovery, I wanted to change it as little as possible. I could use twrp-3.2.1-x_blu_spark_v8.70-op5_op5t.img perfectly, via "flashboot boot /path/to/img".

Today I decided to upgrade. I had an OTA notification for months, for OOS 5.1.4 (Android 8.1). I made a full backup with TWRP. Read about latest ROMs, seems 5.1.6 has some Bluetooth issues.

I opened the 5.1.4 OTA notification, curious to read the Changelog. A message said I had a rooted phone, and the update would install a full version instead of the OTA. Also, I'd lose root. Nothing I didn't expect, but I still wanted to flash with TWRP. Unfortunately, the updater had already downloaded the ROM and suddenly restarted. I did not want to interrupt the update, so let it go through.

Phone booted on OOS 5.1.4 without problems. Unrooted, of course. I went into fastboot mode, and tried loading TWRP with "flashboot boot /path/to/twrp-3.2.1-x_blu_spark_v8.70-op5_op5t.img". The image gets downloaded, and boot takes about 10s (most of which is a warning message that my bootloader is unlocked). After that, I just drop back to the 1+ bootloader screen. If I then try repeating the same "flashboot boot ..." command, I get the error "FAILED (remote: 'Failed to load/authenticate boot image: 00000050')".

I tried:

Several TWRP versions, from Blu_Spark, Codeworkx and siankatabg. Don't remember all of them, but at least the latest ones, i.e. twrp-3.2.3-x_blu_spark_v9.85_treble-op5_op5t.img, twrp-3.2.3-0-20180822-codeworkx-dumpling.img and TWRP_3.2.3-Pie_siankatabg_OP5.img.

Removing the screen pattern, lock screen None.

Upgrading to OOS 5.1.6 via stock recovery.

Different USB cable.

I can boot the phone to the OS just fine. But without TWRP I cannot flash Magisk, to root the phone. I have not tried flashing TWRP to recovery, because at least I know stock recovery works and don't want to lose that.

This is what I have from device-info (did not change from before the upgrade, when TWRP was working):

$ ./fastboot oem device-info
(bootloader) Verity mode: enforcing
(bootloader) Device unlocked: true
(bootloader) Device critical unlocked: false
(bootloader) Charger screen enabled: true
(bootloader) enable_dm_verity: true
(bootloader) have_console: false
(bootloader) selinux_type: SELINUX_TYPE_INVALID
(bootloader) boot_mode: NORMAL_MODE
(bootloader) kmemleak_detect: false
(bootloader) force_training: 0
(bootloader) mount_tempfs: 0
OKAY [ 0.001s]
Finished. Total time: 0.001s

This is how I'm trying to boot TWRP. From the OS, I hold the power button, press Reboot, then choose Bootloader (I have advanced reboot enabled). That takes me to a pretty screen with a menu and options to start the OS, boot to recovery, or restart bootloader. I can select using the volume up and volume down keys, but I just ignore the options and type the fastboot commands:

$ ./fastboot devices
<redacted> fastboot
$ ./fastboot boot ~/Android/OnePlus5t/TWRP/twrp-3.2.3-x_blu_spark_v9.85_treble-op5_op5t.img
Downloading 'boot.img' OKAY [ 0.819s]
booting OKAY [ 10.106s]
Finished. Total time: 10.936s

The 10s is basically the Android warning message saying the boot loader is unlocked and the data cannot be trusted. I get that when booting to normal OS as well. After that warning goes away, instead of getting TWRP, I just get a black screen with "1+ Bootloader".

In that screen, I can still type fastboot commands, but now I have a different error:

$ ./fastboot boot ~/Android/OnePlus5t/TWRP/twrp-3.2.3-x_blu_spark_v9.85_treble-op5_op5t.img
Downloading 'boot.img' OKAY [ 0.822s]
booting FAILED (remote: 'Failed to load/authenticate boot image: 00000050')
Finished. Total time: 0.835s

I'm at a loss here. Can someone help? I've considered trying out the RedWolf alternative thing, but am worried it might mess with encryption or something. I could flash TWRP, but really worry that 1) if it doesn't work, I won't have any recovery at all, and 2) I'd like to keep the stock recovery as a fallback.