You can do a temporary recovery image boot.
You must know fastboot commands and must download a recovery image and a super SU zip.
1. Boot into bootloader
2. Fastboot boot recovery.img
3. Once in recovery, MAKE A BACKUP
4. Install SuperSU.zip
6. Check root once rebooted
You will retain your stock recovery, and your backup will be of the stock system. This will also not give you a tampered flag, at least in my experience
Sent from my HTC One_M8