How to get root back after OTA update

Question

When I updated to Android 4.1.2 on my Galaxy Nexus, I lost root. Everything is still there, but the permissions on system/bin/su no longer include the setuid bit, which prevents su from working.

I don't want to go through the whole re-rooting process if possible since I used root to re-lock the bootloader and unlocking it without root will wipe the entire phone, including /storage/sdcard0.

As far as I can tell, the only thing I need to do to get root back is to fix the permissions of su. How should I do this?

Of possible relevance:

• I'm using the stock ROM.
• The phone is the international GSM version.
• I tried to get into recovery and got an android with a red exclamation mark. Is this due to the bootloader being locked or is it a different problem?
• Since the update and losing root, a process called system_service has frequently started comsuming 100% of my CPU and slowing my phone to a crawl, forcing me to reboot.

Answer 1

What I wanted doesn't seem to be possible. It makes sense, really, as if it were possible it would represent a major security hole that would allow any app to gain root.

I ended up wiping my phone and testing the quality of my backups. (They weren't as good as I'd hoped.)

Answer 2

There are apps for that: Use OTA RootKeeper (Free) or SuperSU Pro (Paid).

If the OTA update hasn't touched your /system partition, these apps will work flawlessly. Another requirement is ext2, ext3, ext4 file system. As you are with stock ROM of Galaxy Nexus, this requirement has met.