Arch Linux

41418

Member

Registered: 2015-01-13

Posts: 11

Acer C720P - dev_boot_legacy set back to 0

I've been running a successful installation of Arch on this chromebook for a couple of months now.  I never set SeaBIOS an the default (I didn't mind pressing ctrl+l everytime), but recently, a friend booted up the chromebook and  did not know what to do at the splash screen. I don't know what he did, but now I cannot get into SeaBIOS anymore. At the splash screen, when I press ctrl+i, I can see that dev_boot_usb, and dev_boot_legacy have been set back to 0.  Since ChromeOS is not installed, I am unable to get back to the root shell to reset these.

How could those settings have been reverted? More importantly, is there a way to change those settings back without wiping/reinstalling ChromeOS?

Thank you in advance.

dhead

Member

Registered: 2012-09-26

Posts: 190

Re: Acer C720P - dev_boot_legacy set back to 0

From what I understand your current situation now is that you can only boot a kernel which was signed by Google.
Without accessing the fw chip directly you can only boot Chrome OS or the recovery media, I never tried it but I'm guessing the recovery media is fully automated, will automatically wipe your drive  and there's no way to get to a shell with it (thought it worth trying to boot it with the SSD disconnected).

The easiest solution is to grab another SSD (16GB in Amazon goes for 20$) and use the recovery media to install Chrome OS on it and then set the GBB flags or dev_boot_usb, and dev_boot_legacy.
You can of course also get a M.2 adapter to SATA/PCIe/USB (~15$ for SATA adapter on amazon) and backup your SSD before recovering Chrome OS.

Another solution would be flashing a custom fw with the use of BBB/RPi/Bus Blaster...(I'm guessing probably any SPI capable hw, google for guides on the process), you can backup the original fw in case you might want to revert to Chrome OS, the upstream flashrom should get the job done, just don't forget to remove the write protection screw.

It might be also possible to chroot the Chrome OS recovery media and use the relevant tools (crossystem, gbb flags script, flashrom) to manually set the GBB flags or dev_boot_usb, and dev_boot_legacy, you might need to manually replicate the way the gbb flag script is working, your best friend is the actual code, google "vboot_reference" (I guess you can also read the fw, change the correct values with an hex editor and write.it back). Edit: of course this would be done on another machine using SPI hardware to read the fw.
Notice that the firmware tools in Chrome OS are able to detect the structure of the fw and partially write to specific "partition" which the upstream tools (Coreboot's flashrom) can't.

Last edited by dhead (2015-04-29 06:44:08)

41418

Member

Registered: 2015-01-13

Posts: 11

Re: Acer C720P - dev_boot_legacy set back to 0

Ah! The SSD swap seems promising.  Any clues on how those flags could have reset?  I'll be setting the GBB flags this time around, but the curiosity will eat me alive.

Thanks.

dhead

Member

Registered: 2012-09-26

Posts: 190

Re: Acer C720P - dev_boot_legacy set back to 0

I'm guessing...

http://cdn3.howtogeek.com/wp-content/up … 8N4HFz.jpg

rikn00

Member

Registered: 2014-06-26

Posts: 21

Re: Acer C720P - dev_boot_legacy set back to 0

Those seabios settings are battery-backed so removing battery or letting it run completely out disables those settings. There's a guide on chromium wiki to restore those settings using a modded recovery media. http://dev.chromium.org/chromium-os/dev … n-dev-mode

dhead

Member

Registered: 2012-09-26

Posts: 190

Re: Acer C720P - dev_boot_legacy set back to 0

@rikn00

Nice, should be added to the wiki.
Edit: I added it to the Chrome OS devices page.

Last edited by dhead (2015-04-29 23:02:38)