Disable UI-based OTA updates if root file system is writable

[Tony Espy]

As it's possible for end-users of pre-installed Touch devices to make their file system writable *and* we know that this can lead to broken systems if new packages are installed via apt, we should prevent UI-based OTA updates from being applied. The UI should provide a clear explanation of why OTA updates are non-installable.

As discussed, it should be possible to use the command-line client to apply updates, and if used, a simple warning should be output.

This is being filed as of OTA4, based on the vivid-overlay PPA.

[Steve Langasek (vorlon)]

First, the problem isn't specific to the rootfs being *currently* writable. The problem exists if the rootfs has *ever* been marked writable (and modified).

Second, as discussed elswhere in mail, it would be better to force a full update in this case, *not* refuse to update the system. The Ubuntu Phone model is that users should receive updates automatically and seamlessly; while users can fiddle with their filesystem, we probably /don't/ want to leave those users islanded on the currently-installed image version forever, but instead propose a full update, possibly with appropriate caveats. This will clobber any (unsupported) local modifications but at least won't leave them with an insecure and unsupportable device.

[Mateo Salta (mateo-salta)]

yeah, I only made mine writable to do some ad blocking with the hosts file. something I can re-apply after an ota wipes it out, but nothing crazy like installing things with apt. also I have heard some add new ring tones this way too.