ubuntu-phone team mailing list archive

[Martin Pitt <martin.pitt@xxxxxxxxxx>]

Hello Iain,

Iain Lane [2015-05-20 15:19 +0100]:
> We sometimes come across bugs where software expects to be able to
> rename files to work properly (either to atomically update, or perhaps
> to disable some functionality). This doesn't work with our read-only
> partition layout. We know how to work around this limitation by making
> code changes, for example
> 
>   https://code.launchpad.net/~seb128/whoopsie-preferences/touch-writable-image/+merge/259531

Indeed, I have similar hacks in systemd. Our current approach to
making writable parts of /etc via file bind mounts is hopelessly
doomed, as it breaks file system semantics like the one you mentioned.

> to make use of a symlink into a writable directory. It's not a great
> solution, not least because we have to implement it on a case-by-case
> basis and we don't have any good way of proactively spotting the bugs -
> they typically manifest themselves by settings not being saved but
> people usually do not check for rename failures or surface these to the
> user.
> 
> Can anyone think of a way to fix this class of problem and remove the
> need for us to have to go and patch code? Or at least to detect these
> rename failures (check for EROFS) and aggregate them somewhere so that
> they can be fixed. This is on touch - I'm not sure if snappy does
> anything different here.

TBH, the only clean way that I see is to flip the whole thing around.
Keep /etc (and /var) writable, and instead deal with the things that
should *not* be modified. I. e. ship them in e. g. /usr/etc/ and
symlink/bind-mount them to /etc, or longer-term, get rid of them
completely. IMHO this would be a much saner architecture than trying
to play eternal catch-up with bugs like this and getting an
ever-growing spaghetti heap of file bind mounts which we'll never be
able to revert/change.

Martin

-- 
Martin Pitt                        | http://www.piware.de
Ubuntu Developer (www.ubuntu.com)  | Debian Developer  (www.debian.org)

Attachment: signature.asc
Description: Digital signature