[yocto] Support for writable files with a read-only root file system

[Matt Schuckmann]

> -----Original Message-----
> From: yocto-bounces at yoctoproject.org [mailto:yocto-
> bounces at yoctoproject.org] On Behalf Of Jim Rafert
> Sent: Wednesday, November 26, 2014 8:33 AM
> To: Mike Looijmans; yocto at yoctoproject.org
> Subject: Re: [yocto] Support for writable files with a read-only root
> file system
> 
> Hi Mike,
> 
> It sounds like using volatiles is the opposite of what you want.
> Unless I miss the point, what you want is to have your root file system
> mounted read-only with only select files being writable, and
> persistent.  Volatiles are by definition not persistent. Volatiles may
> be a part of your solution for files that can be non-persistent, but
> can't be the whole solution.

> 
> If you can make a small number of entire directory hierarchies writable
> (like /etc, /mnt), you can make small partitions that are mounted by
> /etc/fstab.  If you want to be more selective, you need to have a
> writable file system where the writable files will reside, with links
> in the root file system that point to them.
> 

Hi Jim, 

Yes you are correct I want a small number of files into to reside in a writeable partition with links in the root file system to point to them. I guess I latched on to the volatiles mechanism because it seems close to meeting my needs.

Making entire directory hierarchies like /etc writeable seems risky, things that you never want changed could get changed. Furthermore it makes it harder to deal with setting up default versions of files. 

Is there really no mechanism for this type of thing in OE? It seems like such a common thing for anybody creating a system with a ro root file system to need or am I missing something. 

If there is not such a mechanism then I will probably recreate the VOLATILE_BINDS mechanism that was in the MentorEmbedded layer and maybe call it PERSISTENT_BINDS or some such. I'm pretty new to OE so whatever I come up with will be pretty rough but I'd be willing to share my solution if there was interest. 

Thanks,
Matt S. 


> -Jim-
> ________________________________________
> From: Mike Looijmans [mike.looijmans at topic.nl]
> Sent: Wednesday, November 26, 2014 7:38 AM
> To: yocto at yoctoproject.org
> Subject: Re: [yocto] Support for writable files with a read-only root
> file system
> 
> The one-size-fits-all solution is to use an unionfs, and a writable
> part in RAM or mounted elsewhere, e.g. in flash (typically a tiny jffs2
> partition).
> 
> It requires quite some scripting to get right, you have to boot the
> system and then mount the overlay, the unionfs to join it with the
> rootfs, and then "pivot" it to make it the new root.
> 
> After that you get a root where you can modify any file, and even
> persist that change (if you use a persistent overlay).
> 
> 
> 
> On 11/26/2014 08:51 AM, Matt Schuckmann wrote:
> > I've been investigating the support for read only root file systems
> and trying to suss out how to support persistent writeable files in my
> image where the root file system is read only, this seems like a very
> common thing for an embedded Linux system to need but I'm not seeing
> the support for it.
> >
> > I should note that I'm working off of the Dylan branch, I'm not sure
> if things have changed in a newer branch.
> >
> > So far I've had to read the code to figure out as much as I have, and
> I haven't found any documentation for this, am I missing something?
> >
> > It appears that my recipe(s) are supposed to install a volatiles file
> under ${D}${sysconfdir}/default/volatiles/ that lists out the volatile
> directories, links and files that need to be created in the image.
> >
> > For writeable persistent files I would presume that I should specify
> links that point to some place on a write partition.
> >
> > What I don't see is any sort of mechanism for setting up the default
> files on that writeable partition, am I on my own for this or is there
> some other recommended mechanism.
> >
> > In my early research I found the MentorEmbedded/meta-ro-rootfs layer
> that had a nifty way of specifying VOLATILE_BINDS to create a list of
> binds that should occur at boot up and in the process creating the
> mounts if the target of the bind didn't exist a copy was made from the
> read only location, this appears to have provided a sort of default to
> go into the writeable location. Was this functionality abandoned when
> support for ro-rootfs was brought into oe-core? If so why?
> >
> > A good example for the kind of writeable persistent file that I
> intend
> > to have is /etc/network/interfaces How do I go about letting the
> init_ifupdown recipe install it normally and then have another recipe
> or even the image configure it to be either a link or a bind mount at
> another location while still preserving the default contents.
> >
> > I hope I'm making sense here.
> >
> > Thanks,
> > Matt S.
> >
> 
> 
> 
> Met vriendelijke groet / kind regards,
> 
> Mike Looijmans
> System Expert
> 
> 
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