> -Original Message-
> From: yocto-boun...@yoctoproject.org [mailto:yocto-
> boun...@yoctoproject.org] On Behalf Of Ulf Samuelsson
> Sent: den 9 juni 2018 11:51
> To: yocto@yoctoproject.org
> Subject: [yocto] How handle files needing updates in read-only
> filesystem
>
> We have a rootfs which is read-only.
> For files that needs to be updated, like "/etc/localtime"
> we will create a symbolic link to a partition (/persistent) which
> is retained between rootfs image updates.
>
> /etc/localtime -> /persistent/etc/localtime
>
> The original recipe will generate the real /etc/localtime of course.
>
> To support the file beeing updated, the following needs to be done in a
> rootfs postprocess.
>
> mv /etc/localtime /update/etc/localtime
> ln -sf /persistent/etc/localtime /etc/localtime
> install -m 0644 /update/etc/localtime /persistent/etc/localtime
> chown : /persistent/etc/localtime
>
> The last two commands needs to be executed on the running system,
> since /persistent is not part of the rootfs.
>
> To ensure that the file can be updated is an ad-hoc activity for each
> affected file. It would be better if there was a class where you
> declare
> a file to be writeable, and then a post process
> would move the file to /update (or similar) and create a symbolic link
>
> SETTINGS ?= "/persistent"
> =
> inherit writeable
>
> WRITEABLE = "/etc/localtime"
> =
>
> This would generate the first two commands for each listed file.
>
> mv /etc/localtime /update/etc/localtime
> ln -sf ${SETTINGS}/etc/localtime /etc/localtime
>
> The file would also be added to a file indicating that it may need
> to be copied to the ${SETTINGS}
>
> Is it a good solution to modify "populate_volatiles.sh"
> to support a second file parameter for the f (file) command
> in the /etc/default/volatiles file?
>
> Today the file create command looks like:
> f
>
> An idea would be to have the following syntax:
> f
>
> If is a valid filepath, then copy this file to
> If is not a valid filepath, create using "touch"
>
> This would break any build which has a "funny" volatiles file.
> Otherwise a script called populate_persistent.sh could be
> created with such an extension.
>
> Comments?
>
> BR
> Ulf Samuelsson
Since a lot of files in /etc typically need to be writable, one way to
handle this is by using an overlayfs for /etc. That is what we do and
it works very well.
An alternative, more along your suggestion, is to use bind mounts. The
advantage of using a bind mount instead of a symbolic link is that it
will look as a normal file.
You should look into the volatile-binds recipe and the VOLATILE_BINDS
variable. It is used to handle directories that need to be writable.
It will create systemd service files that copies the non-volatile
directory to the volatile directory (if it does not already exists),
and the bind mounts it back.
I am not sure volatile-binds works out-of-the-box for files, but I
believe it does. If not, it should not be too hard to modify it so
that it can handle files as well.
//Peter
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