[yocto] How handle files needing updates in read-only filesystem

[Peter Kjellerstedt]

> -----Original Message-----
> From: yocto-bounces at yoctoproject.org [mailto:yocto-
> bounces at yoctoproject.org] On Behalf Of Ulf Samuelsson
> Sent: den 9 juni 2018 11:51
> To: yocto at 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 <user>:<group> /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 <mode> <uid> <gid> <filename> <ignored>
> 
> An idea would be to have the following syntax:
> f <mode> <uid> <gid> <filename> <source>
> 
> If <source> is a valid filepath, then copy this file to <filename>
> If <source> is not a valid filepath, create <filename> 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