> -----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 <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 -- _______________________________________________ yocto mailing list yocto@yoctoproject.org https://lists.yoctoproject.org/listinfo/yocto
