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When the rootfs is readonly, systemd will expect /var to be writable.
Because we do not really have a R/W filesystem to mount on /var, we make
it a tmpfs [*], and use the systemd-tmpfiles feature to populate it with
"factory" defaults.
We obtain those factory defaults by redirecting /var to that location at
build time, using a symlink /var -> /usr/share/factory which is the
location in which systemd-tmpfiles will look for when instructed to
"recursively copy" a directory.
With a line like:
C /var/something - - - -
it will look for /usr/share/factory/something and copy it (recursively
if it is a directory) to /var/something, but only if it does not already
exist there.
We also mark this copy with the exclamation mark, as it is only safe to
copy on boot, not when changing targets.
To be noted: the real format for such lines are:
C /var/something - - - - /from/where/to/copy/something
But if the source is not given, then it is implicitly taken from
/usr/share/factory (which in our case is as-good a location as whatever
else, so we use it, and thus we need not specify the source of the
copy).
Note that we treat symlinks a little bit specially, by creating symlinks
to the factory defaults rather than copying them.
Finally, /var at build time is a symlink, but at runtime, it must be a
directory (so we can mount the tmpfs over there). We can't change that
as a target-finalize hook, because:
- some packages may want to set ownership and/or access rights on
files or directories in /var, and that only happens while assembling
the filesystem images; changing /var from a symlink to a (then
empty) directory would break this;
- /var would be a directory on sub-sequent builds (until the next
"make clean").
Instead, we use the newly-introduce pre- and post-rootfs command hooks,
to turn /var into a directory before assembling the image, and back to a
symlink after assembling the image.
[*] People who want the factory-defaults only on first boot will have
to tweak the fstab to mount something else than a tmpfs on /var.
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Buildroot is a simple, efficient and easy-to-use tool to generate embedded Linux systems through cross-compilation. The documentation can be found in docs/manual. You can generate a text document with 'make manual-text' and read output/docs/manual/manual.text. Online documentation can be found at http://buildroot.org/docs.html To build and use the buildroot stuff, do the following: 1) run 'make menuconfig' 2) select the target architecture and the packages you wish to compile 3) run 'make' 4) wait while it compiles 5) find the kernel, bootloader, root filesystem, etc. in output/images You do not need to be root to build or run buildroot. Have fun! Buildroot comes with a basic configuration for a number of boards. Run 'make list-defconfigs' to view the list of provided configurations. Please feed suggestions, bug reports, insults, and bribes back to the buildroot mailing list: buildroot@buildroot.org You can also find us on #buildroot on Freenode IRC. If you would like to contribute patches, please read https://buildroot.org/manual.html#submitting-patches