path: root/system
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authorGravatar Yann E. MORIN <yann.morin.1998@free.fr>2017-08-02 00:52:23 +0200
committerGravatar Thomas Petazzoni <thomas.petazzoni@free-electrons.com>2017-08-02 20:59:27 +0200
commit26085bbbd50083e87b8540fa043d712ce8e85f61 (patch)
tree4b736f3a385f3bf7b037c7de7b3ec4464d9800d8 /system
parent4628b6f3b4512ec7cfb3c08ad4fe0ec40e1c5cc1 (diff)
system: make systemd work on a read-only rootfs
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>
Diffstat (limited to 'system')
1 files changed, 0 insertions, 1 deletions
diff --git a/system/Config.in b/system/Config.in
index 5716abd3cf..59d8eab271 100644
--- a/system/Config.in
+++ b/system/Config.in
@@ -136,7 +136,6 @@ config BR2_INIT_SYSTEMD
comment "systemd needs a glibc toolchain, headers >= 3.10"