From 26085bbbd50083e87b8540fa043d712ce8e85f61 Mon Sep 17 00:00:00 2001 From: "Yann E. MORIN" Date: Wed, 2 Aug 2017 00:52:23 +0200 Subject: [PATCH] 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" Reviewed-by: Romain Naour Signed-off-by: Arnout Vandecappelle (Essensium/Mind) Signed-off-by: Thomas Petazzoni --- package/skeleton-systemd/skeleton-systemd.mk | 50 +++++++++++++++++++- system/Config.in | 1 - 2 files changed, 48 insertions(+), 3 deletions(-) diff --git a/package/skeleton-systemd/skeleton-systemd.mk b/package/skeleton-systemd/skeleton-systemd.mk index 384715e1c9..860ebea2cd 100644 --- a/package/skeleton-systemd/skeleton-systemd.mk +++ b/package/skeleton-systemd/skeleton-systemd.mk @@ -15,11 +15,57 @@ SKELETON_SYSTEMD_DEPENDENCIES = skeleton-common SKELETON_SYSTEMD_PROVIDES = skeleton +ifeq ($(BR2_TARGET_GENERIC_REMOUNT_ROOTFS_RW),y) + +define SKELETON_SYSTEMD_ROOT_RO_OR_RW + echo "/dev/root / auto rw 0 1" >$(TARGET_DIR)/etc/fstab + mkdir -p $(TARGET_DIR)/var +endef + +else + +# On a R/O rootfs, /var is a tmpfs filesystem. So, at build time, we +# redirect /var to the "factory settings" location. Just before the +# filesystem gets created, the /var symlink will be replaced with +# a real (but empty) directory, and the "factory files" will be copied +# back there by the tmpfiles.d mechanism. +define SKELETON_SYSTEMD_ROOT_RO_OR_RW + mkdir -p $(TARGET_DIR)/etc/systemd/tmpfiles.d + mkdir -p $(TARGET_DIR)/usr/share/factory/var + ln -s usr/share/factory/var $(TARGET_DIR)/var + echo "/dev/root / auto ro 0 1" >$(TARGET_DIR)/etc/fstab + echo "tmpfs /var tmpfs mode=1777 0 0" >>$(TARGET_DIR)/etc/fstab +endef + +define SKELETON_SYSTEMD_PRE_ROOTFS_VAR + rm -f $(TARGET_DIR)/var + mkdir $(TARGET_DIR)/var + for i in $(TARGET_DIR)/usr/share/factory/var/*; do \ + j="$${i#$(TARGET_DIR)/usr/share/factory}"; \ + if [ -L "$${i}" ]; then \ + printf "L+! %s - - - - %s\n" \ + "$${j}" "../usr/share/factory/$${j}" \ + || exit 1; \ + else \ + printf "C! %s - - - -\n" "$${j}" \ + || exit 1; \ + fi; \ + done >$(TARGET_DIR)/etc/tmpfiles.d/var-factory.conf +endef +SKELETON_SYSTEMD_ROOTFS_PRE_CMD_HOOKS += SKELETON_SYSTEMD_PRE_ROOTFS_VAR + +define SKELETON_SYSTEMD_POST_ROOTFS_VAR + rm -rf $(TARGET_DIR)/var + ln -s usr/share/factory/var $(TARGET_DIR)/var +endef +SKELETON_SYSTEMD_ROOTFS_POST_CMD_HOOKS += SKELETON_SYSTEMD_POST_ROOTFS_VAR + +endif + define SKELETON_SYSTEMD_INSTALL_TARGET_CMDS mkdir -p $(TARGET_DIR)/home mkdir -p $(TARGET_DIR)/srv - mkdir -p $(TARGET_DIR)/var - echo "/dev/root / auto rw 0 1" >$(TARGET_DIR)/etc/fstab + $(SKELETON_SYSTEMD_ROOT_RO_OR_RW) endef $(eval $(generic-package)) 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 depends on BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_10 select BR2_ROOTFS_MERGED_USR select BR2_PACKAGE_SYSTEMD - select BR2_TARGET_GENERIC_REMOUNT_ROOTFS_RW if BR2_ROOTFS_SKELETON_DEFAULT comment "systemd needs a glibc toolchain, headers >= 3.10" depends on !(BR2_TOOLCHAIN_USES_GLIBC \ -- 2.30.2