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package/skeleton-systemd: systemd-ify mounting /var tmpfs with ro rootfs

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To mount our /var tmpfs when the rootfs is mounted read-only (really,
not remounted read-write), we use an entry in fstab.

However, /etc could also be a tmpfs (for full state-less systems, or
easy factory-reset, see [0]). It also prevents easily ordering other
systemd units until after /var is mounted (not impossible, but less
easy).

So, we register /var as a systemd mount unit, so that we can also have
the /var factory populated and functional even when /etc is empty. The
var.mount unit is heavily modelled after systemd's own tmp.mount one, so
we carry the same license for that file (in case that may apply).

This has two side effects:
  - as hinted previously, it simplifies writing other systemd units to
    order them after /var is mounted
  - user can easily replace it with their own, which mounts an actual
    filesystem

[0] http://0pointer.de/blog/projects/stateless.html

Signed-off-by: Yann E. MORIN <yann.morin@orange.com>
Cc: Norbert Lange <nolange79@gmail.com>
Cc: Romain Naour <romain.naour@smile.fr>
Cc: Jérémy Rosen <jeremy.rosen@smile.fr>
[yann.morin.1998@free.fr:
  - split original patch in two
  - this one only handles converting /var mounting into a systemd unit
  - adapt commit log accordingly
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Reviewed-by: Norbert Lange <nolange79@gmail.com>
[yann.morin.1998@free.fr (as suggested by Norbert):
  - fix Before= dependencies
  - drop [Install] section
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
This commit is contained in:
Yann E. MORIN 2022-10-18 21:43:05 +02:00 committed by Yann E. MORIN
parent ff0fd90c3e
commit 8cbfbe487a
2 changed files with 17 additions and 1 deletions
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3
package/skeleton-init-systemd/skeleton-init-systemd.mk
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@ -30,7 +30,6 @@ else
# back there by the tmpfiles.d mechanism.
define SKELETON_INIT_SYSTEMD_ROOT_RO_OR_RW
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_INIT_SYSTEMD_PRE_ROOTFS_VAR
@ -52,6 +51,8 @@ define SKELETON_INIT_SYSTEMD_PRE_ROOTFS_VAR
|| exit 1; \
fi; \
done >$(TARGET_DIR)/usr/lib/tmpfiles.d/00-buildroot-var.conf
$(INSTALL) -D -m 0644 $(SKELETON_INIT_SYSTEMD_PKGDIR)/var.mount \
$(TARGET_DIR)/usr/lib/systemd/system/var.mount
endef
SKELETON_INIT_SYSTEMD_ROOTFS_PRE_CMD_HOOKS += SKELETON_INIT_SYSTEMD_PRE_ROOTFS_VAR

15
package/skeleton-init-systemd/var.mount Normal file
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@ -0,0 +1,15 @@
# SPDX-License-Identifier: LGPL-2.1-or-later
# Modelled after systemd's tmp.mount
[Unit]
Description=Buildroot /var tmpfs
DefaultDependencies=no
Conflicts=umount.target
Before=local-fs.target umount.target
After=swap.target
[Mount]
What=tmpfs
Where=/var
Type=tmpfs
Options=mode=1777,strictatime,nosuid,nodev,size=50%%,nr_inodes=1m