/var needs to be read-write for a functioning system. Various
systemd services will fail to start if /var is read-only. After we
remount /var (or if we skip remounting /var because / is read-only),
if /var is still readonly, mount a tmpfs file system on /var.
While this isn't strictly part of ostree, keeping it here makes sense
because it keeps twiddling around with the /var mount in one place
for easier coordination and maintenance. This will likely need updating
if systemd gains better support for a readonly root filesystem.
https://bugzilla.gnome.org/show_bug.cgi?id=732979
The previous S_IMMUTABLE commit broke ostree-remount; / is now not
actually writable. All we really wanted to know though was whether it
was *mounted* writable, so check that via statvfs() which is cleaner
anyways (i.e. not via access() which kernel people hate).
https://bugzilla.gnome.org/show_bug.cgi?id=728006
Linux creates a copy of the soure mount flags when creating a bind
mount; if the source is read-only, then the bind mount is.
The problem is that systemd will remount the rootfs read/write, but
each mount (/home, /var etc.) will still be read-only. We need to
remount every bind mount except for /usr to read-write too.
This only "worked" with the old ostree-switch-root because it
effectively force mounted the rootfs read-write always, ignoring the
"ro" flag.
Giuseppe Scrivano
Owen W. Taylor
Colin Walters