If someone reads /run/host/os-release at the exact same time it is being updated, and it
is large enough, they might read a half-written file. This is very unlikely as
os-release is typically small and very rarely changes, but it is not
impossible.
Bind mount a staging directory instead of the file, and symlink the file
into into, so that we can do atomic file updates and close this gap.
Atomic replacement creates a new inode, so existing bind mounts would
continue to see the old file, and only new services would see the new file.
The indirection via the directory allows to work around this, as the
directory is fixed and never changes so the bind mount is always valid,
and its content is shared with all existing services.
Fixes https://github.com/systemd/systemd/issues/28794
Follow-up for 3f37a82545
Otherwise if the os-release file shrinks between updates, there
will be a merge of the two.
Also remove redundant ENOENT check.
Follow-up for 3f37a82545
As it says on the tin, configures the unit to survive a soft reboot.
Currently all the following options have to be set by hand:
Conflicts=reboot.target kexec.target poweroff.target halt.target
Before=reboot.target kexec.target poweroff.target halt.target
After=sysinit.target basic.target
DefaultDependencies=no
IgnoreOnIsolate=yes
This is not very user friendly. If new default dependencies are added,
or new shutdown/reboot types, they also have to be added manually.
The new option is much simpler, easy to find, and does the right thing
by default.
Currently for portable services we automatically add a bind mount
os-release -> /run/host/os-release. This becomes problematic for the
soft-reboot case, as it's likely that portable services will be configured
to survive it, and thus would forever keep a reference to the old host's
os-release, which would be a problem because it becomes outdated, and also
it stops the old rootfs from being garbage collected.
Create a copy when the manager starts under /run/systemd/propagate instead,
and bind mount that for all services using RootDirectory=/RootImage=, so
that on soft-reboot the content gets updated (without creating a new file,
so the existing bind mounts will see the new content too).
This expands the /run/host/os-release protocol to more services, but I
think that's a nice thing to have too.
Closes https://github.com/systemd/systemd/issues/28023
Luca Boccassi
Frantisek Sumsal
Frantisek Sumsal
Lennart Poettering