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When adding a sysext image to the system and manuall merging it, a
later "systemctl (re)start systemd-sysext" won't work because "merge"
refuses to work when something is merged already. Another problem with
"merge" at start plus "unmerge" at stop is that a service restart can't
make use of the new MOVE_MOUNT_BENEATH in the future even which would
only be available in "refresh". It also prepares us for setting up the
merged overlay for the sysroot from the initrd already, which also
would lead to the mentioned start problem of the service (One
optimization could be to skip the loading but only if we are sure that
all images were loaded and weren't modified since - this assumption is
hard because early services could want to inject a sysext, too).
Use "refresh" on service start to fix the problem that the service
can't start as soon as a manual merge was done. Also add a reload
action that allows to issue "systemctl reload systemd-sysext" and it
will make use of MOVE_MOUNT_BENEATH once we implement this in
systemd-sysext refresh (and it's available from the kernel).
sysexts are meant to extend /usr. All extension images and directories are opened and merged in a
single, read-only overlayfs layer, mounted on /usr.
So far, we had fallback storage directories in /usr/lib/extensions and /usr/local/lib/extensions.
This is problematic for three reasons.
Firstly, technically, for directory-based extensions the kernel will reject
creating such an overlay, as there is a recursion problem. It actively
validates that a lowerdir is not a child of another lowerdir, and fails with
-ELOOP if it is. So having a sysext /usr/lib/extensions/myextdir/ would result
in an overlayfs config lowerdir=/usr/lib/extensions/myextdir/usr/:/usr which is
not allowed, as indicated by Christian the kernel performs this check:
/*
* Check if this layer root is a descendant of:
* - another layer of this overlayfs instance
* - upper/work dir of any overlayfs instance
*/
<...>
/* Walk back ancestors to root (inclusive) looking for traps */
while (!err && parent != next) {
if (is_lower && ovl_lookup_trap_inode(sb, parent)) {
err = -ELOOP;
pr_err("overlapping %s path\n", name);
Secondly, there's a confusing aspect to this recursive storage. If you
have /usr/lib/extensions/myext.raw which contains /usr/lib/extensions/mynested.raw
'systemd-sysext merge' will only pick up the first one, but both will appear in
the merged root under /usr/lib/extensions/. So you have two extension images, both
appear in your merged filesystem, but only one is actually in use.
Finally, there's a conceptual aspect: the idea behind sysexts and hermetic /usr
is that the /usr tree is not modified locally, but owned by the vendor. Dropping
extensions in /usr thus goes contrary to this foundational concept.
For shutdown, we queue shutdown.target/start, so in every unit which should be
stopped *before* shutdown, we need both Conflicts and an ordering dependency
with shutdown.target (either Before= or After= would work, because stop jobs
are always ordered before start jobs).
For initrd transition, we queue initrd-switch-root.service/isolate. This
automatically creates a /stop job for every running unit without
IgnoreOnIsolate. But no ordering dependency is created, unless the unit has a
(possibly transitive) ordering dependency on initrd-switch-root.service.
Since most units must stop before the transition, we should add the ordering
dependency. It is nicer to use Before=initrd-switch-root.target for this.
initrd-switch-root.target is ordered before initrd-switch-root.service, so
the effect it the same when both are in a transaction.
Fixes#23745.
To also cover the case where somebody is emergency mode in the initrd and
queues initrd-switch-root.service/start (not isolate), also add
Conflicts=initrd-switch-root.target, so various units are stopped properly.
This extends 2525682565 to cover all the other
services that are touched. It could be consider "operator error", but it's
easy to make and it's nicer if we can make this more foolproof.
The block is reordered and split to have:
1. description + documentation
2. (optionally) conditions
3. all the dependencies
I think it's easier to read the units this way.
Also, the Conflicts+Before is seperated out to separate lines.
The ordering dependency is "fake", because it could just as well be
After=, we are adding it to force ordering wrt. shutdown.target, and
it plays a different role than the other Before=, which are about a
real ordering on boot.
