utils/vmbox

vmbox — a disposable Arch VM sandbox

Run destructive system scripts — shutdown installers, chattr +i hosts guards, pacman hooks — against a throwaway Arch guest, and verify the outcome from the host. Break it as thoroughly as you like; reset takes under a second.

./install.sh          # installs deps (libisoburn), links `vm` onto PATH
vm build              # builds the golden image, once (~100s)
vm new demo           # instant: a ~200 KB overlay on the sealed base
vm run demo 'sudo systemctl poweroff'
vm reset demo         # back to pristine

Why this exists

Much of testsAndMisc/linux_configuration cannot be tested on the real machine. The repo's own CI says so — its Ubuntu job runs only the "side-effect-free" tests and notes the rest "drive pacman, /etc, systemd or adb and cannot run on an Ubuntu runner without root and an Arch container". Locally they probe the live system (test_results.log asserts PASS: /etc/hosts is immutable).

Three things make testing them on the host genuinely unsafe:

• chattr +i immutability — 15 files make /etc/hosts and friends immutable. A bad test is not simply revertable; rm fails with EPERM.
• Auto-poweroff — the shutdown installers write systemd units that power the machine off on a schedule.
• Pacman hooks — boot_recovery/install.sh installs pre/post-transaction hooks and touches mkinitcpio. Blast radius is "your next update".

The hard part: a successful shutdown kills your observer

Running a shutdown script over SSH returns a broken pipe. That is indistinguishable from a crash, a hang, or a network blip — ssh returns 255 for all of them. So "did it actually shut down?" is answered from the host, using two artefacts that outlive the VM:

Channel	What it proves
QMP event log (events.jsonl)	That the machine stopped, and who stopped it
Serial console log	How far the shutdown sequence got

A persistent recorder attaches at launch, before anything runs in the guest. This ordering matters: attach afterwards and QEMU has already exited, the stream is closed, and there is no verdict to read.

Five outcomes, all host-observable

Every row below was measured against a real Arch guest, not inferred:

Guest command	QMP event	Verdict	Exit
systemctl poweroff	SHUTDOWN {guest:true, guest-shutdown} + serial reaches reboot: Power down	clean poweroff	0
a stop that dies mid-sequence	SHUTDOWN {guest:true}, serial truncated	dirty	3
systemctl reboot	SHUTDOWN {guest:true, guest-reset}	rebooted, did not power off	4
echo c > /proc/sysrq-trigger	GUEST_PANICKED	crashed	5
a script that wedges	no event, liveness probe fails	hung	6

ssh returned the identical Connection closed by remote host for the poweroff, the reboot AND the panic. That is the entire argument for reading the verdict from the host rather than from the connection.

guest:true alone is not enough for "shuts down completely" — poweroff -f and a script that kills systemd mid-sequence both produce it. The serial tail is the discriminator, and it is asserted by grep, not read by eye.

A POWERDOWN event is not proof of anything: it means the ACPI request was delivered. With no OS booted, QEMU emits POWERDOWN and then runs forever.

Worked example: testing the real shutdown installer

vm share ~/testsAndMisc                 # read-only bind, not a symlink
vm share ~/utils                        # guard-lib lives here
vm new st --rtc 2026-08-22T20:55:00     # inside the 21:00-05:00 window
vm run st 'git clone --no-hardlinks -q /mnt/hostrepo/testsAndMisc ~/tam'
vm run st 'echo y | sudo bash ~/tam/linux_configuration/scripts/periodic_background/digital_wellbeing/setup_midnight_shutdown.sh enable'
vm run st 'sudo systemctl poweroff'     # -> VERDICT: clean poweroff
vm reset st                             # back to pristine

Verified: the config lands at /etc/shutdown-schedule.conf under guard-lib's immutable protection, and shutdown-timer-monitor.service, day-specific-shutdown.timer and shutdown-timer-monitor-watchdog.timer all come up enabled -- while the host's own copy of that file kept its original timestamp and /etc/hosts kept its +i flag.

Two things that installer taught us, both worth knowing before you run it:

• It is interactive. It ends on read -r -p 'Do you want to proceed?'. vm run closes stdin on purpose (installers that prompt would otherwise hang forever), so pipe echo y | when you actually want it to proceed.
• It is subcommand-driven (enable / status). Bare invocation prints usage and exits 1 without changing anything.

Design notes

• Reset is a backing-file trick. The golden image is built once and sealed (chmod 444 + a sha256 sidecar verified on every launch). Each sandbox is a thin qcow2 overlay, so vm reset is rm + recreate. Booting the base itself would silently invalidate every overlay, so the launcher refuses to.
• No root anywhere. User-mode networking with a per-VM forwarded SSH port; VM-to-VM traffic rides a socket multicast segment. No bridge, no libvirt, no daemon.
• The guest clock is pinned (--rtc) and re-applied on every launch. The shutdown installers gate on a 21:00–05:00 window, so an unpinned clock makes their tests pass or fail depending on the time of day. NTP is masked in the image, or it would quietly undo the pin.
• The guest gets its own clone of the repos from a read-only mount. Real credentials are never shared; the guest carries sentinel values instead.

Commands

Command	Effect
vm build [--force]	Build/rebuild the golden image
vm new <name> [--rtc <ts>]	Create a sandbox
vm run <name> <cmd...>	Run a command, print the verdict
vm ssh <name>	Interactive shell (starts the VM if needed)
vm screenshot <name> [out.png]	Capture the screen (locker/X11 tests) — verified: real 1280x800 PNG of the guest's i3 session, no viewer installed
vm reset <name>	Wipe back to pristine
vm rm <name> / vm list	Delete / list sandboxes

Out of scope

A VM cannot cover everything: adb/phone tests (~36 files), GPU monitors, the i2c RGB in setup_night_lockdown.sh, and the Nextcloud/SearXNG suites (whose fixtures assume a Debian layout) all stay on the host or stay untested.

Tests

bats tests/test_vmbox.bats — 39 host-side unit tests for name validation, meta handling, serial rotation and the full verdict table. They do not boot a VM; booting is covered by the destructive end-to-end demo above.

Speed

Measured on this host, all with the cloud image already downloaded:

	before	after
vm run (cold sandbox)	41s	14s
vm run (warm sandbox)	30s	3s

Neither number was the guest being slow. _run_settle waited up to 30s for a poweroff marker on EVERY run, including the majority that leave the guest running, and the ssh readiness poll used a 5s connect timeout against a forwarded port QEMU accepts before the guest is listening on it.

A cold vm run also waits for the guest's X session, because sshd now answers before tty1's autologin has started Xorg; VMBOX_X_WAIT=<seconds> overrides it.

The build reads packages from the host's own /var/cache/pacman/pkg, mounted read-only, rather than fetching 396 MiB over the internet. The guest keeps its own writable cache, so a package the host lacks is downloaded as usual. See DOCS-session-results.md for the build before/after.