fix(kiosk): stop GDM covering the wall with a greeter after auto-login
Auto-login succeeded and the wall kept streaming, but about ten seconds after the kiosk session came up GDM autospawned a greeter on the same GPU and a login screen covered it. gnome-kiosk ships X-GDM-SessionRegisters=false, so nothing ever registers and GDM concludes the display is still free — which is why every remote check (loginctl, the mirror roster) stayed green while the physical screen showed a password prompt. Override that session file with the flag set to true, keeping the file name and session id identical: inventing an id makes GDM quietly fall back to the plain GNOME session, top bar and all. Add a per-minute guard that reclaims the foreground VT, because this class of failure is invisible from the outside, and have the checker assert both the foreground VT and the greeter-session count. That count had a silent false green: journalctl has no --no-legend, so the check read an empty stream and passed unconditionally. Also keep a copy of the payload in /usr/local/share/mirror-kiosk. /tmp is cleared at every boot, which deletes the checker just after the one event you are most likely to want to re-run it for. Verified on the reference client: a cold boot now creates no greeter session at all, the kiosk compositor and Firefox come up undisturbed, and a console stolen with chvt is taken back within a minute. LLM-Generated: true
This commit is contained in:
@@ -116,15 +116,16 @@ session, auto-logged in as `kiosk`, with Firefox in `--kiosk` mode pointed at
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no monitor:
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```bash
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scp -r docs/kiosk-silverblue client:/tmp/ && ssh client \
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'cd /tmp/kiosk-silverblue && sudo sh install-kiosk.sh && sudo systemctl restart gdm'
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ssh client 'cd /tmp/kiosk-silverblue && sudo sh check-kiosk.sh'
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scp -r docs/kiosk-silverblue client:/var/tmp/ && ssh client \
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'cd /var/tmp/kiosk-silverblue && sudo sh install-kiosk.sh && sudo systemctl restart gdm'
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ssh client 'sudo sh /usr/local/share/mirror-kiosk/check-kiosk.sh'
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```
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Because nobody is standing at the machine, every claim about it is verified from
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the outside: `loginctl` for the session, the server's `/api/streams` for the
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viewer, the sharer's own peer state for the media, and a probe page for the
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rendered resolution. The runbook records why each file lives where it does on an
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the outside: `loginctl` for the session, the foreground VT for "is the wall
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actually on screen", the server's `/api/streams` for the viewer, the sharer's own
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peer state for the media, and a probe page for the rendered resolution. The
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runbook records why each file lives where it does on an
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otherwise read-only OS.
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## Development
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@@ -16,6 +16,9 @@ or to change its URL.
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| `kiosk.conf` | `~kiosk/.config/mirror/kiosk.conf` | **the one per-client value: the URL** |
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| `policies.json` | `/etc/firefox/policies/policies.json` | no updates, accounts, telemetry, first-run pages |
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| `window-config.ini` | `/usr/local/share/gnome-kiosk/window-config.ini` | force the Firefox window fullscreen |
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| `gnome-kiosk-script-wayland.desktop` | `/usr/local/share/wayland-sessions/` | **stops GDM covering the wall with a greeter** |
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| `mirror-kiosk-console-guard` | `/usr/local/libexec/` | reclaims the console if the wall loses it |
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| `mirror-kiosk-console-guard.{service,timer}` | `/etc/systemd/system/` | runs that check every minute |
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| `dim-probe.py` | run on any reachable host | measures what the kiosk actually renders |
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---
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@@ -29,15 +32,18 @@ or to change its URL.
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# 2. Layer the kiosk packages. This is the only step that needs a reboot.
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ssh client 'sudo rpm-ostree install gnome-kiosk gnome-kiosk-script-session && sudo reboot'
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# 3. Wait ~60s, then install everything else.
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scp -r docs/kiosk-silverblue client:/tmp/ && ssh client \
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'cd /tmp/kiosk-silverblue && sudo sh install-kiosk.sh'
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# 3. Wait ~60s, then install everything else. Stage it in /var/tmp, not /tmp:
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# /tmp is wiped at every boot, so a checker staged there is gone exactly when
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# you want it. The installer keeps a permanent copy of the payload in
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# /usr/local/share/mirror-kiosk.
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scp -r docs/kiosk-silverblue client:/var/tmp/ && ssh client \
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'cd /var/tmp/kiosk-silverblue && sudo sh install-kiosk.sh'
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# 4. Hand the console to the kiosk.
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ssh client 'sudo systemctl restart gdm'
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# 5. Prove it, without touching the machine.
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ssh client 'cd /tmp/kiosk-silverblue && sudo sh check-kiosk.sh'
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ssh client 'sudo sh /usr/local/share/mirror-kiosk/check-kiosk.sh'
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```
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`install-kiosk.sh` creates the `kiosk` user if it does not exist, so step 2 can
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@@ -170,6 +176,76 @@ ConditionPathExists=!/var/home/kiosk/.config/gnome-initial-setup-done`.
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---
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## The greeter that eats the wall
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The shipped `gnome-kiosk-script-wayland.desktop` declares:
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```ini
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X-GDM-SessionRegisters=false
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```
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That is true of the session itself — gnome-kiosk is a bare mutter kiosk, so
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nothing ever registers with GDM. But GDM reads "never registered" as "this
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display is still mine", and about ten seconds after a perfectly successful
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auto-login it **autospawns a full greeter session on the same GPU**. A login
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screen covers the wall.
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The part that makes this maddening: the kiosk keeps running underneath. The
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session stays `Active`, Firefox keeps streaming, the mirror roster keeps
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counting the wall as a viewer, `loginctl` reports the kiosk on `seat0` — every
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remote check says nothing is wrong while a password prompt is on the physical
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screen. This is what "it didn't auto-login" actually looks like.
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From a real boot, `journalctl -b`:
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```
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19:07:28 PAM:authentication grantors=pam_permit acct="kiosk" ... res=success
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19:07:28 New session '1' of user 'kiosk' with class 'user' and type 'wayland'
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19:07:30 Started org.gnome.Kiosk@wayland.service ← the wall is up
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19:07:38 New session 'c1' of user 'gdm-greeter' with class 'greeter'
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19:07:39 gnome-shell: Registering session with GDM ← the greeter eats it
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19:08:46 Stopped org.gnome.Shell@gdm.service ← 68s of login screen
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```
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**The fix** is the local override of that one file, changing only
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`X-GDM-SessionRegisters` to `true`, so GDM treats the kiosk session as the owner
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of the display and never brings up a greeter. Two things about it are traps:
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* **Keep the file name and session id identical to the shipped one.** Inventing
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`mirror-kiosk.desktop` and pointing AccountsService at it does *not* work —
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GDM silently falls back to the plain GNOME session, and you get a normal
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desktop with a top bar instead of a kiosk. Verified the hard way.
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* A normal GNOME session already sets `true`, which is why this bug only ever
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shows up on the kiosk.
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After the override, the same boot looks like this: `gdm-greeter` appears **zero**
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times, the session registers, and the kiosk compositor and Firefox come up
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undisturbed.
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## And if the wall loses the console anyway
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Because the failure above is invisible from the outside, the install also drops
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in a guard: once a minute it checks that the kiosk's `seat0` session is still the
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foreground VT, and if not it runs `loginctl activate` on it and writes one line
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to the journal.
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It is deliberately timid — it never restarts gdm and never logs anyone out, and
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if there is no kiosk session at all it only reports that, because a display
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manager is a bad thing to fight automatically. To stop it fighting you while you
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debug the console by hand:
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```bash
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sudo systemctl stop mirror-kiosk-console-guard.timer
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sudo systemctl start mirror-kiosk-console-guard.timer # when finished
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sudo journalctl -u mirror-kiosk-console-guard.service # what it has done
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```
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Tested by stealing the console with `chvt 1`; the wall came back on its own:
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```
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mirror-kiosk-guard[4598]: kiosk session 1 brought back to the foreground
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```
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## Verifying a kiosk you cannot see
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The console is not an interface. Every claim below is checkable over SSH.
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@@ -184,6 +260,8 @@ The console is not an interface. Every claim below is checkable over SSH.
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| Media really flows | the sharer's own page renders each viewer's `RTCPeerConnection.connectionState`; `connected` = ICE+DTLS+SRTP up |
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| It is truly fullscreen | `dim-probe.py`: `viewport == window == screen == available` |
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| The URL is serving *mirror* | `check-kiosk.sh` greps the page for `<title>Watch · mirror` |
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| The kiosk holds the screen | `/sys/class/tty/tty0/active` equals the kiosk session's TTY and the session is `Active` — the one check that catches a covered wall |
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| No greeter is fighting it | `journalctl -b \| grep -c "New session.*gdm-greeter"` is `0`; any non-zero number is a login screen on your wall |
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| It survives a power cut | `sudo systemctl reboot`, then re-run `check-kiosk.sh` |
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Measured on the reference client, which is attached to a 4K output:
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@@ -218,7 +296,9 @@ convenient IP.
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| Symptom | Cause and fix |
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|---|---|
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| Greeter on screen, not the wall | `seat0` logged in as someone else — `sudo systemctl restart gdm`. GDM reads `custom.conf` at start only |
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| **Login screen on the wall, but every remote check passes** | The session is not registered with GDM, so GDM autospawns a greeter over it. See [The greeter that eats the wall](#the-greeter-that-eats-the-wall); confirm with `ls /usr/local/share/wayland-sessions/` |
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| Wall vanished, console on a blank VT | Someone or something switched VT — the guard reclaims it within a minute; `journalctl -u mirror-kiosk-console-guard.service` |
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| Greeter on screen after boot and `seat0` is another user | `sudo systemctl restart gdm`. GDM reads `custom.conf` at start only |
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| Kiosk logs in to a normal desktop | `Session=` empty in `/var/lib/AccountsService/users/kiosk`; re-run the installer (it restarts `accounts-daemon`) |
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| Wall shows a non-mirror page | the URL serves something else. `check-kiosk.sh` calls this out; a stale vhost/DNS looks like a working kiosk showing a login form |
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| Black screen, no browser at all | the URL is unreachable so the loop is waiting. `curl -fsSL <url>` from the client; then look in `~/.local/state/mirror-kiosk.log` |
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@@ -75,6 +75,13 @@ if [ -n "$HOME_DIR" ]; then
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fi
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check_file /etc/firefox/policies/policies.json "DisableAppUpdate" "firefox policy"
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check_file /usr/local/share/gnome-kiosk/window-config.ini "org.mozilla" "kiosk window rules"
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# Without this key the shipped gnome-kiosk session never registers with GDM and
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# GDM autospawns a greeter over the wall ~8s after auto-login. The single most
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# confusing failure this setup has: the kiosk keeps running underneath, so the
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# mirror roster and loginctl both still look perfect while the screen shows a
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# login prompt.
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check_file /usr/local/share/wayland-sessions/gnome-kiosk-script-wayland.desktop \
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"X-GDM-SessionRegisters=true" "GDM session override"
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printf '\n%s\n' '--- session wiring ---'
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ACCT="/var/lib/AccountsService/users/$KIOSK_USER"
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@@ -103,7 +110,34 @@ else
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fi
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SEAT_SESSION=$(loginctl list-sessions --no-legend 2>/dev/null | awk '$4=="seat0"{print $1; exit}')
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if [ -n "$SEAT_SESSION" ]; then
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info "seat0 session $SEAT_SESSION: class=$(loginctl show-session "$SEAT_SESSION" -p Class --value 2>/dev/null) type=$(loginctl show-session "$SEAT_SESSION" -p Type --value 2>/dev/null) vt=$(loginctl show-session "$SEAT_SESSION" -p VTHint --value 2>/dev/null)"
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info "seat0 session $SEAT_SESSION: class=$(loginctl show-session "$SEAT_SESSION" -p Class --value 2>/dev/null) type=$(loginctl show-session "$SEAT_SESSION" -p Type --value 2>/dev/null)"
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fi
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# Does the kiosk actually hold the screen? A session can be Active while another
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# one owns the foreground VT, which is what a covered wall looks like from here.
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KIOSK_SESSION=$(loginctl list-sessions --no-legend 2>/dev/null |
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awk -v u="$KIOSK_USER" '$3==u && $4=="seat0"{print $1; exit}')
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if [ -n "$KIOSK_SESSION" ]; then
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K_ACTIVE=$(loginctl show-session "$KIOSK_SESSION" -p Active --value 2>/dev/null)
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K_TTY=$(loginctl show-session "$KIOSK_SESSION" -p TTY --value 2>/dev/null)
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K_FG=$(sudo cat /sys/class/tty/tty0/active 2>/dev/null)
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if [ "$K_ACTIVE" = "yes" ] && [ -n "$K_TTY" ] && [ "$K_FG" = "$K_TTY" ]; then
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ok "the kiosk holds the console (foreground VT $K_FG)"
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else
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bad "console is on ${K_FG:-?} but the kiosk is on ${K_TTY:-?} (Active=$K_ACTIVE) — the screen is showing something else"
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info "fix: sudo loginctl activate $KIOSK_SESSION, then check the session override above"
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fi
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fi
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# Count the greeter *sessions* GDM created this boot, not every mention: a
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# healthy kiosk creates none, and the autospawn bug creates exactly one soon
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# after auto-login. (journalctl has no --no-legend; using it here once made this
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# test read an empty stream and pass unconditionally — a silent false green.)
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GREETERS=$(sudo journalctl -b --no-pager 2>/dev/null |
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grep -c "New session .* of user 'gdm-greeter'")
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if [ "${GREETERS:-0}" -gt 0 ]; then
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warn "$GREETERS GDM greeter session(s) this boot: the wall may be covered by a login screen"
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info "check X-GDM-SessionRegisters=true in /usr/local/share/wayland-sessions/"
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fi
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if [ -n "$HOME_DIR" ]; then
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@@ -0,0 +1,9 @@
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[Desktop Entry]
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Name=Kiosk Script Session (Wayland Display Server)
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Comment=This session logs you into the session started by ~/.local/bin/gnome-kiosk-script
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Exec=gnome-session --session gnome-kiosk-script
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TryExec=gnome-session
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Type=Application
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DesktopNames=GNOME-Kiosk;GNOME;
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X-GDM-SessionRegisters=true
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X-GDM-CanRunHeadless=true
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@@ -58,7 +58,7 @@ for c in rpm getent restorecon useradd sed awk grep install; do need "$c"; done
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[ -f /etc/os-release ] && grep -q '^ID=fedora' /etc/os-release ||
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say "warning: this does not look like Fedora; continuing anyway"
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say "== 1/6 packages =="
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say "== 1/7 packages =="
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if rpm -q gnome-kiosk gnome-kiosk-script-session >/dev/null 2>&1; then
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say "gnome-kiosk + gnome-kiosk-script-session already present"
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else
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@@ -69,7 +69,7 @@ else
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say "layered the kiosk packages — a reboot is required before they exist"
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fi
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say "== 2/6 kiosk user =="
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say "== 2/7 kiosk user =="
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if id -u "$KIOSK_USER" >/dev/null 2>&1; then
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HOME_DIR=$(getent passwd "$KIOSK_USER" | cut -d: -f6)
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else
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@@ -81,7 +81,7 @@ fi
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KIOSK_GROUP=$(getent passwd "$KIOSK_USER" | cut -d: -f4)
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say "user $KIOSK_USER uid=$(id -u "$KIOSK_USER") home=$HOME_DIR"
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say "== 3/6 payload =="
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say "== 3/7 payload =="
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# Firefox enterprise policy: no update prompts, no account/sync, no telemetry,
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# no first-run pages. /etc is mutable on Silverblue and survives upgrades.
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install -D -m 0644 "$HERE/policies.json" /etc/firefox/policies/policies.json
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@@ -93,6 +93,18 @@ say " /etc/firefox/policies/policies.json"
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install -D -m 0644 "$HERE/window-config.ini" /usr/local/share/gnome-kiosk/window-config.ini
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say " /usr/local/share/gnome-kiosk/window-config.ini"
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# THE GREETER FIX. The shipped gnome-kiosk session sets X-GDM-SessionRegisters=
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# false, so nothing ever registers with GDM and GDM concludes the display is
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# still free: about eight seconds after auto-login it autospawns a full greeter
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# session on the same GPU and a login screen covers the wall. The kiosk keeps
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# running underneath, so loginctl and the mirror roster both still look healthy.
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# Registering makes GDM treat the session as the display's owner. Keep the file
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# name identical to the shipped one and change only that key — inventing a new
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# session id makes GDM quietly fall back to the plain GNOME session.
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install -D -m 0644 "$HERE/gnome-kiosk-script-wayland.desktop" \
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/usr/local/share/wayland-sessions/gnome-kiosk-script-wayland.desktop
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say " /usr/local/share/wayland-sessions/gnome-kiosk-script-wayland.desktop (registers with GDM)"
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# `install -d` creates missing parents but leaves them owned by whoever is
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# running the installer, which is how a kiosk ends up with a root-owned
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# ~/.local and then spends its life logging "Permission denied" about
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@@ -117,7 +129,7 @@ say " $HOME_DIR/.config/mirror/kiosk.conf -> $KIOSK_URL"
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install -m 0644 -o "$KIOSK_USER" -g "$KIOSK_GROUP" /dev/null \
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"$HOME_DIR/.config/gnome-initial-setup-done"
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say "== 4/6 default session =="
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say "== 4/7 default session =="
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ACCT="/var/lib/AccountsService/users/$KIOSK_USER"
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if [ -f "$ACCT" ]; then
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set_ini "$ACCT" User Session "$KIOSK_SESSION"
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@@ -129,7 +141,7 @@ fi
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systemctl restart accounts-daemon.service 2>/dev/null || :
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say " $ACCT Session=$KIOSK_SESSION"
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say "== 5/6 auto-login =="
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say "== 5/7 auto-login =="
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if [ ! -d /etc/gdm ]; then
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say " gdm is not installed — installing it"
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rpm-ostree install gdm
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@@ -144,13 +156,36 @@ if [ -d /etc/gdm ]; then
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say " /etc/gdm/custom.conf: AutomaticLogin=$KIOSK_USER"
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fi
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say "== 6/6 selinux labels =="
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say "== 6/7 console guard =="
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# Cheap insurance against the class of bug above: if the kiosk session exists but
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# something else holds the foreground VT, bring the wall back. Stops short of
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# ever restarting gdm. Pause it while debugging the console with:
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# sudo systemctl stop mirror-kiosk-console-guard.timer
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install -D -m 0755 "$HERE/mirror-kiosk-console-guard" /usr/local/libexec/mirror-kiosk-console-guard
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install -D -m 0644 "$HERE/mirror-kiosk-console-guard.service" /etc/systemd/system/mirror-kiosk-console-guard.service
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install -D -m 0644 "$HERE/mirror-kiosk-console-guard.timer" /etc/systemd/system/mirror-kiosk-console-guard.timer
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systemctl daemon-reload
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systemctl enable --now mirror-kiosk-console-guard.timer
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say " mirror-kiosk-console-guard.timer enabled"
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# Keep a copy of the payload on the box. /tmp is cleared at every boot, so a
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# checker staged there is gone precisely when you want to run it again — after a
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# reboot, or from a rescue session hours later.
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if [ "$HERE" != "/usr/local/share/mirror-kiosk" ]; then
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install -d -m 0755 /usr/local/share/mirror-kiosk
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cp -a "$HERE"/. /usr/local/share/mirror-kiosk/
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say " /usr/local/share/mirror-kiosk (survives reboot; run check-kiosk.sh from here)"
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fi
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say "== 7/7 selinux labels =="
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restorecon -RF "$HOME_DIR/.local" "$HOME_DIR/.config" 2>/dev/null || :
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restorecon -RF /etc/firefox /usr/local/share/gnome-kiosk 2>/dev/null || :
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restorecon -RF /etc/firefox /usr/local/share/gnome-kiosk /usr/local/share/wayland-sessions \
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/usr/local/libexec 2>/dev/null || :
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say ""
|
||||
say "Installed."
|
||||
[ "$NEED_REBOOT" -eq 1 ] && say "NEXT: sudo reboot (new RPMs are only in the new deployment)"
|
||||
say "Then, to start the kiosk now (this closes anything on the console):"
|
||||
say " sudo systemctl restart gdm"
|
||||
say "Check it with: sh check-kiosk.sh"
|
||||
say "Check it with (this path survives reboots):"
|
||||
say " sudo sh /usr/local/share/mirror-kiosk/check-kiosk.sh"
|
||||
|
||||
@@ -0,0 +1,54 @@
|
||||
#!/bin/sh
|
||||
# mirror-kiosk-console-guard — make sure the kiosk owns the console.
|
||||
#
|
||||
# Run by mirror-kiosk-console-guard.timer once a minute as root.
|
||||
#
|
||||
# The failure this exists for is invisible remotely: the kiosk session keeps
|
||||
# running (loginctl says active, the mirror roster counts the wall as a viewer)
|
||||
# while some other session holds the foreground VT, so the screen shows a login
|
||||
# prompt and every check says everything is fine. That is exactly how the
|
||||
# greeter-autospawn bug presented itself.
|
||||
#
|
||||
# It deliberately does very little: if the kiosk HAS a seat0 session and that
|
||||
# session is not the foreground one, activate it. It never restarts gdm and
|
||||
# never logs anyone out, because a display manager is a bad thing to fight
|
||||
# automatically. If there is no kiosk session at all it only says so — that is a
|
||||
# problem for a human, and the message lands in the journal.
|
||||
#
|
||||
# Debugging the console by hand? Stop the fight:
|
||||
# sudo systemctl stop mirror-kiosk-console-guard.timer
|
||||
|
||||
set -eu
|
||||
|
||||
KIOSK_USER="${KIOSK_USER:-kiosk}"
|
||||
FG_VT=/sys/class/tty/tty0/active
|
||||
|
||||
say() { logger -t mirror-kiosk-guard -- "$*"; }
|
||||
|
||||
[ "$(id -u)" -eq 0 ] || { printf 'run me as root\n' >&2; exit 1; }
|
||||
|
||||
# SESSION UID USER SEAT ... — the kiosk's own session on the physical seat. The
|
||||
# greeter also sits on seat0 but under the gdm-greeter user, so the name filter
|
||||
# is what keeps this from chasing the greeter.
|
||||
session=$(loginctl list-sessions --no-legend 2>/dev/null |
|
||||
awk -v u="$KIOSK_USER" '$3 == u && $4 == "seat0" { print $1; exit }')
|
||||
|
||||
if [ -z "$session" ]; then
|
||||
say "no $KIOSK_USER session on seat0: auto-login did not produce one. Not touching gdm."
|
||||
exit 0
|
||||
fi
|
||||
|
||||
active=$(loginctl show-session "$session" -p Active --value 2>/dev/null || echo unknown)
|
||||
tty=$(loginctl show-session "$session" -p TTY --value 2>/dev/null || echo "")
|
||||
foreground=$(cat "$FG_VT" 2>/dev/null || echo "")
|
||||
|
||||
if [ "$active" = "yes" ] && [ -n "$tty" ] && [ "$foreground" = "$tty" ]; then
|
||||
exit 0 # the wall already has the screen
|
||||
fi
|
||||
|
||||
say "console is on ${foreground:-unknown}, kiosk session $session is on ${tty:-unknown} (Active=$active) — activating it"
|
||||
if loginctl activate "$session" 2>/dev/null; then
|
||||
say "kiosk session $session brought back to the foreground"
|
||||
else
|
||||
say "could not activate kiosk session $session — look at journalctl -b -u gdm"
|
||||
fi
|
||||
@@ -0,0 +1,8 @@
|
||||
[Unit]
|
||||
Description=Keep the mirror kiosk on the console
|
||||
# Nothing else may be mid-switch when we look at the console.
|
||||
After=graphical.target
|
||||
|
||||
[Service]
|
||||
Type=oneshot
|
||||
ExecStart=/usr/local/libexec/mirror-kiosk-console-guard
|
||||
@@ -0,0 +1,13 @@
|
||||
[Unit]
|
||||
Description=Check every minute that the mirror kiosk owns the console
|
||||
|
||||
[Timer]
|
||||
# Give the session time to come up first, then check forever.
|
||||
OnBootSec=90s
|
||||
OnUnitActiveSec=60s
|
||||
AccuracySec=15s
|
||||
# If the box was suspended, run once immediately on resume.
|
||||
WakeSystem=false
|
||||
|
||||
[Install]
|
||||
WantedBy=timers.target
|
||||
Reference in New Issue
Block a user