From 5ec220d0a692022a36a558e1ccd3c12ccc5f7412 Mon Sep 17 00:00:00 2001
From: "Michael N. Lipp" <mnl@mnl.de>
Date: Thu, 27 Feb 2025 17:41:57 +0100
Subject: [PATCH] Use gids for id management and isolate home directories.

---
 dev-example/vmop-agent/vmop-agent | 43 +++++++++++++++++++++++++++----
 1 file changed, 38 insertions(+), 5 deletions(-)

diff --git a/dev-example/vmop-agent/vmop-agent b/dev-example/vmop-agent/vmop-agent
index ab4e765..99aa25d 100755
--- a/dev-example/vmop-agent/vmop-agent
+++ b/dev-example/vmop-agent/vmop-agent
@@ -15,28 +15,45 @@ if [ ! -w "$ttyPath" ]; then
   exit 1
 fi
 
+# Create fd for the tty in variable con 
 if ! exec {con}<>"$ttyPath"; then
   echo >&2 "Cannot open device $ttyPath"
   exit 1
 fi
 
+# Temporary file for logging error messages, clear tty and signal ready
 temperr=$(mktemp)
 clear >/dev/tty1
 echo >&${con} "220 Hello"
 
+# This script uses the (shared) home directory as "dictonary" for
+# synchronizing the username and the uid between hosts.
+#
+# Every user has a directory with his username. The directory is
+# owned by root to prevent changes of access rights by the user.
+# The uid and gid of the directory are equal. Thus the name of the
+# directory and the id from the group ownership also provide the
+# association between the username and the uid.
+
+# Add the user with name $1 to the host's "user database". This
+# may not be invoked concurrently. 
 createUser() {
   local missing=$1
   local uid
   local userHome="/home/$missing"
   local createOpts=""
+  
+  # Retrieve or create the uid for the username
   if [ -d "$userHome" ]; then
-    uid=$(ls -ldn "$userHome" | head -n 1 | awk '{print $3}')
+    # If a home directory exists, use the id from the group ownership as uid
+    uid=$(ls -ldn "$userHome" | head -n 1 | awk '{print $4}')
     createOpts="--no-create-home"
   else
-    uid=$(ls -ln "/home" | tail -n +2 | awk '{print $3}' | sort | tail -1)
+    # Else get the maximum of all ids from the group ownership +1 
+    uid=$(ls -ln "/home" | tail -n +2 | awk '{print $4}' | sort | tail -1)
     uid=$(( $uid + 1 ))
-    if [ $uid -lt 1000 ]; then
-      uid=1000
+    if [ $uid -lt 1100 ]; then
+      uid=1100
     fi
     createOpts="--create-home"
   fi
@@ -44,33 +61,45 @@ createUser() {
   useradd $missing -u $uid -g $uid $createOpts
 }
 
+# Login the user, i.e. create a desktopn for the user.
 doLogin() {
   user=$1
   if [ "$user" = "root" ]; then
     echo >&${con} "504 Won't log in root"
     return
   fi
+  
+  # Check if this user is already logged in on tty1
   curUser=$(loginctl -j | jq -r '.[] | select(.tty=="tty1") | .user')
   if [ "$curUser" = "$user" ]; then
     echo >&${con} "201 User already logged in"
     return
   fi
+  
+  # Terminate a running desktop (fail safe)
   attemptLogout
+  
+  # Check if username is known on this host. If not, create user
   uid=$(id -u ${user} 2>/dev/null)
   if [ $? != 0 ]; then
     ( flock 200
       createUser ${user}
     ) 200>/home/.gen-uid-lock
+    
+    # This should now work, else something went wrong
     uid=$(id -u ${user} 2>/dev/null)
     if [ $? != 0 ]; then
       echo >&${con} "451 Cannot determine uid"
       return
     fi
   fi
+  
+  # Start the desktop for the user
   systemd-run 2>$temperr \
     --unit vmop-user-desktop --uid=$uid --gid=$uid \
     --working-directory="/home/$user" -p TTYPath=/dev/tty1 \
     -p PAMName=login -p StandardInput=tty -p StandardOutput=journal \
+    -p Conflicts="gdm.service getty@tty1.service" \
     -E XDG_RUNTIME_DIR="/run/user/$uid" \
     -p ExecStartPre="/usr/bin/chvt 1" \
     dbus-run-session -- gnome-shell --display-server --wayland
@@ -81,6 +110,8 @@ doLogin() {
   fi
 }
 
+# Attempt to log out a user currently using tty1. This is an intermediate
+# operation that can be invoked from other operations
 attemptLogout() {
   systemctl status vmop-user-desktop > /dev/null 2>&1
   if [ $? = 0 ]; then
@@ -93,6 +124,8 @@ attemptLogout() {
   echo >&${con} "102 Desktop stopped"
 }
 
+# Log out any user currently using tty1. This is invoked when executing
+# the logout command and therefore sends back a 2xx return code.
 doLogout() {
   attemptLogout
   echo >&${con} "202 User logged out"
@@ -101,7 +134,7 @@ doLogout() {
 while read line <&${con}; do
   case $line in
   "login "*) IFS=' ' read -ra args <<< "$line"; doLogin ${args[1]};;
-  logout) doLogout;;
+  "logout") doLogout;;
   esac
 done