app_containers: Move to using CONTAINER_BIND_MOUNTS for extra binds

...instead of the overly generic SYSTEMD_NSPAWN_EXTRA_ARGS variable.

app_containers/.local/bin/run_app_container

@@ -5,9 +5,24 @@ die() {
   exit 1
 }
 
+cleanup() {
+  rm -rf "$container_xdg_runtime"
+  # Remove the temporary facl-based permissions
+  setfacl -x u:$((user_on_host)) $XDG_RUNTIME_DIR/$WAYLAND_DISPLAY
+  xhost -si:localuser:\#$((user_on_host))
+
+  for input in $(find /dev/input -type c); do
+    sudo setfacl -x u:$user_on_host $input
+  done
+}
+
 [ -z "$CONTAINER_NAME" ] && die "\$CONTAINER_NAME not set"
 [ -z "$DISPLAY" ] && die "\$DISPLAY not set (you must run the script in a desktop environment"
 
+# Use a GUI-available askpass program for sudo
+# This should be made configurable
+export SUDO_ASKPASS=$HOME/.local/bin/askpass-bemenu 
+
 # Source configuration files if any
 config="$HOME/.config/app_containers/$CONTAINER_NAME.sh"
 if [ -f "$config" ]; then
@@ -15,8 +30,8 @@ if [ -f "$config" ]; then
 fi
 
 # Create a XDG_RUNTIME_DIR for guest on host
-container_xdg_runtime="$(mktemp -d)"
-trap 'rm -rf -- "$container_xdg_runtime"' EXIT
+container_xdg_runtime="$(mktemp -d -p /var/tmp)"
+trap cleanup EXIT
 
 # Link the current wayland session to the container's xdg runtime
 # Note that the session itself must be bind-mounted first
@@ -37,17 +52,52 @@ fi
 
 [ -S $host_pulse ] || die "PulseAudio UNIX socket not found"
 
-# Default username (assume `user` always has the same uid as the host user)
-run_as=user
-homedir=/home/user
+# Default user
+run_as=$UID
 if [ "$CONTAINER_RUN_AS_ROOT" = true ]; then
-  run_as=root
-  homedir=/root
+  run_as=0
+fi
+
+homedir=/
+for line in $(sudo -A cat /var/lib/machines/$CONTAINER_NAME/etc/passwd); do
+  if [ "$(echo "$line" | cut -d: -f3)" == "$run_as" ]; then
+    homedir="$(echo "$line" | cut -d: -f6)"
+    break
+  fi
+done
+
+# Userns-related config
+# Default to identity mapping, which does not provide uid isolation but does for capabilities
+private_users=identity
+bind_opts=""
+user_on_host=$run_as
+if [ "$CONTAINER_USE_USERNS" = true ]; then
+  private_users=$(shuf -i 65536-$((2147483647 - 65536)) -n1) # Pick a random starting offset
+  bind_opts="idmap"
+  user_on_host=$((private_users + run_as))
 fi
 
-SUDO_ASKPASS=$HOME/.local/bin/askpass-bemenu sudo -A systemd-nspawn -M $CONTAINER_NAME \
-    `# This doesn't provide userns isolation, but it does provide capability isolation` \
-    --private-users=identity \
+# Grant the user inside the container access to the Wayland / Xorg display
+# For the Wayland socket, a simple facl rule would suffice
+# For Xorg, we need to use the `xhost` facilities
+setfacl -m u:$user_on_host:rwx $XDG_RUNTIME_DIR/$WAYLAND_DISPLAY
+xhost +si:localuser:\#$user_on_host
+
+# Grant the user inside the container access to input devices
+# Note: any new device plugged in when the container is running would not
+# be added properly here.
+for input in $(find /dev/input -type c); do
+  sudo -A setfacl -m u:$user_on_host:rw- $input
+done
+
+# Bind-mounts defined by the user (possibly in the container-specific config file)
+# Format should be "src:target". target cannot be omitted
+for mount in ${CONTAINER_BIND_MOUNTS[@]}; do
+  SYSTEMD_NSPAWN_EXTRA_ARGS+=" --bind=$mount:$bind_opts"
+done
+
+sudo -A systemd-nspawn -M $CONTAINER_NAME \
+    --private-users=$private_users --private-users-ownership=map \
     `# DNS (when containers do not have their own netns)` \
     --bind-ro=/run/systemd/resolve/stub-resolv.conf:/etc/resolv.conf \
     `# GPU` \
@@ -57,7 +107,7 @@ SUDO_ASKPASS=$HOME/.local/bin/askpass-bemenu sudo -A systemd-nspawn -M $CONTAINE
     --bind-ro=/dev/input \
     --property=DeviceAllow='char-input r' \
     `# Xdg runtime` \
-    --bind=$container_xdg_runtime:/run/xdg \
+    --bind=$container_xdg_runtime:/run/xdg:$bind_opts \
     --setenv=XDG_RUNTIME_DIR=/run/xdg \
     `# Xorg / Xwayland` \
     --bind=/tmp/.X11-unix \