app_containers: Optionally support user namespaces

using the env variable CONTAINER_USE_USERNS

app_containers/.local/bin/run_app_container

@@ -5,6 +5,14 @@ die() {
   exit 1
 }
 
+exit() {
+  rm -rf "$container_xdg_runtime"
+  if [ "$CONTAINER_USE_USERNS" = true ]; then
+    setfacl -x u:$((private_users + 1000)) $XDG_RUNTIME_DIR/$WAYLAND_DISPLAY
+    xhost -si:localuser:\#$((private_users + 1000))
+  fi
+}
+
 [ -z "$CONTAINER_NAME" ] && die "\$CONTAINER_NAME not set"
 [ -z "$DISPLAY" ] && die "\$DISPLAY not set (you must run the script in a desktop environment"
 
@@ -15,8 +23,8 @@ 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 exit EXIT
 
 # Link the current wayland session to the container's xdg runtime
 # Note that the session itself must be bind-mounted first
@@ -45,9 +53,20 @@ if [ "$CONTAINER_RUN_AS_ROOT" = true ]; then
   homedir=/root
 fi
 
+# Userns-related config
+# Default to identity mapping, which does not provide uid isolation but does for capabilities
+private_users=identity
+bind_opts=""
+if [ "$CONTAINER_USE_USERNS" = true ]; then
+  private_users=$(shuf -i 65536-$((2147483647 - 65536)) -n1) # Pick a random starting offset
+  bind_opts="idmap" # Note: custom mounts specified by the user are expected to incldue the idmap option as well
+  # Grant the user inside the namespace access to the Wayland / Xorg display
+  setfacl -m u:$((private_users + 1000)):rwx $XDG_RUNTIME_DIR/$WAYLAND_DISPLAY
+  xhost +si:localuser:\#$((private_users + 1000))
+fi
+
 SUDO_ASKPASS=$HOME/.local/bin/askpass-bemenu sudo -A systemd-nspawn -M $CONTAINER_NAME \
-    `# This doesn't provide userns isolation, but it does provide capability isolation` \
-    --private-users=identity \
+    --private-users=$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 +76,7 @@ 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 \