Merge lp:~lbssousa/lightdm/guest-account-union-mount-skel into lp:lightdm

[Laércio de Sousa]

This patch proposes an alternative mechanism to customize guest sessions, based on union-mounting /etc/guest-session/skel with guest home directory, rather than just copying over contents from one to the other.

It *may* turn guest logins a bit faster (I've never measured it, however), and it avoids copying large files from /etc/guest-session/skel (e.g. a whole .wine directory) to a limited tmpfs home for guest accounts.

For each guest account created, it wraps /etc/guest-session/skel in a dedicated BindFS mount, so that the account in question will see itself as the owner of that directory's contents. If BindFS is not available, it will fall back directly to current copy over method.

If OverlayFS kernel module is available in the system, use it. However, if only AuFS is available, use it instead. If none of them is available, fall back to current copy over method. If /etc/guest-session/skel is not available, nothing changes: it'll continue copying over from /etc/skel (no union-mounting will be performed in this case).

[lp:~lbssousa/lightdm/guest-account-union-mount-skel]

2198. By Laércio de Sousa on 2015-10-09

Revert some changes and make a per-user bindfs mount, rather than a shared bindfs mount for all guest accounts. Multi-seat guest logins break if we have a single shared bindfs mount.

2199. By Laércio de Sousa on 2015-10-09

Fix missing redefinition of PRE_HOME variable.

[Robert Ancell (robert-ancell)]

Looks good!

review: Approve

[Laércio de Sousa (lbssousa)]

Robert, could you please update this merge with a last-minute patch to add a -r (read-only option) for bindfs mounts? Consider also including packages "bindfs" and "linux-image>=3.18 (to ensure overlayfs is available) | aufs-tools" in "Suggests:" field for lightdm package.

[lp:~lbssousa/lightdm/guest-account-union-mount-skel]

2200. By Laércio de Sousa on 2015-10-13

Enforce read-only mode for all BindFS mounts to minimize risk of /etc/guest-session/skel corruption.

[Robert Ancell (robert-ancell)]

Can you do another MP for these changes? It will be easier as you know the exact changes.

Unmerged revisions

=== modified file 'debian/guest-account.sh'
--- debian/guest-account.sh	2015-01-17 18:18:30 +0000
+++ debian/guest-account.sh	2015-10-09 19:39:23 +0000
@@ -22,6 +22,7 @@
 {
   HOME=`mktemp -td guest-XXXXXX`
   USER=`echo $HOME | sed 's/\(.*\)guest/guest/'`
+  PRE_HOME="/tmp/.pre-${USER}"
 
   # if $USER already exists, it must be a locked system account with no existing
   # home directory
@@ -49,20 +50,72 @@
     adduser --system --no-create-home --home / --gecos $(gettext "Guest") --group --shell /bin/bash $USER || {
         umount "$HOME"
         rm -rf "$HOME"
+        umount "$PRE_HOME"
+        rm -rf "$PRE_HOME"
         exit 1
     }
   fi
 
-  # create temporary home directory
-  mount -t tmpfs -o mode=700 none "$HOME" || { rm -rf "$HOME"; exit 1; }
-  chown $USER:$USER "$HOME"
   gs_skel=/etc/guest-session/skel/
+
   if [ -d "$gs_skel" ] && [ -n "`find $gs_skel -type f`" ]; then
-    cp -rT $gs_skel "$HOME"
+    # Only perform union-mounting if BindFS is available
+    if [ -x /usr/bin/bindfs ]; then
+      # create temporary home directory
+      mkdir "$PRE_HOME"
+      mount -t tmpfs -o mode=700 none "$PRE_HOME" || { rm -rf "$PRE_HOME" "$HOME"; exit 1; }
+      mkdir ${PRE_HOME}/lower ${PRE_HOME}/upper
+      chown -R $USER:$USER "$PRE_HOME"
+
+      # Wrap ${gs_skel} in a BindFS mount, so that
+      # guest account will see itself as the owner of ${gs_skel}'s contents.
+      bindfs -M $USER $gs_skel ${PRE_HOME}/lower || {
+        rm -rf "$PRE_HOME"
+        rm -rf "$HOME"
+        exit 1
+      }
+
+      # Try OverlayFS first
+      if modinfo -n overlay >/dev/null 2>&1; then
+        mkdir ${PRE_HOME}/work
+        chown $USER:$USER ${PRE_HOME}/work
+        mount -t overlay -o lowerdir=${PRE_HOME}/lower,upperdir=${PRE_HOME}/upper,workdir=${PRE_HOME}/work overlay $HOME || {
+          umount ${PRE_HOME}/lower
+          umount "$PRE_HOME"
+          rm -rf "$PRE_HOME"
+          rm -rf "$HOME"
+          exit 1
+        }
+      # If OverlayFS is not available, try AuFS
+      elif [ -x /sbin/mount.aufs ]; then
+        mount -t aufs -o br=${PRE_HOME}/upper:${PRE_HOME}/lower none $HOME || {
+          umount ${PRE_HOME}/lower
+          umount "$PRE_HOME"
+          rm -rf "$PRE_HOME"
+          rm -rf "$HOME"
+          exit 1
+        }
+      # If none of them is available, fall back to copy over
+      else
+        umount ${PRE_HOME}/lower
+        umount "$PRE_HOME"
+        rm -rf "$PRE_HOME"
+        mount -t tmpfs -o mode=700 none "$HOME" || { rm -rf "$HOME"; exit 1; }
+        cp -rT $gs_skel "$HOME"
+        chown -R $USER:$USER "$HOME"
+      fi
+    # If BindFS is not available, just fall back to copy over
+    else
+      mount -t tmpfs -o mode=700 none "$HOME" || { rm -rf "$HOME"; exit 1; }
+      cp -rT $gs_skel "$HOME"
+      chown -R $USER:$USER "$HOME"
+    fi
   else
+    mount -t tmpfs -o mode=700 none "$HOME" || { rm -rf "$HOME"; exit 1; }
     cp -rT /etc/skel/ "$HOME"
+    chown -R $USER:$USER "$HOME"
   fi
-  chown -R $USER:$USER "$HOME"
+
   usermod -d "$HOME" "$USER"
 
   #
@@ -127,6 +180,7 @@
   }
   GUEST_UID=`echo "$PWENT" | cut -f3 -d:`
   GUEST_HOME=`echo "$PWENT" | cut -f6 -d:`
+  GUEST_PRE_HOME=/tmp/.pre-$GUEST_USER
 
   if [ "$GUEST_UID" -ge 500 ]; then
     echo "Error: user $GUEST_USER is not a system user."
@@ -146,6 +200,9 @@
 
   umount "$GUEST_HOME" || umount -l "$GUEST_HOME" || true
   rm -rf "$GUEST_HOME"
+  umount ${GUEST_PRE_HOME}/lower || umount -l ${GUEST_PRE_HOME}/lower || true
+  umount "$GUEST_PRE_HOME" || umount -l "$GUEST_PRE_HOME" || true
+  rm -rf "$GUEST_PRE_HOME"
 
   # remove leftovers in /tmp
   find /tmp -mindepth 1 -maxdepth 1 -uid "$GUEST_UID" -print0 | xargs -0 rm -rf || true