Index: /trunk/src/VBox/Additions/linux/installer/install.sh.in
===================================================================
--- /trunk/src/VBox/Additions/linux/installer/install.sh.in	(revision 66503)
+++ /trunk/src/VBox/Additions/linux/installer/install.sh.in	(revision 66504)
@@ -17,4 +17,11 @@
 #
 
+# Testing:
+# * After successful installation, 0 is returned if the vboxguest module version
+#   built matches the one loaded and 2 is returned otherwise.  E.g. VBoxClient
+#   running will prevent vboxguest reloading.
+# * If the kernel modules cannot be built (run the installer with KERN_DIR=/) or
+#   loaded (run with KERN_VER=<installed non-current version>) then 1 is returned.
+
 PATH=$PATH:/bin:/sbin:/usr/sbin
 
@@ -36,4 +43,7 @@
 SELF=$1
 LOGFILE="/var/log/vboxadd-install.log"
+
+## Were we able to stop any previously running Additions kernel modules?
+MODULES_STOPPED=1
 
 . "./$ROUTINES"
@@ -320,4 +330,7 @@
 test "$ACTION" = "install" || exit 0
 
+# Now check whether the kernel modules were stopped.
+lsmod | grep -q vboxguest && MODULES_STOPPED=
+
 # Choose a proper umask
 umask 022
@@ -564,2 +577,13 @@
     echo >&2 "  ldconfig"
 fi
+
+# And do a final test as to whether the kernel modules were properly created
+# and loaded.  Return 0 if both are true, 1 if the modules could not be built
+# or loaded (except due to already running older modules) and 2 if already
+# running modules probably prevented new ones from loading.  vboxvideo is
+# currently not tested.
+# Assume that we have already printed enough messages by now and stay silent.
+modinfo vboxguest >/dev/null 2>&1 || exit 1
+lsmod | grep -q vboxguest || exit 1
+test -n "${MODULES_STOPPED}" || exit 2
+exit 0