diff --git a/tools/post_process_props.py b/tools/post_process_props.py
index 4fa15bc..397526f 100755
--- a/tools/post_process_props.py
+++ b/tools/post_process_props.py
@@ -16,8 +16,8 @@
 
 import sys
 
-# Usage: post_process_props.py file.prop [blacklist_key, ...]
-# Blacklisted keys are removed from the property file, if present
+# Usage: post_process_props.py file.prop [disallowed_key, ...]
+# Disallowed keys are removed from the property file, if present
 
 # See PROP_VALUE_MAX in system_properties.h.
 # The constant in system_properties.h includes the terminating NUL,
@@ -29,8 +29,8 @@
 def mangle_build_prop(prop_list):
   # If ro.debuggable is 1, then enable adb on USB by default
   # (this is for userdebug builds)
-  if prop_list.get("ro.debuggable") == "1":
-    val = prop_list.get("persist.sys.usb.config")
+  if prop_list.get_value("ro.debuggable") == "1":
+    val = prop_list.get_value("persist.sys.usb.config")
     if "adb" not in val:
       if val == "":
         val = "adb"
@@ -40,52 +40,121 @@
   # UsbDeviceManager expects a value here.  If it doesn't get it, it will
   # default to "adb". That might not the right policy there, but it's better
   # to be explicit.
-  if not prop_list.get("persist.sys.usb.config"):
+  if not prop_list.get_value("persist.sys.usb.config"):
     prop_list.put("persist.sys.usb.config", "none");
 
 def validate(prop_list):
   """Validate the properties.
 
+  If the value of a sysprop exceeds the max limit (91), it's an error, unless
+  the sysprop is a read-only one.
+
+  Checks if there is no optional prop assignments.
+
   Returns:
     True if nothing is wrong.
   """
   check_pass = True
-  for p in prop_list.get_all():
+  for p in prop_list.get_all_props():
     if len(p.value) > PROP_VALUE_MAX and not p.name.startswith("ro."):
       check_pass = False
       sys.stderr.write("error: %s cannot exceed %d bytes: " %
                        (p.name, PROP_VALUE_MAX))
       sys.stderr.write("%s (%d)\n" % (p.value, len(p.value)))
+
+    if p.is_optional():
+      check_pass = False
+      sys.stderr.write("error: found unresolved optional prop assignment:\n")
+      sys.stderr.write(str(p) + "\n")
+
   return check_pass
 
+def override_optional_props(prop_list):
+  """Override a?=b with a=c, if the latter exists
+
+  Overriding is done by deleting a?=b
+  When there are a?=b and a?=c, then only the last one survives
+  When there are a=b and a=c, then it's an error.
+
+  Returns:
+    True if the override was successful
+  """
+  success = True
+  for name in prop_list.get_all_names():
+    props = prop_list.get_props(name)
+    optional_props = [p for p in props if p.is_optional()]
+    overriding_props = [p for p in props if not p.is_optional()]
+    if len(overriding_props) > 1:
+      # duplicated props are allowed when the all have the same value
+      if all(overriding_props[0].value == p.value for p in overriding_props):
+        continue
+      success = False
+      sys.stderr.write("error: found duplicate sysprop assignments:\n")
+      for p in overriding_props:
+        sys.stderr.write("%s\n" % str(p))
+    elif len(overriding_props) == 1:
+      for p in optional_props:
+        p.delete("overridden by %s" % str(overriding_props[0]))
+    else:
+      if len(optional_props) > 1:
+        for p in optional_props[:-1]:
+          p.delete("overridden by %s" % str(optional_props[-1]))
+      # Make the last optional one as non-optional
+      optional_props[-1].optional = False
+
+  return success
+
 class Prop:
 
-  def __init__(self, name, value, comment=None):
+  def __init__(self, name, value, optional=False, comment=None):
     self.name = name.strip()
     self.value = value.strip()
-    self.comment = comment
+    if comment != None:
+      self.comments = [comment]
+    else:
+      self.comments = []
+    self.optional = optional
 
   @staticmethod
   def from_line(line):
     line = line.rstrip('\n')
     if line.startswith("#"):
-      return Prop("", "", line)
+      return Prop("", "", comment=line)
+    elif "?=" in line:
+      name, value = line.split("?=", 1)
+      return Prop(name, value, optional=True)
     elif "=" in line:
       name, value = line.split("=", 1)
-      return Prop(name, value)
+      return Prop(name, value, optional=False)
     else:
       # don't fail on invalid line
       # TODO(jiyong) make this a hard error
-      return Prop("", "", line)
+      return Prop("", "", comment=line)
 
   def is_comment(self):
-    return self.comment != None
+    return bool(self.comments and not self.name)
+
+  def is_optional(self):
+    return (not self.is_comment()) and self.optional
+
+  def make_as_comment(self):
+    # Prepend "#" to the last line which is the prop assignment
+    if not self.is_comment():
+      assignment = str(self).rsplit("\n", 1)[-1]
+      self.comments.append("#" + assignment)
+      self.name = ""
+      self.value = ""
+
+  def delete(self, reason):
+    self.comments.append("# Removed by post_process_props.py because " + reason)
+    self.make_as_comment()
 
   def __str__(self):
-    if self.is_comment():
-      return self.comment
-    else:
-      return self.name + "=" + self.value
+    assignment = []
+    if not self.is_comment():
+      operator = "?=" if self.is_optional() else "="
+      assignment.append(self.name + operator + self.value)
+    return "\n".join(self.comments + assignment)
 
 class PropList:
 
@@ -94,25 +163,35 @@
       self.props = [Prop.from_line(l)
                     for l in f.readlines() if l.strip() != ""]
 
-  def get_all(self):
+  def get_all_props(self):
     return [p for p in self.props if not p.is_comment()]
 
-  def get(self, name):
+  def get_all_names(self):
+    return set([p.name for p in self.get_all_props()])
+
+  def get_props(self, name):
+    return [p for p in self.get_all_props() if p.name == name]
+
+  def get_value(self, name):
+    # Caution: only the value of the first sysprop having the name is returned.
     return next((p.value for p in self.props if p.name == name), "")
 
   def put(self, name, value):
-    index = next((i for i,p in enumerate(self.props) if p.name == name), -1)
+    # Note: when there is an optional prop for the name, its value isn't changed.
+    # Instead a new non-optional prop is appended, which will override the
+    # optional prop. Otherwise, the new value might be overridden by an existing
+    # non-optional prop of the same name.
+    index = next((i for i,p in enumerate(self.props)
+                  if p.name == name and not p.is_optional()), -1)
     if index == -1:
-      self.props.append(Prop(name, value))
+      self.props.append(Prop(name, value,
+                             comment="# Auto-added by post_process_props.py"))
     else:
+      self.props[index].comments.append(
+          "# Value overridden by post_process_props.py. Original value: %s" %
+          self.props[index].value)
       self.props[index].value = value
 
-  def delete(self, name):
-    index = next((i for i,p in enumerate(self.props) if p.name == name), -1)
-    if index != -1:
-      new_comment = "# removed by post_process_props.py\n#" + str(self.props[index])
-      self.props[index] = Prop.from_line(new_comment)
-
   def write(self, filename):
     with open(filename, 'w+') as f:
       for p in self.props:
@@ -127,12 +206,15 @@
 
   props = PropList(filename)
   mangle_build_prop(props)
+  if not override_optional_props(props):
+    sys.exit(1)
   if not validate(props):
     sys.exit(1)
 
-  # Drop any blacklisted keys
+  # Drop any disallowed keys
   for key in argv[2:]:
-    props.delete(key)
+    for p in props.get_props(key):
+      p.delete("%s is a disallowed key" % key)
 
   props.write(filename)