For llGetMass(), return the mass of the avatar is the object is attached.

As per http://lslwiki.net/lslwiki/wakka.php?wakka=llGetMass
This is the mass as used by the physics engine (ODE or Bullet).

OpenSim/Region/Framework/Scenes/ScenePresence.cs

@@ -3514,6 +3514,22 @@ namespace OpenSim.Region.Framework.Scenes
             });
         }
 
+        /// <summary>
+        /// Gets the mass.
+        /// </summary>
+        /// <returns>
+        /// The mass.
+        /// </returns>
+        public float GetMass()
+        {
+            PhysicsActor pa = PhysicsActor;
+
+            if (pa != null)
+                return pa.Mass;
+            else
+                return 0;
+        }
+
         internal void PushForce(Vector3 impulse)
         {
             if (PhysicsActor != null)

OpenSim/Region/ScriptEngine/Shared/Api/Implementation/LSL_Api.cs

@@ -2907,10 +2907,31 @@ namespace OpenSim.Region.ScriptEngine.Shared.Api
         public LSL_Float llGetMass()
         {
             m_host.AddScriptLPS(1);
-            if (m_host.IsRoot)
+
-                return m_host.ParentGroup.GetMass();
+            if (m_host.ParentGroup.IsAttachment)
+            {
+                ScenePresence attachedAvatar = World.GetScenePresence(m_host.ParentGroup.AttachedAvatar);
+
+                if (attachedAvatar != null)
+                {
+                    return attachedAvatar.GetMass();
+                }
+                else
+                {
+                    return 0;
+                }
+            }
             else
-                return m_host.GetMass();
+            {
+                if (m_host.IsRoot)
+                {
+                    return m_host.ParentGroup.GetMass();
+                }
+                else
+                {
+                    return m_host.GetMass();
+                }
+            }
         }
 
         public void llCollisionFilter(string name, string id, int accept)