Implement function to validate user memory pointers w/ S.

src/userprog/syscall.c

@@ -1,5 +1,6 @@
 #include "userprog/syscall.h"
 #include "devices/shutdown.h"
+#include "threads/vaddr.h"
 #include <stdio.h>
 #include <syscall-nr.h>
 #include "threads/interrupt.h"
@@ -51,4 +52,15 @@ static void
 exit (int status)
 {
   //TODO
+}
+
+/* A function to validate if a block of memory starting at PTR and of
+   size SIZE bytes is fully contained within user virtual memory.  */
+static void *
+validate_user_pointer (void *ptr, size_t size)
+{
+  if (ptr == NULL || !is_user_vaddr (ptr) || !is_user_vaddr (ptr + size - 1))
+    thread_exit ();
+
+  return ptr;
 }