Only check user pages rather than all bytes in-between, for known-size pointers

src/userprog/syscall.c

@@ -421,7 +421,8 @@ validate_user_pointer (const void *ptr, size_t size, bool check_write)
   void *last = ptr + size - 1;
   if (!is_user_vaddr (last))
     thread_exit ();
-  for (; ptr <= last; ptr++)
+  ptr = pg_round_down (ptr);
+  while (ptr <= last)
     {
       int result;
       /* Check read access to pointer. */
@@ -430,6 +431,7 @@ validate_user_pointer (const void *ptr, size_t size, bool check_write)
       /* Check write access to pointer (if required). */
       if (check_write && !put_user (ptr, result))
         thread_exit ();
+      ptr += PGSIZE;
     }
 }