Update validate_user_pointer to start from the beginning of the page rather than the given ptr

src/userprog/syscall.c

@@ -475,7 +475,7 @@ validate_user_pointer (const void *start, size_t size)
 
   /* We now need to check if the entire memory block is mapped to physical
      memory by the page table. */
-  for (const void *ptr = start; ptr <= end; ptr += PGSIZE)
+  for (const void *ptr = pg_round_down (start); ptr <= end; ptr += PGSIZE)
     if (pagedir_get_page (thread_current ()->pagedir, ptr) == NULL)
       syscall_exit (EXIT_FAILURE);
 }