add file_deny_write and file_allow_write to process creation and exiting to make executable read-only

src/threads/thread.h

@@ -127,6 +127,7 @@ struct thread
     struct process_result *result;      /* Result of the process. */
     struct list child_results;          /* List of children's of this thread
                                            process results. */
+    struct file *exec_file;             /* Thread's currently running file */
 
     /* Shared between thread.c and synch.c. */
     struct list_elem elem;              /* List element. */

src/userprog/process.c

@@ -181,6 +181,10 @@ start_process (void *file_name_)
   if (!success) 
     thread_exit ();
 
+  struct file *exec_file = filesys_open (file_name);
+  thread_current ()->exec_file = exec_file;
+  file_deny_write (exec_file);
+
   /* Start the user process by simulating a return from an
      interrupt, implemented by intr_exit (in
      threads/intr-stubs.S).  Because intr_exit takes all of its
@@ -246,6 +250,7 @@ process_exit (void)
   uint32_t *pd;
 
   printf ("%s: exit(%d)\n", cur->name, cur->exit_status);
+  file_close (cur->exec_file);
 
   /* Update process result. */
   if (cur->result != NULL)