Fix bug in userprog-merge where file writes were denied in the wrong thread

src/userprog/process.c

@@ -126,9 +126,16 @@ start_process (void *proc_start_data)
   if_.gs = if_.fs = if_.es = if_.ds = if_.ss = SEL_UDSEG;
   if_.cs = SEL_UCSEG;
   if_.eflags = FLAG_IF | FLAG_MBS;
-
+  
   lock_acquire (&filesys_lock);
+  
+  /* Prevent writing to the file being executed. */
+  struct file *exec_file = filesys_open (data->file_name);
+  thread_current ()->exec_file = exec_file;
+  file_deny_write(exec_file);
+
   success = load (data->file_name, &if_.eip, &if_.esp);
+  
   lock_release (&filesys_lock);
 
   /* If load failed, quit. */