Still a lot of bugs :(

os/src/fs/stdio.rs

@@ -14,6 +14,7 @@ impl File for Stdin {
     }
     fn read(&self, mut user_buf: UserBuffer) -> usize {
         assert_eq!(user_buf.len(), 1);
+        println!("before UART.read() in Stdin::read()");
         let ch = UART.read();
         unsafe {
             user_buf.buffers[0].as_mut_ptr().write_volatile(ch);