diff --git a/kernel_module/fops.c b/kernel_module/fops.c
index 95cd2e1..1f7588a 100644
--- a/kernel_module/fops.c
+++ b/kernel_module/fops.c
@@ -30,13 +30,18 @@ int fop_open(struct inode *inode, struct file *file)
 ssize_t fop_read(struct file *file, char __user *buf, size_t len, loff_t *off)
 {
 	printk(KERN_INFO "read\n");
+	printk(KERN_INFO "len = %zu\n", len);
+	/* TODO. */
+	/*copy_to_user(arg, &msg, size);*/
 	return 0;
 }
 
 ssize_t fop_write(struct file *file, const char __user *buf, size_t len, loff_t *off)
 {
 	printk(KERN_INFO "write\n");
-	return 0;
+	printk(KERN_INFO "buf = %.*s\n", (int)len, buf);
+	printk(KERN_INFO "len = %zu\n", len);
+	return len;
 }
 
 /*
diff --git a/rootfs_overlay/fops.sh b/rootfs_overlay/fops.sh
index 7a7836d..e394d0e 100755
--- a/rootfs_overlay/fops.sh
+++ b/rootfs_overlay/fops.sh
@@ -4,9 +4,16 @@ insmod /fops.ko
 mkdir -p /fops
 mount -t debugfs none /fops
 cd /fops/kernel_module_cheat
+
 cat fops
 # => open
 # => read
+# => len = [0-9]+
+# => close
+
+printf a >fops
+# => open
+# => write
+# => len = a
+# => len = 1
 # => close
-# TODO.
-#echo a >fops