From 3207df143a96c86e406c3c9dc1990a0642b9f98e Mon Sep 17 00:00:00 2001
From: Kyle K <kylek389@gmail.com>
Date: Tue, 25 Dec 2012 21:51:47 -0600
Subject: implement write, use semaphores

---
 Makefile |  5 ++++-
 debug.c  |  3 +--
 main.c   | 70 +++++++++++++++++++++++++++++++++++++++++++++++++++++-----------
 main.h   |  2 ++
 4 files changed, 65 insertions(+), 15 deletions(-)

diff --git a/Makefile b/Makefile
index 4b897db..eba35e6 100644
--- a/Makefile
+++ b/Makefile
@@ -24,5 +24,8 @@ KERNELDIR ?= /lib/modules/$(shell uname -r)/build
 all:
 	make -C $(KERNELDIR) KCPPFLAGS="-I$(CURDIR)" M=$(PWD) modules
 
-clean:
+kclean:
 	make -C $(KERNELDIR) M=$(PWD) clean
+
+clean:
+	@rm -rf *.o *~ core .depend .*.cmd *.ko *.mod.c .tmp_versions Module.symvers modules.order
diff --git a/debug.c b/debug.c
index 5c746d1..ab0add8 100644
--- a/debug.c
+++ b/debug.c
@@ -1,5 +1,4 @@
-/*
- * debug.c
+/* debug.c
  *
  * utilize debufs
  * FIXME: R/W from /sys/kernel/debug/hello/magic doesn't work
diff --git a/main.c b/main.c
index d791d62..fcd5732 100644
--- a/main.c
+++ b/main.c
@@ -1,3 +1,23 @@
+/* main.c
+ *
+ * r/w /dev/helloc0 character device
+ *
+ * to insert module and call mknod run:
+ * # ./hello_load.sh
+ *
+ * to remove module and cleanup run:
+ * # ./hello_unload.sh
+ *
+ * # cat /dev/helloc0
+ * # echo -n foobar > /dev/helloc0
+ *
+ * notes:
+ * - most notes are handwritten in my book, Device Drivers 3rd
+ * - ssize_t is an int, size_t is unsigned int, be careful
+ * - compile kernel with frame pointers to get better stack readout in case of an oops
+ *
+ */
+
 #include <linux/module.h>
 #include <linux/version.h>
 #include <linux/kernel.h>
@@ -37,31 +57,56 @@ static int hello_create_debugfs(void)
 static ssize_t hello_read(struct file *filp, char __user *buff, size_t count, loff_t *f_pos)
 {
     struct hello_dev *dev = filp->private_data;
+    ssize_t ret = 0; /* end of file, also in case of out of bound */
+
+    if (down_interruptible(&dev->sem))
+        return -ERESTARTSYS;
+
+    if (*f_pos >= HELLO_KERNEL_BUFF_LEN)
+        goto out; /* out of bound read */
 
     if (*f_pos + count > HELLO_KERNEL_BUFF_LEN)
-        count = HELLO_KERNEL_BUFF_LEN - *f_pos;
+        count = HELLO_KERNEL_BUFF_LEN - *f_pos; /* truncate the read */
 
-    if (count > 0)
+    if (copy_to_user(buff, dev->hello_buffer + *f_pos, count)) /* write to userspace memory */
     {
-        copy_to_user(buff, dev->hello_buffer + *f_pos, count);
-        *f_pos += count; /* yes we have to keep track */
-        return count;
+        ret = -EFAULT;
+        goto out;
     }
 
-    return 0; /* end of file, also in case of out of bound */
+    *f_pos += count; /* yes we have to keep track */
+    dev->buff_index += count;
+    ret= count;
+
+  out:
+    up(&dev->sem);
+    return ret;
 }
 
 static ssize_t hello_write(struct file *filp, const char __user *buff, size_t count, loff_t *f_pos)
 {
     struct hello_dev *dev = filp->private_data;
+    ssize_t ret = 0;
 
-#if 0
-    /* we will truncate hello_buffer */
-    copy_from_user(dev->hello_buffer + *f_pos, buff, count);
-    return count;
-#endif
+    if (down_interruptible(&dev->sem))
+        return -ERESTARTSYS;
 
-    return 0;
+    if (count > HELLO_KERNEL_BUFF_LEN)
+        goto out;
+
+    if (*f_pos + count > HELLO_KERNEL_BUFF_LEN)
+        count = HELLO_KERNEL_BUFF_LEN - *f_pos;
+
+    if (copy_from_user(dev->hello_buffer + *f_pos, buff, count))
+    {
+        ret = -EFAULT;
+        goto out;
+    }
+    ret = count;
+
+  out:
+    up(&dev->sem);
+    return ret;
 }
 
 /* there's a count in filp that gets incremented every time our char device gets opened, but note that
@@ -127,6 +172,7 @@ static int __init hello_init(void)
     }
     memset(hello_device, 0, sizeof(struct hello_dev));
     hello_device->devnum = devnum;
+    sema_init(&hello_device->sem, 1);
     strcpy(hello_device->hello_buffer, "write to me\n");
 
     /* setup char_dev structure */
diff --git a/main.h b/main.h
index e437853..db38aa8 100644
--- a/main.h
+++ b/main.h
@@ -16,7 +16,9 @@ extern char *magicstr;
 
 struct hello_dev {
     dev_t devnum;
+    struct semaphore sem;
     struct cdev cdev; /* char device */
+
     /* for r/w operations */
     char hello_buffer[HELLO_KERNEL_BUFF_LEN];
     size_t buff_index;
-- 
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