Input: serio_raw - signal EFAULT even if read/write partially succeeds

When copy_to/from_user fails in the middle of transfer we should not
report to the user that read/write partially succeeded but rather
report -EFAULT right away, so that application will know that it got
its buffers all wrong.

If application messed up its buffers we can't trust the data fetched
from userspace and successfully written to the device or if data read
from the device and transferred to userspace ended up where application
expected it to end.

If serio_write() fails we still going to report partial writes if failure
happens in the middle of the transfer.

This is basically a revert of 7a0a27d2
and 4fa07711.
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

drivers/input/serio/serio_raw.c

@@ -165,9 +165,9 @@ static ssize_t serio_raw_read(struct file *file, char __user *buffer,
 	struct serio_raw *serio_raw = client->serio_raw;
 	char uninitialized_var(c);
 	ssize_t read = 0;
-	int error = 0;
+	int error;
 
-	do {
+	for (;;) {
 		if (serio_raw->dead)
 			return -ENODEV;
 
@@ -179,24 +179,24 @@ static ssize_t serio_raw_read(struct file *file, char __user *buffer,
 			break;
 
 		while (read < count && serio_raw_fetch_byte(serio_raw, &c)) {
-			if (put_user(c, buffer++)) {
-				error = -EFAULT;
-				goto out;
-			}
+			if (put_user(c, buffer++))
+				return -EFAULT;
 			read++;
 		}
 
 		if (read)
 			break;
 
-		if (!(file->f_flags & O_NONBLOCK))
+		if (!(file->f_flags & O_NONBLOCK)) {
 			error = wait_event_interruptible(serio_raw->wait,
 					serio_raw->head != serio_raw->tail ||
 					serio_raw->dead);
-	} while (!error);
+			if (error)
+				return error;
+		}
+	}
 
-out:
-	return read ?: error;
+	return read;
 }
 
 static ssize_t serio_raw_write(struct file *file, const char __user *buffer,
@@ -204,8 +204,7 @@ static ssize_t serio_raw_write(struct file *file, const char __user *buffer,
 {
 	struct serio_raw_client *client = file->private_data;
 	struct serio_raw *serio_raw = client->serio_raw;
-	ssize_t written = 0;
-	int retval;
+	int retval = 0;
 	unsigned char c;
 
 	retval = mutex_lock_interruptible(&serio_raw_mutex);
@@ -225,16 +224,20 @@ static ssize_t serio_raw_write(struct file *file, const char __user *buffer,
 			retval = -EFAULT;
 			goto out;
 		}
+
 		if (serio_write(serio_raw->serio, c)) {
-			retval = -EIO;
+			/* Either signal error or partial write */
+			if (retval == 0)
+				retval = -EIO;
 			goto out;
 		}
-		written++;
+
+		retval++;
 	}
 
 out:
 	mutex_unlock(&serio_raw_mutex);
-	return written ?: retval;
+	return retval;
 }
 
 static unsigned int serio_raw_poll(struct file *file, poll_table *wait)