tty: fix up iterate_tty_read() EOVERFLOW handling

When I converted the tty_ldisc_ops 'read()' function to take a kernel
pointer, I was a bit too aggressive about the ldisc returning EOVERFLOW.

Yes, we want to have EOVERFLOW override any partially read data (because
the whole point is that the buffer was too small for the whole packet,
and we don't want to see partial packets), but it shouldn't override a
previous EFAULT.

And in fact, it really is just EOVERFLOW that is special and should
throw away any partially read data, not "any error".  Admittedly
EOVERFLOW is currently the only one that can happen for a continuation
read - and if the first read iteration returns an error we won't have this issue.

So this is more of a technicality, but let's just make the intent very
explicit, and re-organize the error handling a bit so that this is all
clearer.

Reported-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/tty/tty_io.c

@@ -431,14 +431,12 @@ struct tty_driver *tty_find_polling_driver(char *name, int *line)
 EXPORT_SYMBOL_GPL(tty_find_polling_driver);
 #endif
 
-static ssize_t hung_up_tty_read(struct file *file, char __user *buf,
-				size_t count, loff_t *ppos)
+static ssize_t hung_up_tty_read(struct kiocb *iocb, struct iov_iter *to)
 {
 	return 0;
 }
 
-static ssize_t hung_up_tty_write(struct file *file, const char __user *buf,
-				 size_t count, loff_t *ppos)
+static ssize_t hung_up_tty_write(struct kiocb *iocb, struct iov_iter *from)
 {
 	return -EIO;
 }
@@ -505,8 +503,8 @@ static const struct file_operations console_fops = {
 
 static const struct file_operations hung_up_tty_fops = {
 	.llseek		= no_llseek,
-	.read		= hung_up_tty_read,
-	.write		= hung_up_tty_write,
+	.read_iter	= hung_up_tty_read,
+	.write_iter	= hung_up_tty_write,
 	.poll		= hung_up_tty_poll,
 	.unlocked_ioctl	= hung_up_tty_ioctl,
 	.compat_ioctl	= hung_up_tty_compat_ioctl,
@@ -863,13 +861,20 @@ static int iterate_tty_read(struct tty_ldisc *ld, struct tty_struct *tty,
 		if (!size)
 			break;
 
-		/*
-		 * A ldisc read error return will override any previously copied
-		 * data (eg -EOVERFLOW from HDLC)
-		 */
 		if (size < 0) {
-			memzero_explicit(kernel_buf, sizeof(kernel_buf));
-			return size;
+			/* Did we have an earlier error (ie -EFAULT)? */
+			if (retval)
+				break;
+			retval = size;
+
+			/*
+			 * -EOVERFLOW means we didn't have enough space
+			 * for a whole packet, and we shouldn't return
+			 * a partial result.
+			 */
+			if (retval == -EOVERFLOW)
+				offset = 0;
+			break;
 		}
 
 		copied = copy_to_iter(kernel_buf, size, to);
@@ -925,8 +930,10 @@ static ssize_t tty_read(struct kiocb *iocb, struct iov_iter *to)
 	/* We want to wait for the line discipline to sort out in this
 	   situation */
 	ld = tty_ldisc_ref_wait(tty);
+	if (!ld)
+		return hung_up_tty_read(iocb, to);
 	i = -EIO;
-	if (ld && ld->ops->read)
+	if (ld->ops->read)
 		i = iterate_tty_read(ld, tty, file, to);
 	tty_ldisc_deref(ld);
 
@@ -1103,7 +1110,9 @@ static ssize_t tty_write(struct kiocb *iocb, struct iov_iter *from)
 	if (tty->ops->write_room == NULL)
 		tty_err(tty, "missing write_room method\n");
 	ld = tty_ldisc_ref_wait(tty);
-	if (!ld || !ld->ops->write)
+	if (!ld)
+		return hung_up_tty_write(iocb, from);
+	if (!ld->ops->write)
 		ret = -EIO;
 	else
 		ret = do_tty_write(ld->ops->write, tty, file, from);