cdc-acm: implement TIOCSSERIAL to avoid blocking close(2)

Some devices (ex Nokia C7) simply don't respond at all when data is sent
to some of their USB interfaces.  The data gets stuck in the TTYs queue
and sits there until close(2), which them blocks because closing_wait
defaults to 30 seconds (even though the fd is O_NONBLOCK).  This is
rarely desired.  Implement the standard mechanism to adjust closing_wait
and let applications handle it how they want to.

See also 02303f73 for usb_wwan.c.
Signed-off-by: Dan Williams <dcbw@redhat.com>
Cc: stable <stable@vger.kernel.org>
Acked-by: Oliver Neukum <oneukum@suse.de>
Tested-by: Aleksander Morgado <aleksander@gnu.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/class/cdc-acm.c

@@ -787,6 +787,10 @@ static int get_serial_info(struct acm *acm, struct serial_struct __user *info)
 	tmp.flags = ASYNC_LOW_LATENCY;
 	tmp.xmit_fifo_size = acm->writesize;
 	tmp.baud_base = le32_to_cpu(acm->line.dwDTERate);
+	tmp.close_delay	= acm->port.close_delay / 10;
+	tmp.closing_wait = acm->port.closing_wait == ASYNC_CLOSING_WAIT_NONE ?
+				ASYNC_CLOSING_WAIT_NONE :
+				acm->port.closing_wait / 10;
 
 	if (copy_to_user(info, &tmp, sizeof(tmp)))
 		return -EFAULT;
@@ -794,6 +798,37 @@ static int get_serial_info(struct acm *acm, struct serial_struct __user *info)
 		return 0;
 }
 
+static int set_serial_info(struct acm *acm,
+				struct serial_struct __user *newinfo)
+{
+	struct serial_struct new_serial;
+	unsigned int closing_wait, close_delay;
+	int retval = 0;
+
+	if (copy_from_user(&new_serial, newinfo, sizeof(new_serial)))
+		return -EFAULT;
+
+	close_delay = new_serial.close_delay * 10;
+	closing_wait = new_serial.closing_wait == ASYNC_CLOSING_WAIT_NONE ?
+			ASYNC_CLOSING_WAIT_NONE : new_serial.closing_wait * 10;
+
+	mutex_lock(&acm->port.mutex);
+
+	if (!capable(CAP_SYS_ADMIN)) {
+		if ((close_delay != acm->port.close_delay) ||
+		    (closing_wait != acm->port.closing_wait))
+			retval = -EPERM;
+		else
+			retval = -EOPNOTSUPP;
+	} else {
+		acm->port.close_delay  = close_delay;
+		acm->port.closing_wait = closing_wait;
+	}
+
+	mutex_unlock(&acm->port.mutex);
+	return retval;
+}
+
 static int acm_tty_ioctl(struct tty_struct *tty,
 					unsigned int cmd, unsigned long arg)
 {
@@ -804,6 +839,9 @@ static int acm_tty_ioctl(struct tty_struct *tty,
 	case TIOCGSERIAL: /* gets serial port data */
 		rv = get_serial_info(acm, (struct serial_struct __user *) arg);
 		break;
+	case TIOCSSERIAL:
+		rv = set_serial_info(acm, (struct serial_struct __user *) arg);
+		break;
 	}
 
 	return rv;