linux-kernel - [PATCH] serial: fix TIOCSRS485 locking

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Date:   Wed, 12 Apr 2023 14:48:11 +0200
From:   Johan Hovold <johan@...nel.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>
Cc:     Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        Johan Hovold <johan@...nel.org>, stable@...r.kernel.org,
        Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Subject: [PATCH] serial: fix TIOCSRS485 locking

The RS485 multipoint addressing support for some reason added a new
ADDRB termios cflag which is (only!) updated from one of the RS485
ioctls.

Make sure to take the termios rw semaphore for the right ioctl (i.e.
set, not get).

Fixes: ae50bb275283 ("serial: take termios_rwsem for ->rs485_config() & pass termios as param")
Cc: stable@...r.kernel.org	# 6.0
Cc: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Signed-off-by: Johan Hovold <johan@...nel.org>
---

I did not have time to review the multipoint addressing patches at the
time and only skimmed the archives now, but I can't seem to find any
motivation for why a precious termios bit was seemingly wasted on ADDRB
when it is only updated from the RS485 ioctls.

I hope it wasn't done just to simplify the implementation of
tty_get_frame_size()? Or was it a left-over from the RFC which
apparently actually used termios to enable this feature?

Should we consider dropping the Linux-specific ADDRB bit again?

Johan

 drivers/tty/serial/serial_core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 2bd32c8ece39..728cb72be066 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -1552,7 +1552,7 @@ uart_ioctl(struct tty_struct *tty, unsigned int cmd, unsigned long arg)
 		goto out;

 	/* rs485_config requires more locking than others */
-	if (cmd == TIOCGRS485)
+	if (cmd == TIOCSRS485)
 		down_write(&tty->termios_rwsem);

 	mutex_lock(&port->mutex);
@@ -1595,7 +1595,7 @@ uart_ioctl(struct tty_struct *tty, unsigned int cmd, unsigned long arg)
 	}
 out_up:
 	mutex_unlock(&port->mutex);
-	if (cmd == TIOCGRS485)
+	if (cmd == TIOCSRS485)
 		up_write(&tty->termios_rwsem);
 out:
 	return ret;
-- 
2.39.2