linux-kernel - Re: [PATCH] Bluetooth: hci_uart: Switch pty driver to slave side in tty_set

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Date:   Thu, 31 Jan 2019 16:43:24 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Myungho Jung <mhjungk@...il.com>
Cc:     Johan Hovold <johan@...nel.org>,
        Marcel Holtmann <marcel@...tmann.org>,
        Johan Hedberg <johan.hedberg@...il.com>,
        linux-bluetooth@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Bluetooth: hci_uart: Switch pty driver to slave side in
 tty_set_termios()

On Wed, Jan 30, 2019 at 09:13:53PM -0800, Myungho Jung wrote:
> On Wed, Jan 30, 2019 at 11:07:38AM +0100, Johan Hovold wrote:
> > On Sun, Jan 27, 2019 at 10:53:02PM -0800, Myungho Jung wrote:
> > > tty_set_termios() should be called with slave side of pty driver. So, If
> > > tty driver is pty master, it needs to be switched to ->link.
> > 
> > I'm not sure that's the right solution. PTYs are virtual devices used
> > for IPC and neither end (master or slave) have support for modem
> > control or baud rates.
> > 
> > > Reported-by: syzbot+a950165cbb86bdd023a4@...kaller.appspotmail.com
> > > Signed-off-by: Myungho Jung <mhjungk@...il.com>
> > > ---
> > >  drivers/bluetooth/hci_ldisc.c | 20 +++++++++++++++-----
> > >  1 file changed, 15 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/drivers/bluetooth/hci_ldisc.c b/drivers/bluetooth/hci_ldisc.c
> > > index fbf7b4df23ab..90c5ea8c399b 100644
> > > --- a/drivers/bluetooth/hci_ldisc.c
> > > +++ b/drivers/bluetooth/hci_ldisc.c
> > > @@ -299,10 +299,18 @@ static int hci_uart_send_frame(struct hci_dev *hdev, struct sk_buff *skb)
> > >  	return 0;
> > >  }
> > >  
> > > +/* If driver is pty master, return slave side */
> > > +static struct tty_struct *hci_uart_get_real_tty(struct tty_struct *tty)
> > > +{
> > > +	return  (tty->driver->type == TTY_DRIVER_TYPE_PTY &&
> > > +		 tty->driver->subtype == PTY_TYPE_MASTER) ? tty->link : tty;
> > > +}
> > > +
> > >  /* Flow control or un-flow control the device */
> > >  void hci_uart_set_flow_control(struct hci_uart *hu, bool enable)
> > >  {
> > >  	struct tty_struct *tty = hu->tty;
> > > +	struct tty_struct *real_tty;
> > >  	struct ktermios ktermios;
> > >  	int status;
> > >  	unsigned int set = 0;
> > > @@ -314,11 +322,12 @@ void hci_uart_set_flow_control(struct hci_uart *hu, bool enable)
> > >  		return;
> > >  	}
> > >  
> > > +	real_tty = hci_uart_get_real_tty(tty);
> > >  	if (enable) {
> > >  		/* Disable hardware flow control */
> > > -		ktermios = tty->termios;
> > > +		ktermios = real_tty->termios;
> > >  		ktermios.c_cflag &= ~CRTSCTS;
> > > -		status = tty_set_termios(tty, &ktermios);
> > > +		status = tty_set_termios(real_tty, &ktermios);
> > >  		BT_DBG("Disabling hardware flow control: %s",
> > >  		       status ? "failed" : "success");
> > 
> > So instead of these pointless calls to set the slave termios and
> > modem-control state, you might as well bail out early above (and
> > similarly in set_baudrate()).
> > 
> > Using n_hci for a master pty really makes no sense at all, so we could
> > even bail out at ldisc open, but perhaps that can be discussed and
> > addressed later.

> I fixed it to just return -EOPNOTSUPP if NULL in ath_setup().

That was the other fix. For this one you need to bail out if the tty is
a pty master so that tty_set_termios never gets called here or in
set_baudrate(). Possibly you can do that already at open().

This is also being discussed here:

	https://lkml.kernel.org/r/20190130103227.GR3691@localhost

Johan