linux-kernel - Re: [PATCH v3 3/6] serial: core: fix sanitizing check for RTS settings

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Message-ID: <63b62f6c-d97d-8d45-1612-968ef74b6365@linux.intel.com>
Date:   Fri, 13 Oct 2023 13:24:50 +0300 (EEST)
From:   Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To:     Lino Sanfilippo <l.sanfilippo@...bus.com>
cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>, shawnguo@...nel.org,
        s.hauer@...gutronix.de, mcoquelin.stm32@...il.com,
        alexandre.torgue@...s.st.com, cniedermaier@...electronics.com,
        LKML <linux-kernel@...r.kernel.org>,
        linux-serial <linux-serial@...r.kernel.org>,
        LinoSanfilippo@....de, Lukas Wunner <lukas@...ner.de>,
        p.rosenberger@...bus.com, stable@...r.kernel.org
Subject: Re: [PATCH v3 3/6] serial: core: fix sanitizing check for RTS
 settings

On Thu, 12 Oct 2023, Lino Sanfilippo wrote:
> On 12.10.23 15:10, Ilpo Järvinen wrote:
> > On Wed, 11 Oct 2023, Lino Sanfilippo wrote:
> > 
> >> Among other things uart_sanitize_serial_rs485() tests the sanity of the RTS
> >> settings in a RS485 configuration that has been passed by userspace.
> >> If RTS-on-send and RTS-after-send are both set or unset the configuration
> >> is adjusted and RTS-after-send is disabled and RTS-on-send enabled.
> >>
> >> This however makes only sense if both RTS modes are actually supported by
> >> the driver.
> >>
> >> With commit be2e2cb1d281 ("serial: Sanitize rs485_struct") the code does
> >> take the driver support into account but only checks if one of both RTS
> >> modes are supported. This may lead to the errorneous result of RTS-on-send
> >> being set even if only RTS-after-send is supported.
> >>
> >> Fix this by changing the implemented logic: First clear all unsupported
> >> flags in the RS485 configuration, then adjust an invalid RTS setting by
> >> taking into account which RTS mode is supported.
> >>
> >> Cc: stable@...r.kernel.org
> >> Fixes: be2e2cb1d281 ("serial: Sanitize rs485_struct")
> >> Signed-off-by: Lino Sanfilippo <l.sanfilippo@...bus.com>
> >> ---
> >>  drivers/tty/serial/serial_core.c | 28 ++++++++++++++++++----------
> >>  1 file changed, 18 insertions(+), 10 deletions(-)
> >>
> >> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> >> index 697c36dc7ec8..f4feebf8200f 100644
> >> --- a/drivers/tty/serial/serial_core.c
> >> +++ b/drivers/tty/serial/serial_core.c
> >> @@ -1370,19 +1370,27 @@ static void uart_sanitize_serial_rs485(struct uart_port *port, struct serial_rs4
> >>               return;
> >>       }
> >>
> >> +     rs485->flags &= supported_flags;
> >> +
> >>       /* Pick sane settings if the user hasn't */
> >> -     if ((supported_flags & (SER_RS485_RTS_ON_SEND|SER_RS485_RTS_AFTER_SEND)) &&
> >> -         !(rs485->flags & SER_RS485_RTS_ON_SEND) ==
> >> +     if (!(rs485->flags & SER_RS485_RTS_ON_SEND) ==
> >>           !(rs485->flags & SER_RS485_RTS_AFTER_SEND)) {
> >> -             dev_warn_ratelimited(port->dev,
> >> -                     "%s (%d): invalid RTS setting, using RTS_ON_SEND instead\n",
> >> -                     port->name, port->line);
> >> -             rs485->flags |= SER_RS485_RTS_ON_SEND;
> >> -             rs485->flags &= ~SER_RS485_RTS_AFTER_SEND;
> >> -             supported_flags |= SER_RS485_RTS_ON_SEND|SER_RS485_RTS_AFTER_SEND;
> >> -     }
> >> +             if (supported_flags & SER_RS485_RTS_ON_SEND) {
> >> +                     rs485->flags |= SER_RS485_RTS_ON_SEND;
> >> +                     rs485->flags &= ~SER_RS485_RTS_AFTER_SEND;
> >>
> >> -     rs485->flags &= supported_flags;
> >> +                     dev_warn_ratelimited(port->dev,
> >> +                             "%s (%d): invalid RTS setting, using RTS_ON_SEND instead\n",
> >> +                             port->name, port->line);
> >> +             } else {
> >> +                     rs485->flags |= SER_RS485_RTS_AFTER_SEND;
> >> +                     rs485->flags &= ~SER_RS485_RTS_ON_SEND;
> > 
> > So if neither of the flags is supported, what will happen? You might want
> > add if after that else?
> >
> 
> I would consider this a bug in the driver, as at least one of both modes 
> has to be supported. If the driver does not have at least one of both flags
> set in rs485_supported.flags we could print a warning though. Would you prefer that?

8250_exar.c needs to fixed then? I was taking these as things one can 
"configure" even if when there's support only for a one of them there's 
not that much to configure. As there was neither in 8250_exar's code, I 
didn't add either flag.

But I suppose your interpretation of those flag makes more sense.

-- 
 i.