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

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Message-ID: <b06c2fcd-02c0-464b-a7e8-4dfbf9e2befc@gmx.de>
Date:   Sat, 14 Oct 2023 14:23:15 +0200
From:   Lino Sanfilippo <LinoSanfilippo@....de>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        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>,
        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

Hi Ilpo,

On 13.10.23 12:24, Ilpo Järvinen wrote:
> 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.
>

IMHO this is consistent with what we have in uart_get_rs485_mode(). This function
ensures that we have at least one RTS mode set (with default to RTS_ON_SEND). So
concerning 8250_exar.c, I think it should be fixed (havent noticed the missing
RTS mode though until you mentioned it). Would you like to provide a fix for this
or shall I include one into the next version of this series?


BR,
Lino