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

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Date:   Thu, 12 Oct 2023 23:01:01 +0200
From:   Lino Sanfilippo <l.sanfilippo@...bus.com>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.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

Hi,

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?

Regards,
Lino