linux-kernel - RE: [PATCH 1/2] tty: serial: fsl_lpuart: fix the potential bug of division or modulo by zero

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Message-ID: <AM0PR04MB4947A9253CE547BEBE95BE8092429@AM0PR04MB4947.eurprd04.prod.outlook.com>
Date:   Mon, 26 Apr 2021 11:30:47 +0000
From:   Sherry Sun <sherry.sun@....com>
To:     Greg KH <gregkh@...uxfoundation.org>
CC:     "jirislaby@...nel.org" <jirislaby@...nel.org>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        dl-linux-imx <linux-imx@....com>
Subject: RE: [PATCH 1/2] tty: serial: fsl_lpuart: fix the potential bug of
 division or modulo by zero

Hi Greg,

> -----Original Message-----
> From: Greg KH <gregkh@...uxfoundation.org>
> Sent: 2021年4月26日 16:09
> To: Sherry Sun <sherry.sun@....com>
> Cc: jirislaby@...nel.org; linux-serial@...r.kernel.org; linux-
> kernel@...r.kernel.org; dl-linux-imx <linux-imx@....com>
> Subject: Re: [PATCH 1/2] tty: serial: fsl_lpuart: fix the potential bug of division
> or modulo by zero
> 
> On Mon, Apr 26, 2021 at 03:49:34PM +0800, Sherry Sun wrote:
> > This issue is reported by Coverity Check.
> > In lpuart32_console_get_options, division or modulo by zero may
> > results in undefined behavior.
> >
> > Signed-off-by: Sherry Sun <sherry.sun@....com>
> > ---
> >  drivers/tty/serial/fsl_lpuart.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/drivers/tty/serial/fsl_lpuart.c
> > b/drivers/tty/serial/fsl_lpuart.c index 794035041744..777d54b593f8
> > 100644
> > --- a/drivers/tty/serial/fsl_lpuart.c
> > +++ b/drivers/tty/serial/fsl_lpuart.c
> > @@ -2414,6 +2414,9 @@ lpuart32_console_get_options(struct lpuart_port
> > *sport, int *baud,
> >
> >  	bd = lpuart32_read(&sport->port, UARTBAUD);
> >  	bd &= UARTBAUD_SBR_MASK;
> > +	if (!bd)
> > +		return;
> 
> How can this ever happen?
> 
> Not to say this is a bad check, but it feels like this can't really happen in real
> life, what code patch could create this result?
> 
> And have you tested this on real hardware?
> 

Thanks for the reviewing, yes, I have tested the patchset on the real hardware.

Seems the coverity check is static scan, so cannot judge if UARTBAUD Register will be zero.
I just found below statement in the uart reference manual: "When SBR is 1 - 8191, the baud rate equals "baud clock / ((OSR+1) × SBR)"."
Since I am not familiar with uart, do you mean that the value of UARTBAUD Register will never be zero, so this case will not happen in real word?
If yes, I will drop this patch.

Best regards
Sherry


> thanks,
> 
> greg k-h