linux-kernel - Re: [PATCH 09/25] serial: sh-sci: Avoid overflow in sci_baud_calc

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Message-ID: <4295635.bqyeD1RN5z@avalon>
Date:	Thu, 19 Nov 2015 22:37:19 +0200
From:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:	Geert Uytterhoeven <geert+renesas@...der.be>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Simon Horman <horms@...ge.net.au>,
	Magnus Damm <magnus.damm@...il.com>,
	Yoshinori Sato <ysato@...rs.sourceforge.jp>,
	linux-serial@...r.kernel.org, linux-sh@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 09/25] serial: sh-sci: Avoid overflow in sci_baud_calc_hscif()

Hi Geert,

Thank you for the patch.

On Thursday 19 November 2015 19:38:48 Geert Uytterhoeven wrote:
> If bps >= 1048576, the multiplication of "(sr * (1 << (2 * c + 1))" and
> "bps" will overflow, and both br and err will contain bogus values.
> Skip the current and all higher clock select predividers when overflow
> is detected.  Simplify the calculations using intermediates while we're
> at it.
> 
> Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
> ---
>  drivers/tty/serial/sh-sci.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/tty/serial/sh-sci.c b/drivers/tty/serial/sh-sci.c
> index fba1e1eea15dc3a1..97a0f8ef5adc55a2 100644
> --- a/drivers/tty/serial/sh-sci.c
> +++ b/drivers/tty/serial/sh-sci.c
> @@ -1870,7 +1870,7 @@ static unsigned int sci_scbrr_calc(struct sci_port *s,
> unsigned int bps, static void sci_baud_calc_hscif(unsigned int bps,
> unsigned long freq, int *brr, unsigned int *srr, unsigned int *cks)
>  {
> -	unsigned int sr, br, c;
> +	unsigned int sr, br, a, b, c;
>  	int err, recv_margin;
>  	int min_err = 1000; /* 100% */
>  	int recv_max_margin = 0;
> @@ -1880,12 +1880,14 @@ static void sci_baud_calc_hscif(unsigned int bps,
> unsigned long freq, int *brr, for (sr = 8; sr <= 32; sr++) {
>  		for (c = 0; c <= 3; c++) {
>  			/* integerized formulas from HSCIF documentation */
> -			br = DIV_ROUND_CLOSEST(freq, (sr *
> -					      (1 << (2 * c + 1)) * bps));
> +			a = sr * (1 << (2 * c + 1));
> +			if (bps > UINT_MAX / a)
> +				break;
> +
> +			b = a * bps;

This is becoming unreadable. Could you please use proper variable names ? A 
comment that explains the calculation would also be useful.

> +			br = DIV_ROUND_CLOSEST(freq, b);
>  			br = clamp(br, 1U, 256U);
> -			err = DIV_ROUND_CLOSEST(freq, (br * bps * sr *
> -					       (1 << (2 * c + 1)) / 1000)) -
> -					       1000;
> +			err = DIV_ROUND_CLOSEST(freq, (br * b) / 1000) - 1000;
>  			/* Calc recv margin
>  			 * M: Receive margin (%)
>  			 * N: Ratio of bit rate to clock (N = sampling rate)

-- 
Regards,

Laurent Pinchart

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