netdev - Re: [PATCH] ptp_qoriq: fix overflow in ptp_qoriq

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Date:   Thu, 25 Mar 2021 03:23:07 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Yangbo Lu <yangbo.lu@....com>
Cc:     netdev@...r.kernel.org, Richard Cochran <richardcochran@...il.com>
Subject: Re: [PATCH] ptp_qoriq: fix overflow in ptp_qoriq_adjfine() u64
 calcalation

On Tue, Mar 23, 2021 at 04:02:29PM +0800, Yangbo Lu wrote:
> Current calculation for diff of TMR_ADD register value may have
> 64-bit overflow in this code line, when long type scaled_ppm is
> large.
> 
> adj *= scaled_ppm;
> 
> This patch is to resolve it by using mul_u64_u64_div_u64().
> 
> Signed-off-by: Yangbo Lu <yangbo.lu@....com>
> Acked-by: Richard Cochran <richardcochran@...il.com>
> ---
>  drivers/ptp/ptp_qoriq.c | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/ptp/ptp_qoriq.c b/drivers/ptp/ptp_qoriq.c
> index 68beb1bd07c0..f7f220700cb5 100644
> --- a/drivers/ptp/ptp_qoriq.c
> +++ b/drivers/ptp/ptp_qoriq.c
> @@ -189,15 +189,16 @@ int ptp_qoriq_adjfine(struct ptp_clock_info *ptp, long scaled_ppm)
>  	tmr_add = ptp_qoriq->tmr_add;
>  	adj = tmr_add;
>  
> -	/* calculate diff as adj*(scaled_ppm/65536)/1000000
> -	 * and round() to the nearest integer
> +	/*
> +	 * Calculate diff and round() to the nearest integer
> +	 *
> +	 * diff = adj * (ppb / 1000000000)
> +	 *      = adj * scaled_ppm / 65536000000
>  	 */
> -	adj *= scaled_ppm;
> -	diff = div_u64(adj, 8000000);
> -	diff = (diff >> 13) + ((diff >> 12) & 1);
> +	diff = mul_u64_u64_div_u64(adj, scaled_ppm, 32768000000);

mul_u64_u64_div_u64() is not exported. As result, every build with
CONFIG_PTP_1588_CLOCK_QORIQ=m (ie every allmodconfig build) fails with:

ERROR: modpost: "mul_u64_u64_div_u64" [drivers/ptp/ptp-qoriq.ko] undefined!

or a similar error.

Guenter