linux-kernel - Re: [PATCH] staging: rtl8192e: Fix divide fault when calculating beacon age

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Message-ID: <c5ae56d9-e07c-2ca7-6f42-640c330b5d78@infradead.org>
Date:   Thu, 3 Nov 2022 16:08:06 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Larry Finger <Larry.Finger@...inger.net>,
        gregkh@...uxfoundation.org
Cc:     phil@...lpotter.co.uk, linux-staging@...ts.linux.dev,
        linux-kernel@...r.kernel.org, linux-wireless@...r.kernel.org
Subject: Re: [PATCH] staging: rtl8192e: Fix divide fault when calculating
 beacon age



On 11/3/22 13:05, Larry Finger wrote:
> When the configuration parameter CONFIG_HZ is less that 100, the compiler
> generates an error as follows:
> 
> ../drivers/staging/rtl8192e/rtllib_wx.c: In function 'rtl819x_translate_scan':
> ../drivers/staging/rtl8192e/rtllib_wx.c:220:57: warning: division by zero [-Wdiv-by-zero]
>   220 |                       (jiffies - network->last_scanned) / (HZ / 100));
>       |                                                         ^
> In file included from ../include/linux/skbuff.h:45,
>                  from ../include/linux/if_ether.h:19,
>                  from ../include/linux/etherdevice.h:20,
>                  from ../drivers/staging/rtl8192e/rtllib_wx.c:18:
> ../drivers/staging/rtl8192e/rtllib_wx.c: In function 'rtllib_wx_get_scan':
> ../drivers/staging/rtl8192e/rtllib_wx.c:261:70: warning: division by zero [-Wdiv-by-zero]
>   261 |                                    (jiffies - network->last_scanned) /
>       |
> 
> In fact, is HZ is not a multiple of 100, the calculation will be wrong, but it
> will compile correctly.
> 
> The fix is to get rid of the (HZ / 100) portion. To decrease any round-off
> errors, the compiler is forced to perform the 100 * jiffies-difference before
> dividing by HX. This patch is only compile tested.

              HZ.

> 
> Reported-by: Randy Dunlap <rdunlap@...radead.org>
> Signed-off-by: Larry Finger <Larry.Finger@...inger.net>

Acked-by: Randy Dunlap <rdunlap@...radead.org> # build-tested

Thanks.

> ---
>  drivers/staging/rtl8192e/rtllib_wx.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/staging/rtl8192e/rtllib_wx.c b/drivers/staging/rtl8192e/rtllib_wx.c
> index da2c41c9b92f..7013425102dd 100644
> --- a/drivers/staging/rtl8192e/rtllib_wx.c
> +++ b/drivers/staging/rtl8192e/rtllib_wx.c
> @@ -217,7 +217,7 @@ static inline char *rtl819x_translate_scan(struct rtllib_device *ieee,
>  	p = custom;
>  	p += scnprintf(p, MAX_CUSTOM_LEN - (p - custom),
>  		      " Last beacon: %lums ago",
> -		      (jiffies - network->last_scanned) / (HZ / 100));
> +		      (100 *(jiffies - network->last_scanned)) / HZ);

                      (100 * (jiffies ...

>  	iwe.u.data.length = p - custom;
>  	if (iwe.u.data.length)
>  		start = iwe_stream_add_point_rsl(info, start, stop,
> @@ -258,8 +258,8 @@ int rtllib_wx_get_scan(struct rtllib_device *ieee,
>  				   escape_essid(network->ssid,
>  						network->ssid_len),
>  				   network->bssid,
> -				   (jiffies - network->last_scanned) /
> -				   (HZ / 100));
> +				   (100 * (jiffies - network->last_scanned)) /
> +				   HZ);
>  	}
>  
>  	spin_unlock_irqrestore(&ieee->lock, flags);

-- 
~Randy