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

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Message-ID: <0c0773ed-8a16-3082-0b13-38ed37058f44@lwfinger.net>
Date:   Thu, 3 Nov 2022 20:28:58 -0500
From:   Larry Finger <Larry.Finger@...inger.net>
To:     Philipp Hortmann <philipp.g.hortmann@...il.com>,
        gregkh@...uxfoundation.org
Cc:     phil@...lpotter.co.uk, linux-staging@...ts.linux.dev,
        linux-kernel@...r.kernel.org, linux-wireless@...r.kernel.org,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH] staging: rtl8192e: Fix divide fault when calculating
 beacon age

On 11/3/22 17:01, Philipp Hortmann wrote:
> On 11/3/22 21:05, 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.
>>
>> Reported-by: Randy Dunlap <rdunlap@...radead.org>
>> Signed-off-by: Larry Finger <Larry.Finger@...inger.net>
>> ---
>>   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);
>>       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);
> 
> Line length of the description is to long.

Addressed in v2.

Larry