2022-11-03 20:34:28

by Larry Finger

Subject: [PATCH] staging: rtl8192e: Fix divide fault when calculating beacon age

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 <[email protected]>
Signed-off-by: Larry Finger <[email protected]>
---
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);
--
2.38.0

2022-11-03 22:04:20

by Philipp Hortmann

Subject: Re: [PATCH] staging: rtl8192e: Fix divide fault when calculating beacon age

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 <[email protected]>
> Signed-off-by: Larry Finger <[email protected]>
> ---
> 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.

Tested-by: Philipp Hortmann <[email protected]>

2022-11-04 01:35:06

by Larry Finger

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 <[email protected]>
>> Signed-off-by: Larry Finger <[email protected]>
>> ---
>>   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