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Re: [PATCH v2] staging: rtl8192e: Fix divide fault when calculating beacon age

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On 11/4/22 02:27, 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 HZ. This patch is only compile tested.

Reported-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Acked-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx> # build-tested
Signed-off-by: Larry Finger <Larry.Finger@xxxxxxxxxxxx>
---
v2 - some commit log lines are shortened
    - add space after * operator
---
  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..217426ee2e92 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);


Tested-by: Philipp Hortmann <philipp.g.hortmann@xxxxxxxxx>



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