linux-kernel - [PATCH v2] staging: rtl8192e: Fix divide fault when calculating beacon age

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Date:   Thu,  3 Nov 2022 20:27:50 -0500
From:   Larry Finger <Larry.Finger@...inger.net>
To:     gregkh@...uxfoundation.org
Cc:     phil@...lpotter.co.uk, linux-staging@...ts.linux.dev,
        linux-kernel@...r.kernel.org, linux-wireless@...r.kernel.org,
        Larry Finger <Larry.Finger@...inger.net>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: [PATCH v2] 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 HZ. This patch is only compile tested.

Reported-by: Randy Dunlap <rdunlap@...radead.org>
Acked-by: Randy Dunlap <rdunlap@...radead.org> # build-tested
Signed-off-by: Larry Finger <Larry.Finger@...inger.net>
---
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);
-- 
2.38.0