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

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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@xxxxxxxxxxxxx>
Signed-off-by: Larry Finger <Larry.Finger@xxxxxxxxxxxx>
---
  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




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