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

[Larry Finger]

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