linux-kernel - Re: [PATCH] fbdev: i740: Fix potential divide by zero

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Message-ID: <20250615215736.3d850bdd@pumpkin>
Date: Sun, 15 Jun 2025 21:57:36 +0100
From: David Laight <david.laight.linux@...il.com>
To: Jin D <alexguo1023@...il.com>
Cc: deller@....de, linux-fbdev@...r.kernel.org,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fbdev: i740: Fix potential divide by zero

On Sun, 15 Jun 2025 16:43:58 -0400
Jin D <alexguo1023@...il.com> wrote:

> >       bpp = var->bits_per_pixel;
> > +     if (!var->pixclock){
> > +             dev_err(info->device, "pixclock must not be zero\n");
> > +             return -EINVAL;
> > +     }
> >       switch (bpp) {
> >       case 1 ... 8:
> >               bpp = 8;  
> 
> The value used in the switch condition is var->bits_per_pixel. I can not
> find a deterministic relationship between var->bits_per_pixel and
> var->pixclock.

Brain-fade ...

> 
> On Sun, Jun 15, 2025 at 4:30 AM David Laight <david.laight.linux@...il.com>
> wrote:
> 
> > On Sat, 14 Jun 2025 01:18:37 -0400
> > Alex Guo <alexguo1023@...il.com> wrote:
> >  
> > > Variable var->pixclock can be set by user. In case it equals to
> > > zero, divide by zero would occur in 4 switch branches in
> > > i740fb_decode_var.
> > > Similar crashes have happened in other fbdev drivers. We fix this
> > > by checking whether 'pixclock' is zero.  
> >
> > Doesn't it already hit the 'default' clause of the switch statement?
> >
> >         David
> >  
> > >
> > > Similar commit: commit 16844e58704 ("video: fbdev: tridentfb:
> > > Error out if 'pixclock' equals zero")
> > >
> > > Signed-off-by: Alex Guo <alexguo1023@...il.com>
> > > ---
> > >  drivers/video/fbdev/i740fb.c | 4 ++++
> > >  1 file changed, 4 insertions(+)
> > >
> > > diff --git a/drivers/video/fbdev/i740fb.c b/drivers/video/fbdev/i740fb.c
> > > index 9b74dae71472..861e9e397b4e 100644
> > > --- a/drivers/video/fbdev/i740fb.c
> > > +++ b/drivers/video/fbdev/i740fb.c
> > > @@ -419,6 +419,10 @@ static int i740fb_decode_var(const struct  
> > fb_var_screeninfo *var,  
> > >
> > >
> > >       bpp = var->bits_per_pixel;
> > > +     if (!var->pixclock){
> > > +             dev_err(info->device, "pixclock must not be zero\n");
> > > +             return -EINVAL;
> > > +     }
> > >       switch (bpp) {
> > >       case 1 ... 8:
> > >               bpp = 8;  
> >
> >