Re: [PATCH v2] fbmon: prevent division by zero in fb_videomode_from_videomode()

[Roman Smirnov]

On Mon, 18 Mar 2024 20:15:55 +0100 Helge Deller wrote:
> On 3/18/24 09:11, Roman Smirnov wrote:
> > On Fri, 15 Mar 2024 09:44:08 +0100 Helge Deller wrote:
> > > On 3/5/24 14:51, Roman Smirnov wrote:
> > > > The expression htotal * vtotal can have a zero value on
> > > > overflow.
> > >
> > > I'm not sure if those always results in zero in kernel on overflow.
> > > Might be architecture-depended too, but let's assume it
> > > can become zero, ....
> > >
> > > > It is necessary to prevent division by zero like in
> > > > fb_var_to_videomode().
> > > >
> > > > Found by Linux Verification Center (linuxtesting.org) with Svace.
> > > >
> > > > Signed-off-by: Roman Smirnov <r.smirnov@xxxxxx>
> > > > Reviewed-by: Sergey Shtylyov <s.shtylyov@xxxxxx>
> > > > ---
> > > >    V1 -> V2: Replaced the code of the first version with a check.
> > > >
> > > >    drivers/video/fbdev/core/fbmon.c | 2 +-
> > > >    1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/drivers/video/fbdev/core/fbmon.c b/drivers/video/fbdev/core/fbmon.c
> > > > index 79e5bfbdd34c..b137590386da 100644
> > > > --- a/drivers/video/fbdev/core/fbmon.c
> > > > +++ b/drivers/video/fbdev/core/fbmon.c
> > > > @@ -1344,7 +1344,7 @@ int fb_videomode_from_videomode(const struct videomode *vm,
> > > >         vtotal = vm->vactive + vm->vfront_porch + vm->vback_porch +
> > > >                  vm->vsync_len;
> > > >         /* prevent division by zero */
> > > > -     if (htotal && vtotal) {
> > > > +     if (htotal && vtotal && (vm->pixelclock / htotal >= vtotal)) {
> > >
> > > why don't you then simply check for
> > >          if .. ((htotal * vtotal) == 0) ...
> > > instead?
> > >
> > > Helge
> >
> > Thomas Zimmermann from the previous discussion said:
> > On Tue, 5 Mar 2024 11:18:05 +0100 Thomas Zimmerman wrote:
> > > Maybe use
> > >
> > >     if (htotal && vtotal && (vm->pixelclock / htotal >= vtotal))
> > >
> > > for the test. That rules out overflowing multiplication and sets
> > > refresh to 0 in such cases.
> >
> > This prevents overflow, which is also a problematic case.
>
> I don't like adding another division here and I doubt we have
> a problem with possible overflow.
> So, I suggest to keep it simple, something like:
>        ...
>        total = htotal * vtotal;
>        if (total)
>                fbmode->refresh = vm->pixelclock / total;
>        else...

Okay, I'll prepare a third version with that change:

    if (htotal && vtotal && (htotal * vtotal))

I think that will be enough.