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

[Helge Deller]

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...
Helge