linux-kernel - Re: [PATCH] fbdev: au1200fb: Fix potential divide by zero

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Date:   Wed, 15 Mar 2023 20:27:47 +0800
From:   Wei Chen <harperchen1110@...il.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>, deller@....de
Cc:     linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH] fbdev: au1200fb: Fix potential divide by zero

Dear Thomas,

Thank you for the kind advice.

In fact, I notice there was a discussion regarding whether to put the
check in each individual driver or solve this problem as a whole as
you suggested. The conclusion is that it is better to keep the check per
driver rather than in the caller.

Related discussions are here:
https://lore.kernel.org/all/YXclZQGFTr1NFjbc@ravnborg.org/
https://lore.kernel.org/all/YPgbHMtLQqb1kP0l@ravnborg.org/
https://lore.kernel.org/all/20220404084723.79089-1-zheyuma97@gmail.com/

Thanks,
Wei

------ Original Message ------
>From "Thomas Zimmermann" <tzimmermann@...e.de>
To "Wei Chen" <harperchen1110@...il.com>; deller@....de
Cc linux-fbdev@...r.kernel.org; linux-kernel@...r.kernel.org;
dri-devel@...ts.freedesktop.org
Date 2023/3/15 18:25:52
Subject Re: [PATCH] fbdev: au1200fb: Fix potential divide by zero


On Wed, 15 Mar 2023 at 18:25, Thomas Zimmermann <tzimmermann@...e.de> wrote:
>
> Hi,
>
> thanks for looking through these drivers.
>
> Am 15.03.23 um 10:22 schrieb Wei Chen:
> > var->pixclock can be assigned to zero by user. Without
> > proper check, divide by zero would occur when invoking
> > macro PICOS2KHZ in au1200fb_fb_check_var.
> >
> > Error out if var->pixclock is zero.
> >
> > Signed-off-by: Wei Chen <harperchen1110@...il.com>
> > ---
> >   drivers/video/fbdev/au1200fb.c | 3 +++
> >   1 file changed, 3 insertions(+)
> >
> > diff --git a/drivers/video/fbdev/au1200fb.c b/drivers/video/fbdev/au1200fb.c
> > index 81c315454428..b6b22fa4a8a0 100644
> > --- a/drivers/video/fbdev/au1200fb.c
> > +++ b/drivers/video/fbdev/au1200fb.c
> > @@ -1040,6 +1040,9 @@ static int au1200fb_fb_check_var(struct fb_var_screeninfo *var,
> >       u32 pixclock;
> >       int screen_size, plane;
> >
> > +     if (!var->pixclock)
> > +             return -EINVAL;
> > +
>
> Instead of the whale-a-mole approach of fixing individual drivers, could
> this be solved by testing in fb_set_var [1] and fb_try_mode.? [2]
>
> Best regards
> Thomas
>
> [1]
> https://elixir.bootlin.com/linux/latest/source/drivers/video/fbdev/core/fbmem.c#L958
> [2]
> https://elixir.bootlin.com/linux/latest/source/drivers/video/fbdev/core/modedb.c#L567
>
> >       plane = fbdev->plane;
> >
> >       /* Make sure that the mode respect all LCD controller and
>
> --
> Thomas Zimmermann
> Graphics Driver Developer
> SUSE Software Solutions Germany GmbH
> Maxfeldstr. 5, 90409 Nürnberg, Germany
> (HRB 36809, AG Nürnberg)
> Geschäftsführer: Ivo Totev