Re: [PATCH] fbdev: pm3fb: Fix potential divide by zero

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Alex,

On Sat, 7 Jun 2025 at 22:14, Alex Guo <alexguo1023@gmail.com> wrote:
> variable var->pixclock can be set by user. In case it equals to
>  zero, divide by zero would occur in pm3fb_check_var. Similar
> crashes have happened in other fbdev drivers. There is no check
> and modification on var->pixclock along the call chain to
> pm3fb_check_var. So we fix this by checking whether 'pixclock'
> is zero.
>
> Similar commit: commit 16844e58704 ("video: fbdev: tridentfb:
> Error out if 'pixclock' equals zero")
>
> Signed-off-by: Alex Guo <alexguo1023@gmail.com>

Thanks for your patch, which is now commit 59d1fc7b3e1ae9d4
("fbdev: pm3fb: fix potential divide by zero") in fbdev/for-next.

> --- a/drivers/video/fbdev/pm3fb.c
> +++ b/drivers/video/fbdev/pm3fb.c
> @@ -998,6 +998,9 @@ static int pm3fb_check_var(struct fb_var_screeninfo *var, struct fb_info *info)
>                 return -EINVAL;
>         }
>
> +       if (!var->pixclock)
> +               return -EINVAL;

While this fixes the crash, this is correct behavior for an fbdev driver.
When a value is invalid, it should be rounded up to a valid value instead,
if possible.

> +
>         if (PICOS2KHZ(var->pixclock) > PM3_MAX_PIXCLOCK) {
>                 DPRINTK("pixclock too high (%ldKHz)\n",
>                         PICOS2KHZ(var->pixclock));

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds