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

[Helge Deller <deller@gmx.de>]

Hi Alex,

On 6/12/25 11:29, Geert Uytterhoeven wrote:
> On Wed, 11 Jun 2025 at 18:12, Alex Guo <alexguo1023@gmail.com> wrote:
>>> 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.
>>
>> Thanks for your confirmation and suggestions.
>>
>> I added this patch based on existing checks on var->pixclock in other drivers, such as savagefb_check_var, nvidiafb_check_var, etc.
>> Are you suggesting that it is better to replace an invalid value (var->pixclock == 0) with a default valid value, instead of returning -EINVAL?
> 
> Indeed.
> 
>> If so, could you advise what a suitable default value would be for this case?
> 
> The answer is hidden in the existing check below:
> 
>>>> +
>>>>          if (PICOS2KHZ(var->pixclock) > PM3_MAX_PIXCLOCK) {
>>>>                  DPRINTK("pixclock too high (%ldKHz)\n",
>>>>                          PICOS2KHZ(var->pixclock));
>>>>                  return -EINVAL;
>>>>          }
> 
> It can be replaced by:
> 
>      if (var->pixclock <= KHZ2PICOS(PM3_MAX_PIXCLOCK))
>              var->pixclock = KHZ2PICOS(PM3_MAX_PIXCLOCK) + 1;
> 
> The "+ 1" is needed because of rounding.

You sent a whole bunch of patches [1] which check pixclock against
zero, but you don't set the default value as Geert pointed out
above. Can you maybe revise your patches accordingly?

Helge

[1] https://patchwork.kernel.org/project/linux-fbdev/list/