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

Hi Alex,

On 6/12/25 11:29, Geert Uytterhoeven wrote:

On Wed, 11 Jun 2025 at 18:12, Alex Guo [off-list ref] wrote:

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On Sat, 7 Jun 2025 at 22:14, Alex Guo [off-list ref] wrote:

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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 <redacted>

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

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--- 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.

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If so, could you advise what a suitable default value would be for this case?

The answer is hidden in the existing check below:

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+
         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/