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

Hi Alex,

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

quoted

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

quoted

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.

quoted

--- 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:

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quoted

+
        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.

Actually, I have found a few similar issues in other functions as well. I would like to make sure I am addressing them in the correct way.

That would be great. Thanks!

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