[PATCH 2/5] fbcon: Fix up user-provided virtual screen size

[Helge Deller]

On 6/30/22 22:00, Geert Uytterhoeven wrote:
> Hi Helge,
>
> On Thu, Jun 30, 2022 at 9:46 PM Helge Deller <deller at gmx.de> wrote:
>> On 6/30/22 21:36, Geert Uytterhoeven wrote:
>>> On Thu, Jun 30, 2022 at 9:31 PM Helge Deller <deller at gmx.de> wrote:
>>>> On 6/30/22 21:00, Geert Uytterhoeven wrote:
>>>>> On Wed, Jun 29, 2022 at 10:00 PM Helge Deller <deller at gmx.de> wrote:
>>>>>> The virtual screen size can't be smaller than the physical screen size.
>>>>>> Based on the general rule that we round up user-provided input if
>>>>>> neccessary, adjust the virtual screen size as well if needed.
>>>>>>
>>>>>> Signed-off-by: Helge Deller <deller at gmx.de>
>>>>>> Cc: stable at vger.kernel.org # v5.4+
>>>>>
>>>>> Thanks for your patch!
>>>>>
>>>>>> --- a/drivers/video/fbdev/core/fbmem.c
>>>>>> +++ b/drivers/video/fbdev/core/fbmem.c
>>>>>> @@ -1106,6 +1106,11 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
>>>>>>                         return -EFAULT;
>>>>>>                 console_lock();
>>>>>>                 lock_fb_info(info);
>>>>>> +               /* adjust virtual screen size if user missed it */
>>>>>> +               if (var.xres_virtual < var.xres)
>>>>>> +                       var.xres_virtual = var.xres;
>>>>>> +               if (var.yres_virtual < var.yres)
>>>>>> +                       var.yres_virtual = var.yres;
>>>>>>                 ret = fb_set_var(info, &var);
>>>>>>                 if (!ret)
>>>>>>                         fbcon_update_vcs(info, var.activate & FB_ACTIVATE_ALL);
>>>>>
>>>>> Given "[PATCH 4/5] fbmem: Prevent invalid virtual screen sizes in
>>>>> fb_set_var", I don't think we need this patch.
>>>>
>>>> We do.
>>>
>>> Why? It will be caught by [PATCH 4/5].
>>
>> Right, it will be caught by patch #4.
>> But if you drop this part, then everytime a user runs
>>         fbset -xres 800 -yres 600 -xvres 200
>> users will get the KERNEL BUG WARNING (from patch #4) including
>> a kernel backtrace in their syslogs.
>
> No, they will only see that warning if they are using a broken fbdev
> driver that implements .fb_check_var(), but fails to validate or
> update the passed geometry.

IMHO this argument is mood.
That way you put pressure on and need such simple code in
each single driver to fix it up, instead of cleaning it up at a central
place.

Helge