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

[Geert Uytterhoeven]

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.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at 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