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

[Geert Uytterhoeven]

Hi Helge,

On Thu, Jun 30, 2022 at 10:10 PM Helge Deller <deller at gmx.de> wrote:
> On 6/30/22 22:00, Geert Uytterhoeven wrote:
> > 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.

Most hardware has restrictions on resolution (e.g. xres must be a
multiple of N), so the driver has to round up the resolution to make
it fit.  And after that the driver has to validate and update the
virtual resolution again anyway...

If a driver does not support changing the video mode, it can leave
out the .fb_check_var() and .fb_set_par() callbacks, so the fbdev
core will ignore the userspace-supplied parameters, and reinstate
the single supported mode. See e.g. "[PATCH] drm/fb-helper:
Remove helpers to change frame buffer config"
(https://lore.kernel.org/all/20220629105658.1373770-1-geert@linux-m68k.org).

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