[PATCH v6 3/4] fbcon: Prevent that screen size is smaller than font size

[Geert Uytterhoeven]

Hi Helge,

On Tue, Jun 28, 2022 at 10:52 PM Helge Deller <deller at gmx.de> wrote:
> On 6/28/22 10:39, Geert Uytterhoeven wrote:
> > On Sun, Jun 26, 2022 at 12:33 PM Helge Deller <deller at gmx.de> wrote:
> >> We need to prevent that users configure a screen size which is smaller than the
> >> currently selected font size. Otherwise rendering chars on the screen will
> >> access memory outside the graphics memory region.
> >>
> >> This patch adds a new function fbcon_modechange_possible() which
> >> implements this check and which later may be extended with other checks
> >> if necessary.  The new function is called from the FBIOPUT_VSCREENINFO
> >> ioctl handler in fbmem.c, which will return -EINVAL if userspace asked
> >> for a too small screen size.
> >>
> >> Signed-off-by: Helge Deller <deller at gmx.de>
> >> Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
> >> Cc: stable at vger.kernel.org # v5.4+
> >
> > Thanks for your patch, which is now commit f0b6a66d33ca6e7e ("fbcon:
> > Prevent that screen size is smaller than font size") in fbdev/for-next

> >> --- a/drivers/video/fbdev/core/fbmem.c
> >> +++ b/drivers/video/fbdev/core/fbmem.c
> >> @@ -1112,7 +1112,9 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
> >>                         return -EFAULT;
> >>                 console_lock();
> >>                 lock_fb_info(info);
> >> -               ret = fb_set_var(info, &var);
> >> +               ret = fbcon_modechange_possible(info, &var);
> >
> > Again, this should be done (if done at all) after the call to
> > fb_ops.check_var(), as it breaks the FBIOPUT_VSCREENINFO rounding rule.
> >
> > What if the user just wants to display graphics, not text?
>
> Yes, I need to go back to an older version here too and check that
> the test is only run on text consoles.
> That check was dropped, due feedback that you could switch
> back from graphics (e.g. X11) to text console at any time....so the
> check for text-only is not correct.
>
> > Can't the text console be disabled instead?
>
> I think the solution is to return failure if switching back to text mode isn't possible if
> fonts are bigger than the screen resolution. That will be another patch.

Isn't the font a per-VC setting? Hence can't you change resolution,
switch to a different VC, and run into this?

I think the only real solution is to set the number of text columns
and/or rows to zero, and make sure that is handled correctly.

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