Re: [PATCH] drm/fb-helper: Fix height, width, and accel_flags in fb_var

[Geert Uytterhoeven]

Hi Daniel,

On Wed, Apr 19, 2023 at 6:38 PM Daniel Vetter <daniel@xxxxxxxx> wrote:
> On Tue, Apr 18, 2023 at 08:42:46PM +0200, Geert Uytterhoeven wrote:
> > Fbtest contains some very simple validation of the fbdev userspace API
> > contract.  When used with shmob-drm, it reports the following warnings
> > and errors:
> >
> >     height changed from 68 to 0
> >     height was rounded down
> >     width changed from 111 to 0
> >     width was rounded down
> >     accel_flags changed from 0 to 1
> >
> > The first part happens because __fill_var() resets the physical
> > dimensions of the first connector, as filled in by drm_setup_crtcs_fb().
> > Fix this by retaining the original values.
> >
> > The last part happens because __fill_var() forces the FB_ACCELF_TEXT
> > flag on, while fbtest disables all acceleration on purpose, so it can
> > draw safely to the frame buffer.  Fix this by setting accel_flags to
> > zero, as DRM does not implement any text console acceleration.
> > Note that this issue can also be seen in the output of fbset, which
> > reports "accel true".
> >
> > Fixes: ee4cce0a8f03a333 ("drm/fb-helper: fix input validation gaps in check_var")
> > Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>

> > --- a/drivers/gpu/drm/drm_fb_helper.c
> > +++ b/drivers/gpu/drm/drm_fb_helper.c
> > @@ -2066,7 +2068,7 @@ static void drm_fb_helper_fill_var(struct fb_info *info,
> >       info->pseudo_palette = fb_helper->pseudo_palette;
> >       info->var.xoffset = 0;
> >       info->var.yoffset = 0;
> > -     __fill_var(&info->var, fb);
> > +     __fill_var(&info->var, info, fb);
>
> Bit a bikeshed since it zeroed-allocated anyway, but I'd pass NULL here
> for info and catch that in __fill_var and then keep the explicit = 0;

Yeah, it's a bit unfortunate this is done in two places, and
info->var.{height,width} are initialized by drm_setup_crtcs_fb()
only later.

Most of the var contents cannot change as mode changes are not
supported, so drm_fb_helper_check_var() should just do

    if (var->foo > info->var.foo)
            return -EINVAL;
    var->foo = info->var.foo;

For the parts that can change, based on earlier discussions I saw pass
by, I believe there should be a call into atomic try-modesetting at
the end of drm_fb_helper_check_var()?

> Either way Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>

Thanks!

Gr{oetje,eeting}s,

                        Geert

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

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