[PATCH] fbdev: Don't spam dmesg on bad userspace ioctl input

[Daniel Vetter]

On Tue, 4 Apr 2023 at 19:04, Geert Uytterhoeven <geert at linux-m68k.org> wrote:
>
> Hi Daniel,
>
> On Tue, Apr 4, 2023 at 5:55 PM Daniel Vetter <daniel at ffwll.ch> wrote:
> > On Tue, 4 Apr 2023 at 16:51, Geert Uytterhoeven <geert at linux-m68k.org> wrote:
> > > On Tue, Apr 4, 2023 at 4:19 PM Daniel Vetter <daniel at ffwll.ch> wrote:
> > > > On Tue, Apr 04, 2023 at 03:59:12PM +0200, Daniel Vetter wrote:
> > > > > On Tue, Apr 04, 2023 at 03:53:09PM +0200, Geert Uytterhoeven wrote:
> > > > > > On Tue, Apr 4, 2023 at 2:36 PM Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
> > > > > > > There's a few reasons the kernel should not spam dmesg on bad
> > > > > > > userspace ioctl input:
> > > > > > > - at warning level it results in CI false positives
> > > > > > > - it allows userspace to drown dmesg output, potentially hiding real
> > > > > > >   issues.
> > > > > > >
> > > > > > > None of the other generic EINVAL checks report in the
> > > > > > > FBIOPUT_VSCREENINFO ioctl do this, so it's also inconsistent.
> > > > > > >
> > > > > > > I guess the intent of the patch which introduced this warning was that
> > > > > > > the drivers ->fb_check_var routine should fail in that case. Reality
> > > > > > > is that there's too many fbdev drivers and not enough people
> > > > > > > maintaining them by far, and so over the past few years we've simply
> > > > > > > handled all these validation gaps by tighning the checks in the core,
> > > > > > > because that's realistically really all that will ever happen.
> > > > > > >
> > > > > > > Reported-by: syzbot+20dcf81733d43ddff661 at syzkaller.appspotmail.com
> > > > > > > Link: https://syzkaller.appspot.com/bug?id=c5faf983bfa4a607de530cd3bb008888bf06cefc
> > > > > >
> > > > > >     WARNING: fbcon: Driver 'vkmsdrmfb' missed to adjust virtual screen
> > > > > > size (0x0 vs. 64x768)
> > > > > >
> > > > > > This is a bug in the vkmsdrmfb driver and/or DRM helpers.
> > > > > >
> > > > > > The message was added to make sure the individual drivers are fixed.
> > > > > > Perhaps it should be changed to BUG() instead, so dmesg output
> > > > > > cannot be drown?
> > > > >
> > > > > So you're solution is to essentially force us to replicate this check over
> > > > > all the drivers which cannot change the virtual size?
> > > > >
> > > > > Are you volunteering to field that audit and type all the patches?
> > > >
> > > > Note that at least efifb, vesafb and offb seem to get this wrong. I didn't
> > > > bother checking any of the non-fw drivers. Iow there is a _lot_ of work in
> > > > your nack.
> > >
> > > Please don't spread FUD: efifb, vesafb and offb do not implement
> > > fb_ops.fb_check_var(), so they are not affected.
> >
> > Hm I missed that early out. I'll do a patch to fix the drm fb helpers,
> > as mentioned in the other thread I don't think we can actually just
> > delete that because it would short-circuit out the fb_set_par call
> > too.
>
> As I said to the other thread earlier today[1], I think we can keep
> the .fb_set_par() implementation.
> There's just no point in providing a .fb_check_var() callback if
> you don't support changing the video mode.

If you don't have check_var then set_par just isn't called. Or am
blind once more, not the first time today.

And see the big comment in the drm set_par implementation, we can't
ditch that because uabi fun in how the fbdev vs kms interactions are
handled.
-Daniel

> [1] https://lore.kernel.org/all/CAMuHMdUaHd1jgrsCSxCqF-HP2rAo2ODM_ZOjhk7Q4vjuqvt36w@mail.gmail.com
> 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



-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch