[PATCH] drm/vkms: Don't warn hrtimer_forward_now failure.

[Daniel Vetter]

On Mon, May 25, 2020 at 5:34 PM Tetsuo Handa
<penguin-kernel at i-love.sakura.ne.jp> wrote:
>
> On 2020/05/26 0:21, Daniel Vetter wrote:
> > On Mon, May 25, 2020 at 11:38:49PM +0900, Tetsuo Handa wrote:
> >> Commit 3a0709928b172a41 ("drm/vkms: Add vblank events simulated by
> >> hrtimers") introduced ret_overrun variable. And that variable was an
> >> unused-but-set-variable until commit 09ef09b4ab95dc40 ("drm/vkms:
> >> WARN when hrtimer_forward_now fails") added WARN_ON(ret_overrun != 1).
> >>
> >> Now, syzbot is hitting this WARN_ON() using a simple reproducer that
> >> does open("/dev/dri/card1") followed by ioctl(DRM_IOCTL_WAIT_VBLANK),
> >> and a debug printk() patch says that syzbot is getting
> >>
> >>    output->vblank_hrtimer.base->get_time()=93531904774 (which is uptime)
> >>    output->period_ns=0
> >>    ret_overrun=216994
> >>
> >> . I can't understand what "verify the hrtimer_forward_now return" in
> >> that commit wants to say. hrtimer_forward_now() must return, and the
> >> return value of hrtimer_forward_now() is not a boolean. Why comparing
> >> with 1 ? Anyway, this failure is not something that worth crashing the
> >> system. Let's remove the ret_overrun variable and WARN_ON() test.
> >
> > Uh we're not crashing the system, it's a warning backtrace.
>
> syzbot uses panic_on_warn=1, and this bug is currently the 8th top crasher.
>
> >
> > And we've spent a few months hunting the races here, so just removing that
> > check isn't really a good idea. The correct thing to do is figure out why
> > we're hitting this. It could be that we're having a missing check
> > somewhere, or missing initialization, and that's what syzbot is hitting.
> > Removing this check here just papers over the bug.
>
> Here is a reproducer which syzbot is using.
>
> ----------
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <fcntl.h>
> #include <sys/ioctl.h>
> #include <drm/drm.h>
>
> int main(int argc, char *argv[])
> {
>         union drm_wait_vblank arg;
>         int fd = open("/dev/dri/card1", O_RDONLY);
>
>         arg.request.type = 0;
>         arg.request.sequence = 0xffff;
>         arg.request.signal = 0x21;
>         ioctl(fd, DRM_IOCTL_WAIT_VBLANK, &arg);
>         return 0;
> }
> ----------
>
> Debug printk() patch shows that hrtimer_forward_now() can return larger than 1.
> What is the reason you are expecting hrtimer_forward_now() to always return 1 ?

The timer simulates display refresh running at 60Hz. If we miss one,
the simulation is not accurate against hardware. Missing a few
thousands either means there's a bug somewhere, or the machine has
missed a few seconds of worth of timers (which is equally bad).
-Daniel

>
> >
> > If the vkms driver is loaded, and there's nothing else going on, then what
> > I expect to happen is that this virtual hw is entirely off. And in that
> > case, the vblank ioctl should be rejected outright. So there's definitely
> > something fishy going on to begin with.
> >
> > If otoh the virtual hw is somehow on (maybe fbcon gets loaded, no idea),
> > then the vblank wait shouldn't just blow up like this.
> > -Daniel
> >
> >>
> >> Link: https://syzkaller.appspot.com/bug?id=0ba17d70d062b2595e1f061231474800f076c7cb
> >> Signed-off-by: Tetsuo Handa <penguin-kernel at I-love.SAKURA.ne.jp>
> >> Reported-by: syzbot+0871b14ca2e2fb64f6e3 at syzkaller.appspotmail.com
> >> Fixes: 09ef09b4ab95dc40 ("drm/vkms: WARN when hrtimer_forward_now fails")
> >> ---
> >>  drivers/gpu/drm/vkms/vkms_crtc.c | 5 +----
> >>  1 file changed, 1 insertion(+), 4 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/vkms/vkms_crtc.c b/drivers/gpu/drm/vkms/vkms_crtc.c
> >> index ac85e17428f8..cc1811ce6092 100644
> >> --- a/drivers/gpu/drm/vkms/vkms_crtc.c
> >> +++ b/drivers/gpu/drm/vkms/vkms_crtc.c
> >> @@ -13,12 +13,9 @@ static enum hrtimer_restart vkms_vblank_simulate(struct hrtimer *timer)
> >>                                                vblank_hrtimer);
> >>      struct drm_crtc *crtc = &output->crtc;
> >>      struct vkms_crtc_state *state;
> >> -    u64 ret_overrun;
> >>      bool ret;
> >>
> >> -    ret_overrun = hrtimer_forward_now(&output->vblank_hrtimer,
> >> -                                      output->period_ns);
> >> -    WARN_ON(ret_overrun != 1);
> >> +    hrtimer_forward_now(&output->vblank_hrtimer, output->period_ns);
> >>
> >>      spin_lock(&output->lock);
> >>      ret = drm_crtc_handle_vblank(crtc);
> >> --
> >> 2.18.2
> >>
> >
>


-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch