[PATCH] drm: assure aux_dev is nonzero before using it

[Daniel Vetter]

On Wed, Aug 12, 2020 at 12:16 AM Zwane Mwaikambo <zwanem at gmail.com> wrote:
>
> On Tue, 11 Aug 2020, Daniel Vetter wrote:
>
> > On Mon, Aug 10, 2020 at 10:11:50AM -0700, Zwane Mwaikambo wrote:
> > > Hi Folks,
> > >     I know this thread eventually dropped off due to not identifying
> > > the underlying issue. It's still occuring on 5.8 and in my case it
> > > happened because the udev device nodes for the DP aux devices were not
> > > cleaned up whereas the kernel had no association with them. I can
> > > reproduce the bug just by creating a device node for a non-existent minor
> > > device and calling open().
> >
> > Hm I don't have that thread anymore, but generally these bugs are solved
> > by not registering the device before it's ready for use. We do have
> > drm_connector->late_register for that stuff. Just a guess since I'm not
> > seeing full details here.
>
> In this particular case, the physical device disappeared before the nodes
> were cleaned up. It involves putting a computer to sleep with a monitor
> plugged in and then waking it up with the monitor unplugged.

We also have early_unregister for the reverse, but yes this sounds
more tricky ... Adding Lyude who's been working on way too much
lifetime fun around dp recently.
-Daniel

>
>
> > >
> > > To me it still makes sense to just check aux_dev because the chardev has
> > > no way to check before calling.
> > >
> > > (gdb) list *drm_dp_aux_dev_get_by_minor+0x29
> > > 0x17b39 is in drm_dp_aux_dev_get_by_minor (drivers/gpu/drm/drm_dp_aux_dev.c:65).
> > > 60      static struct drm_dp_aux_dev *drm_dp_aux_dev_get_by_minor(unsigned index)
> > > 61      {
> > > 62              struct drm_dp_aux_dev *aux_dev = NULL;
> > > 63
> > > 64              mutex_lock(&aux_idr_mutex);
> > > 65              aux_dev = idr_find(&aux_idr, index);
> > > 66              if (!kref_get_unless_zero(&aux_dev->refcount))
> > > 67                      aux_dev = NULL;
> > > 68              mutex_unlock(&aux_idr_mutex);
> > > 69
> > > (gdb) p/x &((struct drm_dp_aux_dev *)(0x0))->refcount
> > > $8 = 0x18
> > >
> > > static int auxdev_open(struct inode *inode, struct file *file)
> > > {
> > >     unsigned int minor = iminor(inode);
> > >     struct drm_dp_aux_dev *aux_dev;
> > >
> > >     aux_dev = drm_dp_aux_dev_get_by_minor(minor);
> > >     if (!aux_dev)
> > >         return -ENODEV;
> > >
> > >     file->private_data = aux_dev;
> > >     return 0;
> > > }
> > >
> > >
> > > _______________________________________________
> > > dri-devel mailing list
> > > dri-devel at lists.freedesktop.org
> > > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> >
> >



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