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

[Zwane Mwaikambo]

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.


> > 
> > 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;
> > }
> > 
> > 
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>