Re: BUG: unable to handle kernel paging request at 6b6b6bcb (v4l2_device_disconnect+0x11/0x30)

[Hans Verkuil]

On Monday, September 05, 2011 11:59:03 Laurent Pinchart wrote:
> Hi Hans,
> 
> On Friday 02 September 2011 09:29:08 Sitsofe Wheeler wrote:
> > On Fri, Sep 02, 2011 at 01:35:49PM +0800, Dave Young wrote:
> > > On Fri, Sep 2, 2011 at 12:59 PM, Dave Young wrote:
> > > > On Fri, Sep 2, 2011 at 3:10 AM, Sitsofe Wheeler wrote:
> > > >> On Thu, Sep 01, 2011 at 05:02:51PM +0800, Dave Young wrote:
> > > >>> On Tue, Aug 30, 2011 at 4:48 AM, Sitsofe Wheeler wrote:
> > > >>> > I managed to produce an oops in 3.1.0-rc3-00270-g7a54f5e by
> > > >>> > unplugging a
> > > >>> 
> > > >>> > USB webcam. See below:
> > > >>> Could you try the attached patch?
> > > >> 
> > > >> This patch fixed the oops but extending the sequence (enable camera,
> > > >> start cheese, disable camera, watch cheese pause, enable camera, quit
> > > >> cheese, start cheese) causes the following "poison overwritten"
> > > >> warning
> > > > 
> > > >> to appear:
> > > > It seems another bug, I can reproduce this as well.
> > > > 
> > > > uvc_device is freed in uvc_delete,
> > > > 
> > > > struct v4l2_device vdev is the member of struct uvc_device, so vdev is
> > > > also freed. Later v4l2_device operations on vdev will overwrite the
> > > > poison memory area.
> > > 
> > > Please try attached patch on top of previous one,  in this patch I
> > > move v4l2_device_put after vdev->release in function
> > > v4l2_device_release
> > > 
> > > Not sure if this is a right fix, comments?
> 
> (inlining the patch's contents)
> 
> > > diff --git a/drivers/media/video/v4l2-dev.c b/drivers/media/video/v4l2-
> > > dev.c
> > > index 98cee19..541dba3 100644
> > > --- a/drivers/media/video/v4l2-dev.c
> > > +++ b/drivers/media/video/v4l2-dev.c
> > > @@ -172,13 +172,14 @@ static void v4l2_device_release(struct device *cd)
> > >  		media_device_unregister_entity(&vdev->entity);
> > >  #endif
> > >  
> > > +	/* Decrease v4l2_device refcount */
> > > +	if (vdev->v4l2_dev)
> > > +		v4l2_device_put(vdev->v4l2_dev);
> > > +
> > >  	/* Release video_device and perform other
> > >  	   cleanups as needed. */
> > >  	vdev->release(vdev);
> > >  
> > > -	/* Decrease v4l2_device refcount */
> > > -	if (vdev->v4l2_dev)
> > > -		v4l2_device_put(vdev->v4l2_dev);
> > >  }
> > >  
> > >  static struct class video_class = {
> 
> v4l2_device_put() got introduced in commit 
> bedf8bcf6b4f90a6e31add3721a2e71877289381 ("v4l2-device: add kref and a release 
> function"). If I understand its purpose correctly, drivers that use a 
> v4l2_device instance should use the v4l2_device release callback to release 
> device structures instead of counting video_device release callbacks manually. 
> In that case I think the v4l2-dev.c code is correct, and all drivers that use 
> v4l2_device should be fixed.
> 
> The above patch fixes the problem in a central location, but seems to defeat 
> the original purpose of v4l2_device_get/put().
> 
> Hans, could you please comment on that ?

The original order is correct, but what I missed is that for drivers that release
(free) everything in the videodev release callback the v4l2_device struct is
also freed and v4l2_device_put will fail.

To fix this, add this code just before the vdev->release call:

	/* Do not call v4l2_device_put if there is no release callback set. */
	if (v4l2_dev->release == NULL)
		v4l2_dev = NULL;

If there is no release callback, then the refcounting is pointless anyway.

This should work.

Regards,

	Hans

> 
> > > 
> > > >> [  191.240695] uvcvideo: Found UVC 1.00 device CNF7129 (04f2:b071)
> > > >> [  191.277965] input: CNF7129 as
> > > >> /devices/pci0000:00/0000:00:1d.7/usb1/1-8/1-8:1.0/input/input9 [
> > > >>  220.287366]
> > > >> =====================================================================
> > > >> ======== [  220.287379] BUG kmalloc-512: Poison overwritten
> > > >> [  220.287384]
> > > >> ---------------------------------------------------------------------
> > > >> -------- [  220.287387]
> > > >> [  220.287394] INFO: 0xec90f150-0xec90f150. First byte 0x6a instead of
> > > >> 0x6b [  220.287410] INFO: Allocated in uvc_probe+0x54/0xd50
> > > >> age=210617 cpu=0 pid=16 [  220.287421]  T.974+0x29d/0x5e0
> > > >> [  220.287427]  kmem_cache_alloc+0x167/0x180
> > > >> [  220.287433]  uvc_probe+0x54/0xd50
> > > >> [  220.287441]  usb_probe_interface+0xd5/0x1d0
> > > >> [  220.287448]  driver_probe_device+0x80/0x1a0
> > > >> [  220.287455]  __device_attach+0x41/0x50
> > > >> [  220.287460]  bus_for_each_drv+0x53/0x80
> > > >> [  220.287466]  device_attach+0x89/0xa0
> > > >> [  220.287472]  bus_probe_device+0x25/0x40
> > > >> [  220.287478]  device_add+0x5a9/0x660
> > > >> [  220.287484]  usb_set_configuration+0x562/0x670
> > > >> [  220.287491]  generic_probe+0x36/0x90
> > > >> [  220.287497]  usb_probe_device+0x24/0x50
> > > >> [  220.287503]  driver_probe_device+0x80/0x1a0
> > > >> [  220.287509]  __device_attach+0x41/0x50
> > > >> [  220.287515]  bus_for_each_drv+0x53/0x80
> > > >> [  220.287522] INFO: Freed in uvc_delete+0xfe/0x110 age=22 cpu=0
> > > >> pid=1645 [  220.287530]  __slab_free+0x1f8/0x300
> > > >> [  220.287536]  kfree+0x100/0x140
> > > >> [  220.287541]  uvc_delete+0xfe/0x110
> > > >> [  220.287547]  uvc_release+0x25/0x30
> > > >> [  220.287555]  v4l2_device_release+0x9d/0xc0
> > > >> [  220.287560]  device_release+0x19/0x90
> > > >> [  220.287567]  kobject_release+0x3c/0x90
> > > >> [  220.287573]  kref_put+0x2c/0x60
> > > >> [  220.287578]  kobject_put+0x1d/0x50
> > > >> [  220.287587]  put_device+0xf/0x20
> > > >> [  220.287593]  v4l2_release+0x56/0x60
> > > >> [  220.287599]  fput+0xcc/0x220
> > > >> [  220.287605]  filp_close+0x44/0x70
> > > >> [  220.287613]  put_files_struct+0x158/0x180
> > > >> [  220.287619]  exit_files+0x40/0x50
> 
> [snip]
> 
> 
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