Re: [PATCH] USB: USBHID: Fix race between disconnect and hiddev_ioctl

[Valentine Barshak]

Dmitry Torokhov wrote:
> On Sat, Dec 04, 2010 at 09:22:23PM +0100, Oliver Neukum wrote:
>   
> > Am Samstag, 4. Dezember 2010, 00:16:12 schrieb Dmitry Torokhov:
> >     
> > > > Since disconnect can happen at any time, we can't initialize
> > > > struct hid_device *hid = hiddev->hid at the beginning of ioctl
> > > > and then use it.
> > > >
> > > > This change checks hiddev->exist flag while holding
> > > > the existancelock and uses hid_device only if it exists.
> > > >         
> > > Why didn't you take the lock and check hiddev->exist at the beginning of
> > > ioctl handler instead of pushing it down into individual command
> > > handlers? I guess it would slow down HIDIOCGVERSION but I think we could
> > > pay this price for code that is more clear ;)
> > >       
> > Strictly speaking you'd change the semantics. Right now you can execute
> > the ioctl even if you know you are holding an fd to a disconnected device
> > open.
> >     
>
> No, I do not think I would. I do not believe that the availability for
> HIDIOCGVERSION on disconnected device is spelled out in API/ABI spec. We
> only know that ioctl will either succeed or appropriate error code is
> returned. The fact that right now HIDIOCGVERSION is available on
> disconnected devices is just an implementation detail subject to change.
>
>   
It's not just HIDIOCGVERSION. A couple of other commands 
(HIDIOCGFLAG/HIDIOCSFLAG) didn't check device existence in the first 
place either.
Current implementation depends on when the device is actually removed.
If it has been removed before the hiddev_ioctl(), hiddev_ioctl() returns 
-EIO.
If the device is removed while hiddev_ioctl() is in progress, we either 
do not notice that and handle HIDIOCGVERSION and HIDIOCGFLAG/HIDIOCSFLAG
just fine, or return -ENODEV.
I'll submit a patch in a bit that applies on top of the "[PATCH] USB: 
USBHID: Fix race between disconnect and hiddev_ioctl" and makes the 
hiddev_ioctl() check
device existence before processing the command and always return -ENODEV 
in case the device has been removed.
Thanks,
Val.

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