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

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