using WdfExecutionLevelPassive for forcing EvtIoDeviceControl to be run at passive_level irql

NTDEV
1

Hi experts,
I am talking about a KMDF disk filter here.
I need to call some zwcreatefile and zwreadfile and kewaitfforsingleobject to wait for some event in my EvtIoDeviceControl routine.

Is it safe to do this in my device add callback to achieve my goal?

WDF_OBJECT_ATTRIBUTES_INIT_CONTEXT_TYPE(&objAtttributes,
                                        FILTER_DEVICE_CONTEXT);

objAtttributes.ExecutionLevel = WdfExecutionLevelPassive;

status = WdfDeviceCreate(&DeviceInit,
                         &objAtttributes,
                         &wdfDevice);

If it does the job, is there any overhead or consequences to forcing WdfExecutionLevelPassive?
Should I consider the device stack I am filtering or other filters too? Or I can freely force my callback being called in passive_level. (looks like kind of magic to me)

2

Is it safe to do this in my device add callback to achieve my goal

Yes. By doing this you’re establishing a “PASSIVE_LEVEL execution constraint.”

I can freely force my callback being called in passive_level.

If you need to be at IRQL PASSIVE_LEVEL, then you need to be at IRQL PASSIVE_LEVEL. The device stack doesn’t matter.

It’s not magic… the Framework simply looks at the current IRQL before it calls your EvtIoXxx Event Processing Callbacks. If the IRQL is > PASSIVE_LEVEL, and you’ve established a PASSIVE_LEVEL execution constraint, then the Framework sends your callback to a worker thread (thus, causing the callback to happen at IRQL PASSIVE_LEVEL as you required).

There’s no extra overhead (then majority of the time) when your EvtIoXxx Event Processing Callback is already called at IRQL PASSIVE_LEVEL.

So, yup… go for it.

Peter

3

Thanks for the answer.
Actually since I was studying the IRQL lately and it was said that one should not lower the IRQL so I thought there might be some magic here.
Well I was aware of the other solution and was planning if this did not work for me I should pass the work to a worker thread. I just do not know why I did not think the Microsoft developers might be as clever as me to know this and concluded they would resort to magic!