This topic describes how a User-Mode Driver Framework (UMDF) driver supports kernel-mode clients, starting in UMDF version 2.
A kernel-mode client is a kernel-mode driver that sends I/O requests to your UMDF driver. The kernel-mode driver might be above the UMDF driver, in the same device stack, or it might be in a different device stack.
The kernel-mode driver can forward I/O requests that it has received from a user-mode application, or can create new I/O requests and send them to the user-mode driver.
To enable a UMDF driver's support for kernel-mode clients, the INF file of the UMDF driver must include a UmdfKernelModeClientPolicy directive in its INF DDInstall.WDF section.
The framework provides two methods that are useful to drivers that support kernel-mode clients. A driver can call the WdfRequestGetRequestorMode method to determine whether an I/O request came from kernel mode or user mode. If the I/O request came from user mode, the driver can call WdfRequestIsFromUserModeDriver to determine whether the request came from an application or another user-mode driver.
Restrictions on kernel-mode drivers
A UMDF driver can process I/O requests from a kernel-mode driver only if the kernel-mode driver meets the following requirements:
The kernel-mode driver must be running at IRQL = PASSIVE_LEVEL when it sends the I/O request.
Unless the driver has set the UmdfFileObjectPolicy INF directive to AllowNullAndUnknownFileObjects, each I/O request that a kernel-mode driver sends to a user-mode driver must have an associated file object. The framework must have previously been notified that the I/O manager created the file object. (Such notification causes the framework to call the user-mode driver's EvtDeviceFileCreate callback function, but that callback function is optional.)
The I/O request cannot contain an IRP_MJ_INTERNAL_DEVICE_CONTROL function code.
The I/O request's buffers must not contain pointers to additional information, because the user-mode driver cannot dereference the pointers.
If the I/O request contains an I/O control code that specifies the "neither" buffer access method, the kernel-mode driver must send the I/O request in the process context of the application that created the I/O request. For more information about how to support the "neither" method in a UMDF driver, see Managing Buffer Access Methods in UMDF Drivers.
The UMDF driver might modify an I/O request's output data, in user mode. Therefore, the kernel-mode driver must validate any output data that it receives from the user-mode driver.
The kernel-mode client should typically validate the Information value that a UMDF driver passes to WdfRequestCompleteWithInformation. If the client is a KMDF driver, it can call WdfRequestGetCompletionParams to obtain this information in an IO_STATUS_BLOCK structure.
Typically, the framework does not validate the information value that a UMDF driver passes to WdfRequestCompleteWithInformation. (This parameter usually specifies the number of transferred bytes.) The framework validates the information value only for output buffers, and only for the buffered I/O data access method. (For example, the framework verifies that the number of transferred bytes does not exceed the output buffer size of a read operation, if the access method is buffered I/O.)