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 [Driver Products]        Driver Technical Tips
  · DriverStudio          Using IOCTL METHOD_NEITHER
  · DriverBundle
  · Previews              If you read our earlier tip on METHOD_IN/OUT_DIRECT, you may have been left
  · Compatibility         wondering about the details of the other IOCTL methods, METHOD_BUFFERED, and
 [Downloads]  ·           METHOD_NEITHER. This tip explores the latter, lesser known of the two,
                          METHOD_NEITHER.
 Wizards
  · Utilities             If an IOCTL code passed to DeviceIoControl is defined using METHOD_NEITHER,
  · NT source             the operating system performs no address translation or validation of
 examples                 pointers passed in parameters InputBuffer and OutputBuffer. Because there is
  · VxD source            relatively less work for the system to do, it can deliver an IRP that uses
 examples                 METHOD_NEITHER to a driver faster than it can IRPs that use the other two
  · WDM source            IOCTL methods. However, the burden is now on the driver to ensure that the
 examples                 pointers passed to it are valid. In addition, the driver can use the raw
 [Resources]  ·           pointers only in the context of the calling process.
 Technical papers
  · Useful links          As shown in the diagram, the pointer passed as parameter InputBuffer appears
  · Technical tips        in field IoGetCurrentIrpStackLocation(pIrp)->
 [Support]  ·             Parameters.DeviceIoControl.Type3InputBuffer. The pointer passed as parameter
                          OutputBuffer appears in pIrp-> UserBuffer.
 Support
  · Knowledge base        Because the operating system has not validated the pointers passed to a
  · Problem               driver in a METHOD_NEITHER IRP, it is crucial to guard against exceptions
 submission               that could result from accessing these pointers. A driver can easily trap an
  · Product               exception that results from dereferencing a bad pointer by bracketing access
 registration             to the pointer in a _try/_except block. Here is some Driver::Works code that
  · Release notes         illustrates the basic idea:
 [Shop NuMega]  ·
 Buy it!                  NTSTATUS MyDevice::DeviceControl(KIrp I)
  · Price list            {
  · How to buy
  · Sales offices                 switch (I.IoctlCode())
                                  {
                                  case IOCTL_NEITHER_OPERATION:
 [Y2K Compliance]                         PINPUT_DATA pInput = PINPUT_DATA(I.IoctlType3InputBuffer());
                                          PULONG pOutput = I.UserBuffer();

 [More information]                       _try
                                          {
                                                  NTSTATUS status=Process(pInput, pOutput);
                                                  return I.Complete(status);
                                          }
                                          _except ( EXCEPTION_EXECUTE_HANDLER )
                                          {
                                                  I.Information()=0;
                                                  return I.Complete(STATUS_ACCESS_DENIED);
                                          }
                                  // etc.
                                  }
                          }

                          The other important thing to remember about pointers in a METHOD_NEITHER IRP
                          is that they may be used only in the context of the calling process. If your
                          driver serializes requests through StartIo, or ever needs to access the
                          buffers from an arbitrary thread context or at IRQL > PASSIVE_LEVEL, then
                          your driver must lock down the buffers and map them to system space. The
                          system services you'll need are MmCreateMdl, MmProbeAndLockPages, and
                          MmGetSystemAddressForMdl.

                          In summary, METHOD_NEITHER can be useful for certain kinds of requests. The
                          precaution of structured exception handling (_try/_except) is required
                          whenever accessing pointers that have not been validated by the system. If
                          the driver can process the request quickly and without synchronization, then
                          METHOD_NEITHER provides an easy and efficient mechanism. However, most of
                          the time it is better to take advantage of the operating system's facilities
                          for buffer management, using either METHOD_IN/OUT_DIRECT, or
                          METHOD_BUFFERED.

                          Back to technical tip start page.

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