Книга: Microsoft Windows Embedded CE 6.0 Exam Preparation Kit

Asynchronous Access

Asynchronous Access

Asynchronous buffer access assumes that multiple caller and driver threads access the buffer sequentially or concurrently. Both scenarios present challenges. In the sequential access scenario, the caller thread might exit before the driver thread has finished its processing. By calling the marshaling helper function CeAllocAsynchronousBuffer, you must re-marshal the buffer after it was marshaled by CeOpenCallerBuffer to ensure in the driver that the buffer remains available even if the caller's address space is unavailable. Do not forget to call CeFreeAsynchronousBuffer after the driver has finished its processing.

To ensure that your device driver works in kernel and user mode, use the following approach to support asynchronous buffer access:

? Pointer parameters Pass pointer parameters as scalar DWORD values and then call CeOpenCallerBuffer and CeAllocAsynchronousBuffer to perform access checks and marshaling. Note that you cannot call CeAllocAsynchronousBuffer on a pointer parameter in user-mode code or perform asynchronous write-back of O_PTR or IO_PTR values.

? Embedded pointers Pass embedded pointers to CeOpenCallerBuffer and CeAllocAsynchronousBuffer to perform access checks and marshaling.

To address the second scenario of concurrent access, you must create a secure copy of the buffer after marshaling, as mentioned earlier. Calling CeOpenCallerBuffer with the ForceDuplicate parameter set to TRUE and CeCloseCallerBuffer is one option. Another is to call CeAllocDuplicateBuffer and CeFreeDuplicateBuffer for buffers referenced by parameter pointers. You can also copy a pointer or buffer into a stack variable or allocate heap memory by using VirtualAlloc and then use memcpy to copy the caller's buffer. Keep in mind that if you do not create a secure copy, you're leaving in a vulnerability that a malicious application could use to take control of the system.

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