將內容與數據流產生關聯

對於在支持資料流的篩選層處理資料的呼叫端，呼叫驅動程式可以將上下文與每個資料流相關聯。 這類內容對篩選引擎而言是不透明的。 圖說文字的 classifyFn 註解函式可以利用這個上下文，在篩選引擎下次為該數據流呼叫時，儲存專屬於該數據流的狀態資訊。 篩選引擎會透過 flowContext 參數，將此上下文傳遞到 callout 的 classifyFn 函式。 如果沒有與數據流相關聯的內容，flowContext 參數為零。

若要將上下文與數據流關聯，標註的 classifyFn 標註函式會呼叫 FwpsFlowAssociateContext0 函式。 例如：

// Context structure to be associated with data flows
typedef struct FLOW_CONTEXT_ {
  .
  .  // Driver-specific content
  .
} FLOW_CONTEXT, *PFLOW_CONTEXT;

#define FLOW_CONTEXT_POOL_TAG 'fcpt'

// classifyFn callout function
VOID NTAPI
 ClassifyFn(
    IN const FWPS_INCOMING_VALUES0  *inFixedValues,
    IN const FWPS_INCOMING_METADATA_VALUES0  *inMetaValues,
    IN OUT VOID  *layerData,
    IN const FWPS_FILTER0  *filter,
    IN UINT64  flowContext,
    IN OUT FWPS_CLASSIFY_OUT  *classifyOut
  )
{
  PFLOW_CONTEXT context;
  UINT64 flowHandle;
  NTSTATUS status;

  ...

  // Check for the flow handle in the metadata
  if (FWPS_IS_METADATA_FIELD_PRESENT(
      inMetaValues,
      FWPS_METADATA_FIELD_FLOW_HANDLE))
  {
    // Get the flow handle
    flowHandle = inMetaValues->flowHandle;

    // Allocate the flow context structure
    context =
      (PFLOW_CONTEXT)ExAllocatePoolWithTag(
        NonPagedPool,
        sizeof(FLOW_CONTEXT),
        FLOW_CONTEXT_POOL_TAG
      );

    // Check the result of the memory allocation
    if (context == NULL) 
    {
 
      // Handle memory allocation error
      ...
    }
    else
    {

      // Initialize the flow context structure
      ...

      // Associate the flow context structure with the data flow
      status = FwpsFlowAssociateContext0(
                flowHandle,
                FWPS_LAYER_STREAM_V4,
                calloutId,
                (UINT64)context
              );

      // Check the result
      if (status != STATUS_SUCCESS)
      {
        // Handle error
        ...
      }
    }
  }

  ...

}

如果內容已經與數據流相關聯，必須先移除該內容，才能將新內容與數據流產生關聯。 若要從數據流中移除上下文，註解的 classifyFn 註解函式會呼叫 FwpsFlowRemoveContext0 函式。 例如：

// Context structure to be associated with data flows
typedef struct FLOW_CONTEXT_ {
  ...
} FLOW_CONTEXT, *PFLOW_CONTEXT;

#define FLOW_CONTEXT_POOL_TAG 'fcpt'

// classifyFn callout function
VOID NTAPI
 ClassifyFn(
    IN const FWPS_INCOMING_VALUES0  *inFixedValues,
    IN const FWPS_INCOMING_METADATA_VALUES0  *inMetaValues,
    IN OUT VOID  *layerData,
    IN const FWPS_FILTER0  *filter,
    IN UINT64  flowContext,
    OUT FWPS_CLASSIFY_OUT  *classifyOut
  )
{
  PFLOW_CONTEXT context;
  UINT64 flowHandle;
  NTSTATUS status;

  ...

  // Check for the flow handle in the metadata
 if (FWPS_IS_METADATA_FIELD_PRESENT(
    inMetaValues,
    FWPS_METADATA_FIELD_FLOW_HANDLE))
  {
    // Get the flow handle
     flowHandle = inMetaValues->flowHandle;

    // Check whether there is a context associated with the data flow
     if (flowHandle != 0) 
     {
        // Get a pointer to the flow context structure
        context = (PFLOW_CONTEXT)flowContext;

        // Remove the flow context structure from the data flow
        status = FwpsFlowRemoveContext0(
                  flowHandle,
                  FWPS_LAYER_STREAM_V4,
                  calloutId
                );

      // Check the result
      if (status != STATUS_SUCCESS)
      {
        // Handle error
        ...
      }

      // Cleanup the flow context structure
      ...

      // Free the memory for the flow context structure
      ExFreePoolWithTag(
        context,
        FLOW_CONTEXT_POOL_TAG
        );
    }
  }

  ...

}

在上述範例中，calloutId 變數包含這個標註的執行時識別符。 執行時識別碼與回呼驅動程式在向篩選引擎註冊回呼時傳回給驅動程式的識別符相同。