Các cấu trúc dữ liệu (Data Structures )

Two data structures are crucial to the handling of I/O requests: the I/O request packet itself and the IO_STACK_LOCATION structure. I’ll describe both structures in this section.

Structure of an IRP

Figure 5-1 illustrates the IRP data structure, with opaque fields shaded in the usual convention of this book. A brief description of the important fields follows.

MdlAddress (PMDL) is the address of a memory descriptor list (MDL) describing the user-mode buffer associated with this request. The I/O Manager creates this MDL for IRP_MJ_READ and IRP_MJ_WRITE requests if the topmost device object’s flags indicate DO_DIRECT_IO. It creates an MDL for the output buffer used with an IRP_MJ_DEVICE_CONTROL request if the control code indicates METHOD_IN_DIRECT or METHOD_OUT_DIRECT. The MDL itself describes the user-mode virtual buffer and also contains the physical addresses of locked pages containing that buffer. A driver has to do additional work, which can be quite minimal, to actually access the user-mode buffer.

Figure 5-1. I/O request packet data structure.

Flags (ULONG) contains flags that a device driver can read but not directly alter. None of these flags are relevant to a Windows Driver Model (WDM) driver.

AssociatedIrp (union) is a union of three possible pointers. The alternative that a typical WDM driver might want to access is named AssociatedIrp.SystemBuffer. The SystemBuffer pointer holds the address of a data buffer in nonpaged kernel-mode memory. For IRP_MJ_READ and IRP_MJ_WRITE operations, the I/O Manager creates this data buffer if the topmost device object’s flags specify DO_BUFFERED_IO. For IRP_MJ_DEVICE_CONTROL operations, the I/O Manager creates this buffer if the I/O control function code indicates that it should. (See Chapter 9.) The I/O Manager copies data sent by user-mode code to the driver into this buffer as part of the process of creating the IRP. Such data includes the data involved in a WriteFile call or the so-called input data for a call to DeviceIoControl. For read requests, the device driver fills this buffer with data; the I/O Manager later copies the buffer back to the user-mode buffer. For control operations that specify METHOD_BUFFERED, the driver places the so-called output data in this buffer, and the I/O Manager copies it to the user-mode output buffer.

IoStatus (IO_STATUS_BLOCK) is a structure containing two fields that drivers set when they ultimately complete a request. IoStatus.Status will receive an NTSTATUS code, while IoStatus.Information is a ULONG_PTR that will receive an information value whose exact content depends on the type of IRP and the completion status. A common use of the Information field is to hold the total number of bytes transferred by an operation such as IRP_MJ_READ that transfers data. Certain Plug and Play (PnP) requests use this field as a pointer to a structure that you can think of as the answer to a query.

RequestorMode will equal one of the enumeration constants UserMode or KernelMode, depending on where the original I/O request originated. Drivers sometimes inspect this value to know whether to trust some parameters.

PendingReturned (BOOLEAN) is meaningful in a completion routine and indicates whether the next lower dispatch routine returned STATUS_PENDING. This chapter contains a disagreeably long discussion of how to use this flag.

Cancel (BOOLEAN) is TRUE if IoCancelIrp has been called to cancel this request and FALSE if it hasn’t (yet) been called. IRP cancellation is a relatively complex topic that I’ll discuss fully later on in this chapter (in “Cancelling I/O Requests”).

CancelIrql (KIRQL) is the interrupt request level (IRQL) at which the special cancel spin lock was acquired. You reference this field in a cancel routine when you release the spin lock.

CancelRoutine (PDRIVER_CANCEL) is the address of an IRP cancellation routine in your driver. You use IoSetCancelRoutine to set this field instead of modifying it directly.

UserBuffer (PVOID) contains the user-mode virtual address of the output buffer for an IRP_MJ_DEVICE_CONTROL request for which the control code specifies METHOD_NEITHER. It also holds the user-mode virtual address of the buffer for read and write requests, but a driver should usually specify one of the device flags DO_BUFFERED_IO or DO_DIRECT_IO and should therefore not usually need to access the field for reads or writes. When handling a METHOD_NEITHER control operation, the driver can create its own MDL using this address.

Tail.Overlay is a structure within a union that contains several members potentially useful to a WDM driver. Refer to Figure 5-2 for a map of the Tail union. In the figure, items at the same level as you read left to right are alternatives within a union, while the vertical dimension portrays successive locations within a structure. Tail.Overlay.DeviceQueueEntry (KDEVICE_QUEUE_ENTRY) and Tail.Overlay.DriverContext (PVOID[4]) are alternatives within an unnamed union within Tail.Overlay. The I/O Manager uses DeviceQueueEntry as a linking field within the standard queue of requests for a device. The cancel-safe queuing routines IoCsqXxx use the last entry in the DriverContext array. If these system usages don’t get in your way, at moments when the IRP is not in some queue that uses this field and when you own the IRP, you can use the four pointers in DriverContext in any way you please. Tail.Overlay.ListEntry (LIST_ENTRY) is available for you to use as a linking field for IRPs in any private queue you choose to implement.

CurrentLocation (CHAR) and Tail.Overlay.CurrentStackLocation (PIO_STACK_LOCATION) aren’t documented for use by drivers because support functions such as IoGetCurrentIrpStackLocation can be used instead. During debugging, however, it might help you to realize that CurrentLocation is the index of the current I/O stack location and CurrentStackLocation is a pointer to it.

Figure 5-2. Map of the Tail union in an IRP.

The I/O Stack

Whenever any kernel-mode program creates an IRP, it also creates an associated array of IO_STACK_LOCATION structures: one stack location for each of the drivers that will process the IRP and sometimes one more stack location for the use of the originator of the IRP. (See Figure 5-3.) A stack location contains type codes and parameter information for the IRP as well as the address of a completion routine. Refer to Figure 5-4 for an illustration of the stack structure.

Figure 5-3. Parallelism between driver and I/O stacks.

A final consideration in calling the two synchronous IRP routines is that you can’t create just any kind of IRP using these routines. See Table 5-1 for the details. A common trick for creating another kind of synchronous IRP is to ask for an IRP_MJ_SHUTDOWN, which has no parameters, and then alter the MajorFunction code in the first stack location.

Table 5-1. Synchronous IRP Types
Support Function	Types of IRP You Can Create
IoBuildSynchronousFsdRequest	IRP_MJ_READ IRP_MJ_WRITE IRP_MJ_FLUSH_BUFFERS IRP_MJ_SHUTDOWN IRP_MJ_PNP IRP_MJ_POWER (but only for IRP_MN_POWER_SEQUENCE)
IoBuildDeviceIoControlRequest	IRP_MJ_DEVICE_CONTROL IRP_MJ_INTERNAL_DEVICE_CONTROL

Creating Asynchronous IRPs

The other two IRP creation functions—IoBuildAsynchronousFsdRequest and IoAllocateIrp—create an asynchronous IRP. Asynchronous IRPs don’t belong to the creating thread, and the I/O Manager doesn’t schedule an APC and doesn’t clean up when the IRP completes. Consequently:

• When a thread terminates, the I/O Manager doesn’t try to cancel any asynchronous IRPs that you happen to have created in that thread.
• It’s OK to create asynchronous IRPs in an arbitrary or nonarbitrary thread.
• Because the I/O Manager doesn’t do any cleanup when the IRP completes, you must provide a completion routine that will release buffers and call IoFreeIrp to release the memory used by the IRP.
• Because the I/O Manager doesn’t automatically cancel asynchronous IRPs, you might have to provide code to do that when you no longer want the operation to occur.
• Because you don’t wait for an asynchronous IRP to complete, you can create and send one at IRQL <= DISPATCH_LEVEL (assuming, that is, that the driver to which you send the IRP can handle the IRP at elevated IRQL—you must check the specifications for that driver!). Furthermore, it’s OK to create and send an asynchronous IRP while owning a fast mutex.

Refer to Table 5-2 for a list of the types of IRP you can create using the two asynchronous IRP routines. Note that IoBuildSynchronousFsdRequest and IoBuildAsynchronousFsdRequest support the same IRP major function codes.

Table 5-2. Asynchronous IRP Types
Support Function	Types of IRP You Can Create
IoBuildAsynchronousFsdRequest	IRP_MJ_READ IRP_MJ_WRITE IRP_MJ_FLUSH_BUFFERS IRP_MJ_SHUTDOWN IRP_MJ_PNP IRP_MJ_POWER (but only for IRP_MN_POWER_SEQUENCE)
IoAllocateIrp	Any (but you must initialize the MajorFunction field of the first stack location)

IRP-handling scenario numbers 5 and 8 at the end of this chapter contain “cookbook” code for using asynchronous IRPs.

Forwarding to a Dispatch Routine

After you create an IRP, you call IoGetNextIrpStackLocation to obtain a pointer to the first stack location. Then you initialize just that first location. If you’ve used IoAllocateIrp to create the IRP, you need to fill in at least the MajorFunction code. If you’ve used another of the four IRP-creation functions, the I/O Manager might have already done the required initialization. You might then be able to skip this step, depending on the rules for that particular type of IRP. Having initialized the stack, you call IoCallDriver to send the IRP to a device driver:

PDEVICE_OBJECT DeviceObject; // <== somebody gives you this

PIO_STACK_LOCATION stack = IoGetNextIrpStackLocation(Irp);

stack->MajorFunction = IRP_MJ_Xxx;

<other initialization of "stack">NTSTATUS status = IoCallDriver(DeviceObject, Irp);

The first argument to IoCallDriver is the address of a device object that you’ve obtained somehow. Often you’re sending an IRP to the driver under yours in the PnP stack. In that case, the DeviceObject in this fragment is the LowerDeviceObject you saved in your device extension after calling IoAttachDeviceToDeviceStack. I’ll describe some other common ways of locating a device object in a few paragraphs.

The I/O Manager initializes the stack location pointer in the IRP to 1 before the actual first location. Because the I/O stack is an array of IO_STACK_LOCATION structures, you can think of the stack pointer as being initialized to point to the “-1” element, which doesn’t exist. (In fact, the stack “grows” from high toward low addresses, but that detail shouldn’t obscure the concept I’m trying to describe here.) We therefore ask for the “next” stack location when we want to initialize the first one.

What IoCallDriver Does

You can imagine IoCallDriver as looking something like this (but I hasten to add that this is not a copy of the actual source code):

NTSTATUS IoCallDriver(PDEVICE_OBJECT DeviceObject, PIRP Irp)

  {

  IoSetNextIrpStackLocation(Irp);

  PIO_STACK_LOCATION stack = IoGetCurrentIrpStackLocation(Irp);

  stack->DeviceObject = DeviceObject;

  ULONG fcn = stack->MajorFunction;

  PDRIVER_OBJECT driver = DeviceObject->DriverObject;

  return (*driver->MajorFunction[fcn])(DeviceObject, Irp);

  }

As you can see, IoCallDriver simply advances the stack pointer and calls the appropriate dispatch routine in the driver for the target device object. It returns the status code that that dispatch routine returns. Sometimes I see online help requests wherein people attribute one or another unfortunate action to IoCallDriver. (For example, “IoCallDriver is returning an error code for my IRP….”) As you can see, the real culprit is a dispatch routine in another driver.

Locating Device Objects

Apart from IoAttachDeviceToDeviceStack, drivers can locate device objects in at least two ways. I’ll tell you here about IoGetDeviceObjectPointer and IoGetAttachedDeviceReference.

IoGetDeviceObjectPointer

If you know the name of the device object, you can call IoGetDeviceObjectPointer as shown here:

PUNICODE_STRING devname; // <== somebody gives you this

ACCESS_MASK access;      // <== more about this later

PDEVICE_OBJECT DeviceObject;

PFILE_OBJECT FileObject;

NTSTATUS status;

ASSERT(KeGetCurrentIrql() == PASSIVE_LEVEL);

status = IoGetDeviceObjectPointer(devname, access,

  &FileObject, &DeviceObject);

This function returns two pointers: one to a FILE_OBJECT and one to a DEVICE_OBJECT.

To help defeat elevation-of-privilege attacks, specify the most restricted access consistent with your needs. For example, if you’ll just be reading data, specify FILE_READ_DATA.

When you create an IRP for a target you discover this way, you should set the FileObject pointer in the first stack location. Furthermore, it’s a good idea to take an extra reference to the file object until after IoCallDriver returns. The following fragment illustrates both these ideas:

PIRP Irp = IoXxx(...);

PIO_STACK_LOCATION stack = IoGetNextIrpStackLocation(Irp);

ObReferenceObject(FileObject);

stack->FileObject = FileObject;<etc.>

IoCallDriver(DeviceObject, Irp);

ObDereferenceObject(FileObject);

After making this call, don’t use either of the file or device object pointers.

IoGetDeviceObjectPointer performs several steps to locate the two pointers that it returns to you:

1. It uses ZwOpenFile to open a kernel handle to the named device object. Internally, this will cause the Object Manager to create a file object and to send an IRP_MJ_CREATE to the target device. ZwOpenFile returns a file handle.
2. It calls ObReferenceObjectByHandle to get the address of the FILE_OBJECT that the handle represents. This address becomes the FileObject return value.
3. It calls IoGetRelatedDeviceObject to get the address of the DEVICE_OBJECT to which the file object refers. This address becomes the DeviceObject return value.
4. It calls ZwClose to close the handle.

Names for Device Objects

For you to use IoGetDeviceObjectPointer, a driver in the stack for the device to which you want to connect must have named a device object. We studied device object naming in Chapter 2. Recall that a driver might have specified a name in the Device folder in its call to IoCreateDevice, and it might have created one or more symbolic links in the DosDevices folder. If you know the name of the device object or one of the symbolic links, you can use that name in your call to IoGetDeviceObjectPointer.

Mechanically, completing an IRP entails filling in the Status and Information members within the IRP’s IoStatus block and calling IoCompleteRequest. The Status value is one of the codes defined by manifest constants in the DDK header file NTSTATUS.H. Refer to Table 5-3 for an abbreviated list of status codes for common situations. The Information value depends on what type of IRP you’re completing and on whether you’re causing the IRP to succeed or to fail. Most of the time, when you’re causing an IRP to fail (that is, completing it with an error status of some kind), you’ll set Information to 0. When you cause an IRP that involves data transfer to succeed, you ordinarily set the Information field equal to the number of bytes transferred.

Table 5-3. Some Commonly Used NTSTATUS Codes
Status Code	Description
STATUS_SUCCESS	Normal completion.
STATUS_UNSUCCESSFUL	Request failed, but no other status code describes the reason specifically.
STATUS_NOT_IMPLEMENTED	A function hasn’t been implemented.
STATUS_INVALID_HANDLE	An invalid handle was supplied for an ­operation.
STATUS_INVALID_PARAMETER	A parameter is in error.
STATUS_INVALID_DEVICE_REQUEST	The request is invalid for this device.
STATUS_END_OF_FILE	End-of-file marker reached.
STATUS_DELETE_PENDING	The device is in the process of being removed from the system.
STATUS_INSUFFICIENT_RESOURCES	Not enough system resources (often ­memory) to perform an operation.

When you call IoCompleteRequest, you supply a priority boost value to be applied to whichever thread is currently waiting for this request to complete. You normally choose a boost value that depends on the type of device, as suggested by the manifest constant names listed in Table 5-4. The priority adjustment improves the throughput of threads that frequently wait for I/O operations to complete. Events for which the end user is directly responsible, such as keyboard or mouse operations, result in greater priority boosts in order to give preference to interactive tasks. Consequently, you want to choose the boost value with at least some care. Don’t use IO_SOUND_INCREMENT for absolutely every operation a sound card driver finishes, for example—it’s not necessary to apply this extraordinary priority increment to a get-driver-version control request.

Table 5-4. Priority Boost Values for IoCompleteRequest
Manifest Constant	Numeric Priority Boost
IO_NO_INCREMENT	0
IO_CD_ROM_INCREMENT	1
IO_DISK_INCREMENT	1
IO_KEYBOARD_INCREMENT	6
IO_MAILSLOT_INCREMENT	2
IO_MOUSE_INCREMENT	6
IO_NAMED_PIPE_INCREMENT	2
IO_NETWORK_INCREMENT	2
IO_PARALLEL_INCREMENT	1
IO_SERIAL_INCREMENT	2
IO_SOUND_INCREMENT	8
IO_VIDEO_INCREMENT	1

At least one of these three flags must be TRUE. Note that IoSetCompletionRoutine is a macro, so you want to avoid arguments that generate side effects. The three flag arguments and the function pointer, in particular, are each referenced twice by the macro.

IoSetCompletionRoutine installs the completion routine address and context argument in the nextIO_STACK_LOCATION—that is, in the stack location in which the next lower driver will find its parameters. Consequently, the lowest-level driver in a particular stack of drivers doesn’t dare attempt to install a completion routine. Doing so would be pretty futile, of course, because—by definition of lowest-level driver—there’s no driver left to pass the request on to.

CAUTION Recall that you are responsible for initializing the next I/O stack location before you call IoCallDriver. Do this initialization before you install a completion routine. This step is especially important if you use IoCopyCurrentIrpStackLocationToNext to initialize the next stack location because that function clears some flags that IoSetCompletionRoutine sets.

A completion routine looks like this:

NTSTATUS CompletionRoutine(PDEVICE_OBJECT fdo, PIRP Irp,

  PVOID context)

  {

  return <some status code>;

  }

It receives pointers to the device object and the IRP, and it also receives whichever context value you specified in the call to IoSetCompletionRoutine. Completion routines can be called at DISPATCH_LEVEL in an arbitrary thread context but can also be called at PASSIVE_LEVEL or APC_LEVEL. To accommodate the worst case (DISPATCH_LEVEL), completion routines therefore need to be in nonpaged memory and must call only service functions that are callable at or below DISPATCH_LEVEL. To accommodate the possibility of being called at a lower IRQL, however, a completion routine shouldn’t call functions such as KeAcquireSpinLockAtDpcLevel that assume they’re at DISPATCH_LEVEL to start with.

There are really just two possible return values from a completion routine:

• STATUS_MORE_PROCESSING_REQUIRED, which aborts the completion process immediately. The spelling of this status code obscures its actual purpose, which is to short-circuit the completion of an IRP. Sometimes, a driver actually does some additional processing on the same IRP. Other times, the flag just means, “Yo, IoCompleteRequest! Like, don’t touch this IRP no more, dude!” Future versions of the DDK will therefore define an enumeration constant, StopCompletion, that is numerically the same as STATUS_MORE_PROCESSING_REQUIRED but more evocatively named. (Future printings of this book may also employ better grammar in describing the meaning to be ascribed the constant, at least if my editors get their way.)
• Anything else, which allows the completion process to continue. Because any value besides STATUS_MORE_PROCESSING_REQUIRED has the same meaning as any other, I usually just code STATUS_SUCCESS. Future versions of the DDK will define STATUS_CONTINUE_COMPLETION and an enumeration constant, Con­tinueCompletion, that are numerically the same as STATUS_SUCCESS.

I’ll have more to say about these return codes a bit further on in this chapter.

Situation 1: Synchronous Subsidiary IRP

The first situation to consider occurs when you create a synchronous IRP to help you process an IRP that someone else has sent you. You intend to complete the main IRP after the subsidiary IRP completes.

You wouldn’t ordinarily use a completion routine with a synchronous IRP, but you might want to if you were going to implement the safe cancel logic discussed later in this chapter. If you follow that example, your completion routine will safely return before you completely finish handling the subsidiary IRP and, therefore, comfortably before you complete the main IRP. The sender of the main IRP is keeping you in memory until then. Consequently, you won’t need to use IoSetCompletionRoutineEx.

Situation 2: Asynchronous Subsidiary IRP

In this situation, you use an asynchronous subsidiary IRP to help you implement a main IRP that someone sends you. You complete the main IRP in the completion routine that you’re obliged to install for the subsidiary IRP.

Here you should use IoSetCompletionRoutineEx if it’s available because the main IRP sender’s protection expires as soon as you complete the main IRP. Your completion routine still has to return to the I/O Manager and therefore needs the protection offered by this new routine.

Situation 3: IRP Issued from Your Own System Thread

The third situation in our analysis of completion routines occurs when a system thread you’ve created (see Chapter 14 for a discussion of system threads) installs completion routines for IRPs it sends to other drivers. If you create a truly asynchronous IRP in this situation, use IoSetCompletionRoutineEx to install the obligatory completion routine and make sure that your driver can’t unload before the completion routine is actually called. You could, for example, claim an IO_REMOVE_LOCK that you release in the completion routine. If you use scenario 8 from the cookbook at the end of this chapter to send a nominally asynchronous IRP in a synchronous way, however, or if you use synchronous IRPs in the first place, there’s no particular reason to use IoSetCompletionRoutineEx because you’ll presumably wait for these IRPs to finish before calling PsTerminateSystemThread to end the thread. Some other function in your driver will be waiting for the thread to terminate before allowing the operating system to finally unload your driver. This combination of protections makes it safe to use an ordinary completion routine.

Situation 4: IRP Issued from a Work Item

Here I hope you’ll be using IoAllocateWorkItem and IoQueueWorkItem, which protect your driver from being unloaded until the work item callback routine returns. As in the previous situation, you’ll want to use IoSetCompletionRoutineEx if you issue an asynchronous IRP and don’t wait (as in scenario 8) for it to finish. Otherwise, you don’t need the new routine unless you somehow return before the IRP completes, which would be against all the rules for IRP handling and not just the rules for completion routines.

Situation 5: Synchronous or Asynchronous IRP for Some Other Purpose

Maybe you have some reason for issuing a synchronous IRP that is not in aid of an IRP that someone else has sent you and is not issued from the context of your own system thread or a work item. I confess that I can’t think of a circumstance in which you’d actually want to do this, but I think you’d basically be toast if you tried. Protecting your completion routine, if any, probably helps a bit, but there’s no bulletproof way for you to guarantee that you’ll still be there when IoCallDriver returns. If you think of a way, you’ll simply move the problem to after you do whatever it is you think of, at which point there has to be at least a return instruction that will get executed without protection from outside your driver.

So don’t do this.