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What Is a File System Filter Driver?
A file system filter driver is an optional driver that adds value to or modifies the behavior of a file system. A file system filter driver is a kernel-mode component that runs as part of the Microsoft Windows NT executive.
A file system filter driver can filter I/O operations for one or more file systems or file system volumes. Depending on the nature of the driver, filter can mean log, observe, modify, or even prevent. Typical applications for file system filter drivers include antivirus utilities, encryption programs, and hierarchical storage management systems.
Filter Manager Concepts
The filter manager is installed with Windows, but it becomes active only when a minifilter driver is loaded. The filter manager attaches to the file system stack for a target volume. A minifilter driver attaches to the file system stack indirectly, by registering with the filter manager for the I/O operations the minifilter driver chooses to filter.
A legacy filter driver's position in the file system I/O stack relative to other filter drivers is determined at system startup by its load order group. For example, an antivirus filter driver should be higher in the stack than a replication filter driver, so it can detect viruses and disinfect files before they are replicated to remote servers. Therefore, filter drivers in the FSFilter Anti-Virus load order group are loaded before filter drivers in the FSFilter Replication group. Each load order group has a corresponding system-defined class and class GUID used in the INF file for the filter driver.
Like legacy filter drivers, minifilter drivers attach in a particular order. However, the order of attachment is determined by a unique identifier called an altitude. The attachment of a minifilter driver at a particular altitude on a particular volume is called an instance of the minifilter driver.
A minifilter driver's altitude ensures that the instance of the minifilter driver is always loaded at the appropriate location relative to other minifilter driver instances, and it determines the order in which the filter manager calls the minifilter driver to handle I/O. Altitudes are allocated and managed by Microsoft.
A minifilter driver can filter IRP-based I/O operations as well as fast I/O and file system filter (FSFilter) callback operations. For each of the I/O operations it chooses to filter, a minifilter driver can register a preoperation callback routine, a postoperation callback routine, or both. When handling an I/O operation, the filter manager calls the appropriate callback routine for each minifilter driver that registered for that operation. When that callback routine returns, the filter manager calls the appropriate callback routine for the next minifilter driver that registered for the operation.
For example, assuming all three minifilter drivers in the above figure registered for the same I/O operation, the filter manager would call their preoperation callback routines in order of altitude from highest to lowest (A, B, C), then forward the I/O request to the next-lower driver for further processing. When the filter manager receives the I/O request for completion, it calls each minifilter driver's postoperation callback routines in reverse order, from lowest to highest (C, B, A).
For interoperability with legacy filter drivers, the filter manager can attach filter device objects to a file system I/O stack in more than one location. Each of the filter manager's filter device objects is called a frame. From the perspective of a legacy filter driver, each filter manager frame is just another legacy filter driver.
Each filter manager frame represents a range of altitudes. The filter manager can adjust an existing frame or create a new frame to allow minifilter drivers to attach at the correct location.
The filter manager cannot attach a minifilter between two attached legacy filters unless there is already a filter manager frame between them. If a minifilter is intended to be attached above a legacy filter, it can be attached below it, depending on the existence of a second attached legacy filter. A minifilter intended to be attached below a legacy filter could, instead, be attached above that legacy filter.
Important Always verify interoperability of legacy filters with minifilters or consider replacing legacy filters with minifilters. For more information, see Guidelines for Porting Legacy Filter Drivers.
If a minifilter driver is unloaded and reloaded, it is reloaded at the same altitude in the same frame from which it was unloaded.
Overview of the Windows I/O Model
Every operating system has an implicit or explicit I/O model for handling the flow of data to and from peripheral devices. One feature of the Microsoft Windows I/O model is its support for asynchronous I/O. In addition, the I/O model has the following general features:
• The I/O manager presents a consistent interface to all kernel-mode drivers, including lowest-level, intermediate, and file system drivers. All I/O requests to drivers are sent as I/O request packets (IRPs).
• I/O operations are layered. The I/O manager exports I/O system services, which user-mode protected subsystems call to carry out I/O operations on behalf of their applications and/or end users. The I/O manager intercepts these calls, sets up one or more IRPs, and routes them through possibly layered drivers to physical devices.
• The I/O manager defines a set of standard routines, some required and others optional, that drivers can support. All drivers follow a relatively consistent implementation model, given the differences among peripheral devices and the differing functionality required of bus, function, filter, and file system drivers.
• Like the operating system itself, drivers are object-based. Drivers, their devices, and system hardware are represented as objects. The I/O manager and other operating system components export kernel-mode support routines that drivers can call to get work done by manipulating the appropriate objects.
In addition to using IRPs to convey traditional I/O requests, the I/O manager works with the PnP and power managers to send IRPs containing PnP and power requests.
Filtering IRPs and Fast I/O
A file system filter driver filters I/O requests for one or more file systems or file system volumes. Each I/O request appears as an I/O request packet (IRP) or fast I/O request. IRPs are I/O system structures that are handled by a driver's IRP dispatch routines. Fast I/O requests are handled by the driver's fast I/O callback routines.
When a filter driver is initialized, its DriverEntry routine registers the filter driver's IRP dispatch routines and fast I/O callback routines. Only one set of these routines can be registered for each filter driver.
Some types of IRPs have fast I/O equivalents, and some fast I/O requests have IRP equivalents. However, IRPs handle many types of I/O that fast I/O cannot. Also, certain specialized fast I/O routines are used to preacquire file system resources for the Cache Manager or Memory Manager without creating an IRP. Thus, for the most part, IRPs and fast I/O requests perform separate roles in I/O operations.
IRP Major Function Codes
Each driver-specific I/O stack location (IO_STACK_LOCATION) for every IRP contains a major function code (IRP_MJ_XXX), which tells the driver what operation it or the underlying device driver should carry out to satisfy the I/O request. Each kernel-mode driver must provide dispatch routines for the major function codes that it must support.
The specific operations a driver carries out for a given IRP_MJ_XXX code depend somewhat on the underlying device, particularly for IRP_MJ_DEVICE_CONTROL and IRP_MJ_INTERNAL_DEVICE_CONTROL requests. For example, the requests sent to a keyboard driver are necessarily somewhat different from those sent to a disk driver. However, the I/O manager defines the parameters and I/O stack location contents for each system-defined major function code.
Every higher-level driver must set up the appropriate I/O stack location in IRPs for the next-lower-level driver and call IoCallDriver, either with each input IRP, or with a driver-created IRP (if the higher-level driver holds on to the input IRP). Consequently, every intermediate driver must supply a dispatch routine for each major function code that the underlying device driver handles. Otherwise, a new intermediate driver will "break the chain" whenever an application or still higher-level driver attempts to send an I/O request down to the underlying device driver.
File system drivers also handle a required subset of system-defined IRP_MJ_XXX function codes, some with subordinate IRP_MN_XXX function codes.
Drivers handle IRPs set with some or all of the following major function codes:
The input and output parameters described in this section are the function-specific parameters in the IRP.