Given our discussion thus far, let’s talk about how someone would write a file system filter driver in this model. In order to get an I/O request to the storage adapter, we need to create a call chain that includes eight different drivers! Legacy File System Filters #COMODO FILESYSTEM FILTER DRIVER IS NOT LOADED DRIVERS#The number of filter drivers present on a default installation of Windows amplifies the issue with the call through model. Figure 2 shows a complete picture of the filters provided with a clean Wind(Redstone 2) installation.įigure 2-Filters in a Win10 1703 Default Installation Windows ships with many filter drivers in these and related stacks. In the “post” processing case, the file system filter would be called only after the drivers below it in the file system Device Stack and the drivers in any other Device Stacks that handle the request have all finished their processing. For example, using “post” processing a file system filter driver could intercept the actual data that’s been read from a file (and not just the request to read that data, that it would see as part of “pre” processing). Similarly, a filter in the volume stack would intercept volume level operations before the volume driver has a chance to process them.įilter drivers can also intercept I/O requests after (“post-“) all lower drivers that handle the request have completed their processing. A filter in the file system stack can intercept file level operations before (“pre-“) the file system driver has a chance to process them. By attaching a filter Device Object to a Device Stack, a filter driver writer can intercept I/O requests as they pass through the Device Stack. Depending on the number of Device Objects and number of Device Stacks involved, this can create significantly long call chains (as we’ll see in a moment).Īn awesome feature of the I/O Manager in Windows is that each individual Device Stack may contain one or more filter Device Objects. The Disk driver then passes the request by calling directly into the Storage Adapter driver. The Volume driver then directly calls into the Disk driver. This means that, using Figure 1 as our example, the File System (NTFS, for example) passes the request along by calling directly into the Volume driver. At the bottom of the disk stack, the I/O request may then be passed on to the top of the storage adapter stack.įigure 1 is a representation of this processing if all these Device Stacks had a single Device Object.Īn important thing to note is that the I/O requests are passed using a call through model. At the bottom of the volume stack, the I/O request may then be passed on to the top of the disk stack. If the request reaches the bottom of the file system stack, the I/O request may then be passed on to the top of the volume stack. For example, if an application attempts to read data from a file, the I/O request is initially presented to the top of the file system stack. If the I/O request reaches the bottom of a Device Stack, the driver at the bottom may choose to pass the request on to the top of another Device Stack. The I/O requests then flow down the Device Stack, being passed from driver to driver until the I/O request is completed. I/O requests are initially presented to the top of a Device Stack, which is a set of attached Device Objects. Each I/O request is represented by a unique I/O Request Packet (IRP), which is sufficient to fully describe any I/O request in the system. In Windows, we use a layered, packet based I/O model. So… Let’s start at the beginning Windows I/O Subsystem Architecture What provided sufficient motivation to create a whole new model? What problems does this new model solve? And, more importantly, what problems does it not solve? Why? Clearly folks were writing file system filters before Filter Manager existed. What is a Windows file system filter, anyway? How does it fit into the overall Windows I/O Subsystem architecture? Plus, Filter Manager wasn’t even released until the Windows Vista/XP SP2 timeframe. To answer this question, I decided that the beginning was the best place to start. While this isn’t exactly unusual, recently my thoughts have revolved around answering a deceivingly difficult question: What does somebody really need to know to be able to write File System Minifilters these days. And this means that I’ve spent a considerable amount of time musing about Windows file systems, file system filters, and the Filter Manager framework. We’ve been running our new and improved Developing File System Minifilters for Windows seminar for some time now. Last reviewed and updated: 10 August 2020
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