© 2004 microsoft corporation. all rights reserved. 1 modifying parameters

© 2004 Microsoft Corporation. All rights reserved. 1

Modifying Parameters


Changing Operation Parameters Only fields in FLT_CALLBACK_DATA.Iopb

structure can be changed Includes TargetInstance, TargetFileObject,

and operation specific parameters (including buffer address and MDL)

TargetInstance can only be changed to an instance of your filter at the same altitude on another volume

TargetFileObject must be to a file on the same volume as TargetInstance

MajorFunction may not be changed – generate another IO


Changing Operation Parameters (cont)

FltSetCallbackDataDirty() Must be called to signify that

parameters have been changed If FLT_CALLBACK_DATA is not marked

DIRTY, changes are ignored FltClearCallbackDataDirty() FltIsCallbackDataDirty()


Changing Operation Parameters (cont) Parameters received in postOperation

callback are the parameters received in preOperation These parameters DO NOT reflect any

parameter changes filter may have made If filter needs to know changes made to

parameters in postOperation callback, it should record the changes in the context passed from the preOperation callback


Changing Operation Parameters (cont) FLT_CALLBACK_DATA.IoStatus may

also be changed This is an exception to the previous rule May be changed in pre operation callback if

completing the operation May be changed in post operation callback

if undoing operation If changing a successful operation to a failure, it is

filters responsibility to undo the operation Do not need to call FLT_SET_CALLBACK_DATA_DIRTY()


Swapping Buffers Always keep the MDL and buffer fields

in sync After making a buffer or MDL change,

always call FltSetCallbackDataDirty()

Never change the state of the FLTFL_CALLBACK_DATA_SYSTEM_BUFFER flag Should only be set by Filter Manager


Swapping Buffers (cont) If you swap buffers you must have a

postOperation callback Do not free the Buffer or MDL you are

replacing Do not free MDLs you have successfully

inserted into a callback data structure FltMgr will free them for you

In your postOperation callback, free the buffer you allocated Again, do not free the MDL


Swapping Buffers (cont) IRP operation

If FLTFL_CALLBACK_DATA_SYSTEM_BUFFER flag is set:

New buffer must be allocated from nonPaged pool Set the MDL field to NULL Call FLT_SET_CALLBACK_DATA_DIRTY()

If FLTFL_CALLBACK_DATA_SYSTEM_BUFFER flag is not set:

New buffer can be from paged or nonPaged pool PagingIO operation buffers must be from nonPaged

pool Always create and set a MDL

See MmBuildMdlForNonPagedPool() Call FLT_SET_CALLBACK_DATA_DIRTY()


Swapping Buffers (cont)

FastIO New buffer can be from paged or

nonPaged pool Set the MDL to NULL

MDL should already be NULL

Call FltSetCallbackDataDirty()


Swapping Buffers (cont) WARNING: Non-Cached IO operations

MM violates the rule that the length of non-cached IO operations must be rounded to a sector boundary

The file systems round the length up to the next sector size below the filter

When swapping buffers in this scenario, the filter must round the length up to the sector size (use SectorSize field in DeviceObject) before allocating/copying data.

If you don’t the system may crash or you may corrupt the file


Swapped Buffer Support Routines FltGetSwappedBufferMdlAddress()

Only callable from postOperation callback Returns the MDL for the buffer that you set in

the preOperation callback This may return NULL

FltRetainSwappedBufferMdlAddress() Only callable from postOperation callback Tells FltMgr to not free the current

operations MDL It is now the filters responsibility to free this MDL


Unloading Filters with Swapped Buffers

These operations are not “drainable”

Instead, the operation is canceled After canceling, FltMgr waits for the

operation to complete. Whether the operation has a cancel

routine or not, it is assumed it will complete soon


Sample

See the SwapBuffers minifilter sample

© 2004 microsoft corporation. all rights reserved. 1 modifying parameters

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