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alternative to FltFlushBuffers2() in OS version prior to 19041

sagi_zarsagi_zar Member - All Emails Posts: 35


I'm implementing a minifilter driver and I need to flush and purge the cache of files that I monitor.
I saw that recently Microsoft added a new API FltFlushBuffers2() that does flush and can also purge the cache.
In the past I used the cache manager API's to do that, but they are not safe to use and cause my driver to hang from time to time.
FltFlushBuffers2() seems to be supported only from Windows build version 19041 which still can't be installed on all computers.
What alternatives I have if I need to flush and purge files that I monitor from within my minifilter?



  • rod_widdowsonrod_widdowson Member - All Emails Posts: 1,224

    I’ve always used FltPerformSynchronousIo (and FltAllocateCallbackData &c).

    There is a minor function (since win7 I think) to turn a MJ_FLUSH into flush & purge.

  • sagi_zarsagi_zar Member - All Emails Posts: 35

    The minifilter is not initiating the write requests to the file.
    I want to flush and purge the cache that was updated by write and read requests that user applications did and the driver monitored

  • Scott_Noone_(OSR)Scott_Noone_(OSR) Administrator Posts: 3,482

    FltFlushBuffers2 is just a wrapper around sending an IRP_MJ_FLUSH_BUFFERS with various different minor function codes:

    // Flush minor function codes
    #define IRP_MN_FLUSH_AND_PURGE           0x01
    #define IRP_MN_FLUSH_DATA_ONLY           0x02    //see FLUSH_FLAGS_FILE_DATA_ONLY for definition of how this works
    #define IRP_MN_FLUSH_NO_SYNC             0x03    //see FLUSH_FLAGS_NO_SYNC for definition of how this works
    #define IRP_MN_FLUSH_DATA_SYNC_ONLY      0x04    //see FLUSH_FLAGS_FILE_DATA_SYNC_ONLY for definition of how this works

    As Rod said you can just build your own callback data instead of using this API.

    Note however that it's up to the file system to actually implement these minor function code. For example, I don't believe FAT uses any of them.


  • MKSMKS Member Posts: 2

    I've got the same issue as Sagi, and while sending the IRP directly does work (thanks Rod and Scott!), my Lead would rather I use the "documented" way of doing things. As such, I'm trying to use FltFlushBuffers2() if it is available. To check the availability, I'm using MmGetSystemRoutineAddress():

    PFN_FLT_FLUSH_BUFFERS_2 pFltFlushBuffers2 = NULL;
    UNICODE_STRING FltFlushBuffers2_Text;
    RtlInitUnicodeString(&FltFlushBuffers2_Text, L"FltFlushBuffers2");
    pFltFlushBuffers2 = (PFN_FLT_FLUSH_BUFFERS_2) (ULONG_PTR)MmGetSystemRoutineAddress(&FltFlushBuffers2_Text);

    Although I'm testing on Windows 10 2H20, this always returns NULL. It also returns NULL for FltFlushBuffers and FltCancelIo. The same code works for MmMapIoSpaceEx() in my driver.

    Is there a different way of getting the addresses of Filter Manager functions?


  • rod_widdowsonrod_widdowson Member - All Emails Posts: 1,224
  • MKSMKS Member Posts: 2

    Thanks Rod, this works perfectly.

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