RE: Problem skipping a header during reads under Windows -2000.

Hi Rob,
my experience is that file header skiping alorithm working under WNT 4.0
works under W2k too. But this algorithm doesn’t comprise only from some file
offset and file length manipulation, it requires break up file object abov
and below your filter (here must be two file obects - one represents file
with header, one the same file without header). A file header stored at the
begining of the file requires also special handling of PAGING I/O (if your
file header is not page size long) .

Majority of developers prefer to store file header (meta data) at the end of
the file, but storing meta data at the begining of a the file is more funny.

Best regards,


-----Original Message-----
From: Rob Linegar []
Sent: Monday, August 21, 2000 1:49 PM
To: File Systems Developers
Subject: [ntfsd] Problem skipping a header during reads under Windows 2000.

Hi All,

I’ve just encountered a problem with my fs filter which it seems only occurs
under Windows 2000.
We have a file with a header, currently this header is 41 bytes in size. The
data after the 41 bytes is a copy of notepad.exe from Windows 98.

When we receive an IRP_MJ_READ for the file that has the header we add the
41 bytes offset to nextIrpStack->Parameters.Read.ByteOffset.QuadPart and set
a completion routine.

The completion routine copies
currentIrpStack->Parameters.Read.ByteOffset.QuadPart over
currentIrpStack->FileObject->CurrentByteOffset.QuadPart then
checks pIrp->IoStatus.Status. If successful
currentIrpStack->FileObject->CurrentByteOffset.QuadPart is then increased by
the amount read (pIrp->IoStatus.Information).

We are trying to call CreateProcess to run the executable after the header,
but even copying the file within explorer exhibits this problem.

Under Windows NT 4 (sp6) this method “works perfectly” ™(c) etc.

Under Windows 2000 something odd is happening…

At pBuffer[470] (byte 471) into the read data a byte is removed, and at
pBuffer[4054] (byte 4055) a byte is added (byte 4097 in the real file).

I changed byte 512 in the file, and the value this is set to does not match
the value added later on. The byte added is always 0x00.

Some interesting observations 471 + 41 = 512, 4055 + 41 = 4096 and 4096 ==

Does anyone have any ideas/suggestions?

Rob Linegar
Software Engineer
Data Encryption Systems Limited

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