Alan Cox <alan@lxorguk.ukuu.org.uk> wrote on 18/08/2008 16:31:48:

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> > Huh? I was never advocating re-scan after each modification and I even 

quoted text
> > explicitly said it does not make sense for AV not only for performance 
but 
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> > because it will be useless most of the time. I thought sending out 
> > modified notification on close makes sense because it is a natural 
point, 
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> > unless someone is trying to subvert which is out of scope. Other have 
> > suggested time delay and lumping up.
> 
> You need a bit more than close I imagine, otherwise I can simply keep 
the
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> file open forever. There are lots of cases where that would be natural
> behaviour - eg if I was to attack some kind of web forum and insert a
> windows worm into the forum which was database backed the file would
> probably never be closed. That seems to be one of the more common attack
> vectors nowdays.

Yes, I agree that modification notifications are needed in some cases.
 
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> > Also, just to double-check, you don't think AV scanning would read the 

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> > whole file on every write?
> 
> So you need the system to accumulate some kind of complete in memory set
> of 'dirty' range lists on all I/O ? That is going to have pretty bad
> performance impacts and serialization.

No, I was just saying scanning is pretty smart, it's not some brute force 
method of scan all data that is there. It has a file type detection and 
what and how to scan is determined by that. If a file does not resemble 
any file type I don't think it gets scanned. For example take couple of 
gigabytes of zeros and try to scan that with some products. I don't think 
they will try to read the whole file.

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Tvrtko A. Ursulin
Senior Software Engineer, Sophos

"Views and opinions expressed in this email are strictly those of the 
author.
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