On Mon, 18 Aug 2008, David Collier-Brown wrote:

quoted text
> tvrtko.ursulin wrote:
>>> Huh? I was never advocating re-scan after each modification and I even 
>>> explicitly said it does not make sense for AV not only for performance but 
>>> 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, 
>>> unless someone is trying to subvert which is out of scope. Other have 
>>> suggested time delay and lumping up.
>
> Alan Cox wrote: 
>> You need a bit more than close I imagine, otherwise I can simply keep the
>> 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.
>
> I suspect we're saying "on close" when what's really meant is
> "opened for write". In the latter case, the notification would tell
> the user-space program to watch for changes, possibly by something as
> simple as doing a stat now and another when it gets around to deciding if it 
> should scan the file. I see lots of room for
> user-space alternatives for change detection, depending on how much
> state it keeps. Rsync-like, perhaps?

trying to have every scanner program monitor every file that any program 
opens for write by doing periodic stat commands on it sounds like a very 
inefficiant process (and unless they then get notified on close as well, 
how do they know when to stop monitoring?)

getting a notification on the transition from scanned -> dirty is much 
less of a load (yes, it does leave open the possiblilty of a file getting 
scanned multiple times as it keeps getting dirtied, but that's a policy 
question of how aggressive the scanner is set to be in scanning files)

David Lang
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