In-Reply-To: <200808062110.58764.nickpiggin@yahoo.com.au>
To: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Eric Paris <eparis@redhat.com>, linux-kernel@vger.kernel.org,
       malware-list@lists.printk.net
Subject: Re: [malware-list] [RFC 0/5] [TALPA] Intro to a linux interface for	on
 access scanning
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From: tvrtko.ursulin@sophos.com
Date: Wed, 6 Aug 2008 12:29:34 +0100
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Nick Piggin <nickpiggin@yahoo.com.au> wrote on 06/08/2008 12:10:58:

> On Wednesday 06 August 2008 19:44, tvrtko.ursulin@sophos.com wrote:
> > Nick Piggin wrote on 05/08/2008 19:08:05:
> > > On Tuesday 05 August 2008 07:00, Eric Paris wrote:
> > > > 5. Define which filesystems are cacheable and which are not
> > >
> > > This is practically impossible to do completely without rewriting a 
lot
> > > of code (which will never be accepted). I don't see why it is needed
> >
> > though
> >
> > > as the filesystem cache is supposed to be kept coherent with disk.
> >
> > Problem is with network filesystems. So could it be a flag somewhere 
per
> > filesystem which would say something like "this filesystem guarantees
> > content of a file cannot change without get_write_access or
> > file_update_time being called locally"? That doesn't sound like a lot 
of
> > code so what am I missing?
> 
> Maybe... but that's not the same as what requirement 5 calls for.

I see what you mean, it should have been worded better. Nevertheless that 
is what was intended by it - to enable caching only on filesystems where 
it is safe to do so.
 
> But depending on exactly what semantics you really call for, it can get
> tricky to account for all of pagecache. Writes can happen through page
> tables or get_user_pages. True that a process has to at some point have
> write permission to the file, but the cache itself could be modified
> even after the file is closed and all mmaps disappear.

I don't have a very good understanding of the VM subsystem I must admit. 
So in other words with the current kernel file modification time is not 
necessarily correct - it represents when the file was last opened for 
modification, not when it was actually modified? (While those two points 
in time can be arbitrarily separated)

How would I use those methods for file modification? I am curious to make 
a test case..

Tvrtko


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