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Subject: Re: [RFC] [PATCH 4/7] Uprobes Implementation
From: Peter Zijlstra <peterz@infradead.org>
To: Jim Keniston <jkenisto@us.ibm.com>
Cc: maneesh@in.ibm.com, Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
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Date: Sat, 16 Jan 2010 11:04:09 +0100
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On Fri, 2010-01-15 at 15:44 -0800, Jim Keniston wrote:
> On Fri, 2010-01-15 at 12:12 +0100, Peter Zijlstra wrote:
> ....
> > 
> > Adding the probe uses the fact that (most) executable mappings are
> > MAP_PRIVATE and CoWs a private copy of the page with the modified ins,
> > right?
> > 
> > What does it do for MAP_SHARED|MAP_EXECUTABLE sections -- simply fail to
> > add the probe?
> 
> If the vma containing the instruction to be probed has the VM_EXEC flag
> set (and it's not in the XOL area) we go ahead and try to probe it.  I'm
> not familar with the implications of MAP_SHARED|MAP_EXECUTABLE -- how
> you would get such a combo, or what access_process_vm() would do with
> it.

I'm not sure how you'd get one, the user has to explicitly create one I
think, regular loaders don't create such things, but maybe JITs do.

The problem is that for MAP_SHARED you cannot CoW the page, you have to
modify the original page, which might get written back into a file if
its file based, not something you'd want to have happen I guess.