Re: [RFC] [PATCH 4/7] Uprobes Implementation

[Peter Zijlstra <peterz@infradead.org>]

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.