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Subject: Re: [RFC] [PATCH 4/7] Uprobes Implementation
From: Peter Zijlstra <peterz@infradead.org>
To: maneesh@in.ibm.com
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
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Date: Fri, 15 Jan 2010 12:18:04 +0100
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On Fri, 2010-01-15 at 12:12 +0100, Peter Zijlstra wrote:
> On Fri, 2010-01-15 at 16:35 +0530, Maneesh Soni wrote:
> > On Fri, Jan 15, 2010 at 11:33:27AM +0100, Peter Zijlstra wrote:
> > > On Fri, 2010-01-15 at 15:56 +0530, Srikar Dronamraju wrote:
> > > > Hi Peter, 
> > > > 
> > > > Or there could be two threads that could be racing to
> > > > insert/delete a breakpoint. These synchronization issues are all handled
> > > > by the Uprobes layer.
> > > 
> > > Shouldn't be hard to put that in the ubp layer, right?
> > > 
> > > > Uprobes layer would need to be notified of process life-time events
> > > > like fork/clone/exec/exit. 
> > > 
> > > No so much the process lifetimes as the vma life times are interesting,
> > > placing a hook in the vm code to track that isn't too hard, 
> > > 
> > 
> > I think similar hooks were given thumbs down in the previous incarnation
> > of uprobes (which was implemented without utrace).  
> > 
> > http://lkml.indiana.edu/hypermail/linux/kernel/0603.2/1254.html
> 
> I wasn't at all proposing to mess with a_ops, nor do you really need to,
> I was more thinking of adding a callback like perf_event_mmap() and a
> corresponding unmap(), that way you can track mapping life-times and
> add/remove probes accordingly.
> 
> 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?

Does it clean up the CoW'ed page on removing the probe? Does that
account for userspace having made other changes in between installing
and removing the probe (for PROT_WRITE mappings obviously)?