linux-kernel - Re: [RFC] [PATCH 4/7] Uprobes Implementation

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Message-ID: <1263636249.4244.525.camel@laptop>
Date:	Sat, 16 Jan 2010 11:04:09 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jim Keniston <jkenisto@...ibm.com>
Cc:	maneesh@...ibm.com, Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	utrace-devel <utrace-devel@...hat.com>,
	Mark Wielaard <mjw@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] [PATCH 4/7] Uprobes Implementation

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.

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