linux-kernel - Re: [PATCH v4 3.0-rc2-tip 7/22] 7: uprobes: mmap and fork hooks.

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Date:	Thu, 16 Jun 2011 08:56:45 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	Linux-mm <linux-mm@...ck.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Hugh Dickins <hughd@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Jonathan Corbet <corbet@....net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Oleg Nesterov <oleg@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...k.frob.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v4 3.0-rc2-tip 7/22]  7: uprobes: mmap and fork hooks.

* Peter Zijlstra <peterz@...radead.org> [2011-06-15 20:11:26]:

> On Tue, 2011-06-07 at 18:29 +0530, Srikar Dronamraju wrote:
> > +       up_write(&mm->mmap_sem);
> > +       mutex_lock(&uprobes_mutex);
> > +       down_read(&mm->mmap_sem); 
> 
> egads, and all that without a comment explaining why you think that is
> even remotely sane.
> 
> I'm not at all convinced, it would expose the mmap() even though you
> could still decide to tear it down if this function were to fail, I bet
> there's some funnies there.

The problem is with lock ordering.  register/unregister operations
acquire uprobes_mutex (which serializes register unregister and the
mmap_hook) and then holds mmap_sem for read before they insert a
breakpoint.

But the mmap hook would be called with mmap_sem held for write. So
acquiring uprobes_mutex can result in deadlock. Hence we release the
mmap_sem, take the uprobes_mutex and then again hold the mmap_sem.

After we re-acquire the mmap_sem, we do check if the vma is valid.

Do we have better solutions?

-- 
Thanks and Regards
Srikar

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