linux-kernel - Re: [RFC] [PATCH 2.6.37-rc5-tip 5/20] 5: Uprobes: register/unregister probes.

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Date:	Wed, 26 Jan 2011 16:45:56 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
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>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Christoph Hellwig <hch@...radead.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	SystemTap <systemtap@...rces.redhat.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC] [PATCH 2.6.37-rc5-tip 5/20]  5: Uprobes:
 register/unregister probes.

On Wed, 2011-01-26 at 21:00 +0530, Srikar Dronamraju wrote:
> * Peter Zijlstra <peterz@...radead.org> [2011-01-26 11:11:48]:
> 
> > On Wed, 2011-01-26 at 13:25 +0530, Srikar Dronamraju wrote:
> > > 
> > > > > +
> > > > > +               list_add(&mm->uprobes_list, &tmp_list);
> > > > > +               mm->uprobes_vaddr = vma->vm_start + offset;
> > > > > +       }
> > > > > +       spin_unlock(&mapping->i_mmap_lock);
> > > > 
> > > > Both this and unregister are racy, what is to say:
> > > >  - the vma didn't get removed from the mm
> > > >  - no new matching vma got added
> > > > 
> > > 
> > > register_uprobe, unregister_uprobe, uprobe_mmap are all synchronized by
> > > uprobes_mutex. So I dont see one unregister_uprobe getting thro when
> > > another register_uprobe is working with a vma.
> > > 
> > > If I am missing something elementary, please explain a bit more.
> > 
> > afaict you're not holding the mmap_sem, so userspace can simply unmap
> > the vma.
> 
> When we do the actual insert/remove of the breakpoint we hold the
> mmap_sem. During the actual insertion/removal, if the vma for the
> specific inode is not found, we just come out without doing the
> actual insertion/deletion.

Right, but then install_uprobe() should:

 - lookup the vma relating to the address you stored,
 - validate that the vma is indeed a map of the right inode
 - validate that the offset of the probe corresponds with the stored
address

Otherwise you can race with unmap/map and end up installing the probe in
a random location.

Also, I think the whole thing goes funny if someone maps the same text
twice ;-)
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