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

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

* Peter Zijlstra <peterz@infradead.org> [2011-06-16 14:00:26]:

> On Thu, 2011-06-16 at 08:56 +0530, Srikar Dronamraju wrote: 
> > * Peter Zijlstra <peterz@infradead.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.
> 
> Sure, I saw why you wanted to do it, I'm just not quite convinced its
> safe to do and something like this definitely wants a comment explaining
> why its safe to drop mmap_sem.  
> 
> > After we re-acquire the mmap_sem, we do check if the vma is valid.
> 
> But you don't on the return path, and if !ret
> mmap_region():unmap_and_free_vma will be touching vma again to remove
> it.
> 

Agree.

> > Do we have better solutions?
> 
> /me kicks the brain into gear and walks off to get a fresh cup of tea.
> 
> So the reason we take uprobes_mutex there is to avoid probes from going
> away while you're installing them, right?

It serializes register/unregister/mmap operations.

> 
> So we start by doing this add_to_temp_list() thing (horrid name), which
> iterates the probes on this inode under uprobes_treelock and adds them
> to a list.
> 
> Then we iterate the list, installing the probles.
> 
> How about we make the initial pass under uprobes_treelock take a
> references on the probe, and then after install_breakpoint() succeeds we
> again take uprobes_treelock and validate the uprobe still exists in the
> tree and drop the extra reference, if not we simply remove the
> breakpoint again and continue like it never existed.
> 
> That should avoid the need to take uprobes_mutex and not require
> dropping mmap_sem, right?

Now since a register and mmap operations can run in parallel, we could
have subtle race conditions like this:

1. register_uprobe inserts the uprobe in RB tree.
2. register_uprobe loops thro vmas and inserts breakpoints.

3. mmap is called for same inode, mmap_uprobe() takes reference; 
4. mmap completes insertion and releases reference.

5. register uprobe tries to install breakpoint on one vma fails and not
due to -ESRCH or -EEXIST.
6. register_uprobe rolls back all install breakpoints except the one
inserted by mmap.

We end up with breakpoints that we have inserted by havent cleared.

Similarly unregister_uprobe might be looping to remove the breakpoints
when mmap comes in installs the breakpoint and returns.
unregister_uprobe might erase the uprobe from rbtree after mmap is done.

-- 
Thanks and Regards
Srikar

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