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

Srikar Dronamraju
2011-06-16T03:34:53Z

* 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.

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

Do we have better solutions?

--
Thanks and Regards
Srikar