Re: [PATCH 2/2] trace_workqueue: add refcnt to struct cpu_workqueue_stats

[Frederic Weisbecker <fweisbec@gmail.com>]

On Tue, Jul 07, 2009 at 02:07:35PM +0800, Li Zefan wrote:
> > The stat entries can be freed when the stat file is being read.
> > The worse is, the ptr can be freed immediately after it's returned
> > from workqueue_stat_start/next().
> > 
> > Add a refcnt to struct cpu_workqueue_stats to avoid use-after-free.
> > 
> > Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
> > Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
> > ---
> ...
> > @@ -175,11 +184,14 @@ static void *workqueue_stat_next(void *prev, int idx)
> >  				return NULL;
> >  		} while (!(ret = workqueue_stat_start_cpu(cpu)));
> >  		return ret;
> > +	} else {
> > +		ret = list_entry(prev_cws->list.next,
> > +				 struct cpu_workqueue_stats, list);
> 
> I just realized accessing prev_cws->list.next can be invalid!
> 
> We can fix it by using list_del_init() to delete cws->list in
> probe_workqueue_destruction(), but then if the race happened,
> the next time stat_next() is called, NULL will be returned.
> I guess this is Ok, since the race is rare.


If you ensure the kref_get/put are under the
workqueue_cpu_stat(cpu)->lock, it should be fine, right?

 
> (I never like the design of trace_stat..Fortunately we'll
> probably switch to perfcounter for this kind of statistics
> reporting)


I don't like its design either. I wrote it specifically for
the branch tracer and didn't think about free-able events :-/


> 
> > +		kref_get(&ret->kref);
> >  	}
> >  	spin_unlock_irqrestore(&workqueue_cpu_stat(cpu)->lock, flags);
> >  
> > -	return list_entry(prev_cws->list.next, struct cpu_workqueue_stats,
> > -			  list);
> > +	return ret;
> >  }
>