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

[Li Zefan <lizf@cn.fujitsu.com>]

> 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.

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

> +		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;
>  }