Re: [for-next][PATCH 4/6] tracing: Return error if ftrace_trace_arrays list is empty

[Namhyung Kim <namhyung@gmail.com>]

On Fri, 06 Jun 2014 12:30:38 -0400, Steven Rostedt wrote:
> From: Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@hitachi.com>
>
> ftrace_trace_arrays links global_trace.list. However, global_trace
> is not added to ftrace_trace_arrays if trace_alloc_buffers() failed.
> As the result, ftrace_trace_arrays becomes an empty list. If
> ftrace_trace_arrays is an empty list, current top_trace_array() returns
> an invalid pointer. As the result, the kernel can induce memory corruption
> or panic.
>
> Current implementation does not check whether ftrace_trace_arrays is empty
> list or not. So, in this patch, if ftrace_trace_arrays is empty list,
> top_trace_array() returns NULL. Moreover, this patch makes all functions
> calling top_trace_array() handle it appropriately.

[SNIP]
> @@ -252,6 +252,9 @@ static inline struct trace_array *top_trace_array(void)
>  {
>  	struct trace_array *tr;
>  
> +	if (list_empty(ftrace_trace_arrays.prev))
> +		return NULL;

It looks weird to me.. why not checking "list_empty(&ftrace_trace_arrays)"?

Thanks,
Namhyung

> +
>  	tr = list_entry(ftrace_trace_arrays.prev,
>  			typeof(*tr), list);
>  	WARN_ON(!(tr->flags & TRACE_ARRAY_FL_GLOBAL));