Re: [PATCH 2/3] function-graph: enable the stack after initialization of other variables

[Frederic Weisbecker <fweisbec@gmail.com>]

On Tue, Jun 02, 2009 at 02:30:38PM -0400, Steven Rostedt wrote:
> From: Steven Rostedt <srostedt@redhat.com>
> 
> The function graph tracer checks if the task_struct has ret_stack defined
> to know if it is OK or not to use it. The initialization is done for
> all tasks by one process, but the idle tasks use the same initialization
> used by new tasks.
> 
> If an interrupt happens on an idle task that just had the ret_stack
> created, but before the rest of the initialization took place, then
> we can corrupt the return address of the functions.
> 
> This patch moves the setting of the task_struct's ret_stack to after
> the other variables have been initialized.
> 
> [ Impact: prevent kernel panic on idle task when starting function graph ]
> 
> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> ---
>  kernel/trace/ftrace.c                |    9 +++++++--
>  kernel/trace/trace_functions_graph.c |    6 ++++++
>  2 files changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index ebff62e..20e0660 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -2739,15 +2739,20 @@ void unregister_ftrace_graph(void)
>  void ftrace_graph_init_task(struct task_struct *t)
>  {
>  	if (atomic_read(&ftrace_graph_active)) {
> -		t->ret_stack = kmalloc(FTRACE_RETFUNC_DEPTH
> +		struct ftrace_ret_stack *ret_stack;
> +
> +		ret_stack = kmalloc(FTRACE_RETFUNC_DEPTH
>  				* sizeof(struct ftrace_ret_stack),
>  				GFP_KERNEL);
> -		if (!t->ret_stack)
> +		if (!ret_stack)
>  			return;
>  		t->curr_ret_stack = -1;
>  		atomic_set(&t->tracing_graph_pause, 0);
>  		atomic_set(&t->trace_overrun, 0);
>  		t->ftrace_timestamp = 0;
> +		/* make curr_ret_stack visable before we add the ret_stack */
> +		smp_wmb();
> +		t->ret_stack = ret_stack;
>  	} else
>  		t->ret_stack = NULL;
>  }


Thanks for fixing this.


> diff --git a/kernel/trace/trace_functions_graph.c b/kernel/trace/trace_functions_graph.c
> index d28687e..baeb5fe 100644
> --- a/kernel/trace/trace_functions_graph.c
> +++ b/kernel/trace/trace_functions_graph.c
> @@ -65,6 +65,12 @@ ftrace_push_return_trace(unsigned long ret, unsigned long func, int *depth)
>  	if (!current->ret_stack)
>  		return -EBUSY;
>  
> +	/*
> +	 * We must make sure the ret_stack is tested before we read
> +	 * anything else.
> +	 */
> +	smp_rmb();


Isn't this part a too much costly for very traced function?


> +
>  	/* The return trace stack is full */
>  	if (current->curr_ret_stack == FTRACE_RETFUNC_DEPTH - 1) {
>  		atomic_inc(&current->trace_overrun);
> -- 
> 1.6.3.1
> 
> --