linux-kernel - Re: [PATCH] tracing/filters: protect current event filter users from filter removal

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Date:	Thu, 2 Apr 2009 09:12:36 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Tom Zanussi <tzanussi@...il.com>
cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, fweisbec@...il.com
Subject: Re: [PATCH] tracing/filters: protect current event filter users from
 filter removal


On Thu, 2 Apr 2009, Tom Zanussi wrote:

> This patch allows event filters to be removed even if tracing is active.
> 
> Signed-off-by: Tom Zanussi <tzanussi@...il.com>
> 
> ---
>  kernel/trace/trace.h                |    9 +++++++--
>  kernel/trace/trace_events_filter.c  |   15 +++++++++------
>  kernel/trace/trace_events_stage_3.h |    3 +--
>  3 files changed, 17 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index 9dcbc97..6bd728f 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -847,7 +847,7 @@ extern int filter_parse(char **pbuf, struct filter_pred *pred);
>  extern int filter_add_pred(struct ftrace_event_call *call,
>  			   struct filter_pred *pred);
>  extern void filter_free_preds(struct ftrace_event_call *call);
> -extern int filter_match_preds(struct ftrace_event_call *call, void *rec);
> +extern int filter_match_preds(struct filter_pred **preds, void *rec);
>  extern void filter_free_subsystem_preds(struct event_subsystem *system);
>  extern int filter_add_subsystem_pred(struct event_subsystem *system,
>  				     struct filter_pred *pred);
> @@ -856,8 +856,13 @@ static inline void
>  filter_check_discard(struct ftrace_event_call *call, void *rec,
>  		     struct ring_buffer_event *event)
>  {
> -	if (unlikely(call->preds) && !filter_match_preds(call, rec))
> +	struct filter_pred **preds;
> +
> +	rcu_read_lock();
> +	preds = rcu_dereference(call->preds);
> +	if (unlikely(preds) && !filter_match_preds(preds, rec))
>  		ring_buffer_event_discard(event);
> +	rcu_read_unlock();
>  }

This looks dangerous, since we do trace rcu_read_lock/unlock.

>  
>  #define __common_field(type, item)					\
> diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c
> index 470ad94..f7f8469 100644
> --- a/kernel/trace/trace_events_filter.c
> +++ b/kernel/trace/trace_events_filter.c
> @@ -83,14 +83,14 @@ static int filter_pred_string(struct filter_pred *pred, void *event)
>  }
>  
>  /* return 1 if event matches, 0 otherwise (discard) */
> -int filter_match_preds(struct ftrace_event_call *call, void *rec)
> +int filter_match_preds(struct filter_pred **preds, void *rec)
>  {
>  	int i, matched, and_failed = 0;
>  	struct filter_pred *pred;
>  
>  	for (i = 0; i < MAX_FILTER_PRED; i++) {
> -		if (call->preds[i]) {
> -			pred = call->preds[i];
> +		if (preds[i]) {
> +			pred = preds[i];
>  			if (and_failed && !pred->or)
>  				continue;
>  			matched = pred->fn(pred, rec);
> @@ -162,13 +162,16 @@ void filter_free_pred(struct filter_pred *pred)
>  
>  void filter_free_preds(struct ftrace_event_call *call)
>  {
> +	struct filter_pred **preds;
>  	int i;
>  
>  	if (call->preds) {
> +		preds = call->preds;
> +		rcu_assign_pointer(call->preds, NULL);
> +		synchronize_rcu();

We get the same effect if you just have preemption disabled instead of 
the rcu_read_lock/unlock and call synchronize_sched() here instead.

This would make it a bit safer in the tracing.

-- Steve



>  		for (i = 0; i < MAX_FILTER_PRED; i++)
> -			filter_free_pred(call->preds[i]);
> -		kfree(call->preds);
> -		call->preds = NULL;
> +			filter_free_pred(preds[i]);
> +		kfree(preds);
>  	}
>  }
>  
> diff --git a/kernel/trace/trace_events_stage_3.h b/kernel/trace/trace_events_stage_3.h
> index 9d2fa78..cca3843 100644
> --- a/kernel/trace/trace_events_stage_3.h
> +++ b/kernel/trace/trace_events_stage_3.h
> @@ -222,8 +222,7 @@ static void ftrace_raw_event_##call(proto)				\
>  									\
>  	assign;								\
>  									\
> -	if (call->preds && !filter_match_preds(call, entry))		\
> -		ring_buffer_event_discard(event);			\
> +	filter_check_discard(call, entry, event);			\
>  									\
>  	trace_nowake_buffer_unlock_commit(event, irq_flags, pc);	\
>  									\
> -- 
> 1.5.6.3
> 
> 
> 
> 
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