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Date: Thu, 04 Jul 2013 21:12:25 +0900
From: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Frederic Weisbecker <fweisbec@...il.com>,
Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org,
Ingo Molnar <mingo@...hat.com>,
Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
Andrew Morton <akpm@...ux-foundation.org>,
jovi.zhangwei@...wei.com, Jiri Olsa <jolsa@...hat.com>,
Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH] tracing: Atomically get refcounts of event_call and
trace_array
Steven, Oleg,
I think your patches are OK, but not enough.
Here is an additional patch to fix the unsafe case which I found.
Could you review this too?
(2013/07/04 20:55), Masami Hiramatsu wrote:
> Currently ftrace_open_generic_file gets an event_file from
> inode->i_private, and then locks event_mutex and gets refcount.
> However, this can cause a race as below scenario;
>
> CPU0 CPU1
> open(kprobe_events)
> trace_remove_event_call() open(enable)
> lock event_mutex get event_file from inode->i_private
> event_remove() wait for unlock event_mutex
> ...
> free event_file
> unlock event_mutex
> lock event_mutex
> add refcount of event_file->call (*)
>
> So, at (*) point, the event_file is already freed and we
> may access the corrupted object.
> The same thing could happen on trace_array because it is also
> directly accessed from event_file.
>
> To avoid this, when opening events/*/*/enable, we must atomically
> do; ensure the ftrace_event_file object still exists on a trace_array,
> and get refcounts of event_file->call and the trace_array.
>
>
> CPU0 CPU1
> open(kprobe_events)
> trace_remove_event_call() open(enable)
> lock event_mutex get event_file from inode->i_private
> event_remove() wait for unlock event_mutex
> ...
> free event_file
> unlock event_mutex
> lock event_mutex
> search the event_file and failed
> unlock event_mutex
> return -ENODEV
>
> Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
> ---
> kernel/trace/trace_events.c | 58 +++++++++++++++++++++++++++++++++----------
> 1 file changed, 45 insertions(+), 13 deletions(-)
>
> diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
> index 1a5547e..db6b107 100644
> --- a/kernel/trace/trace_events.c
> +++ b/kernel/trace/trace_events.c
> @@ -391,15 +391,24 @@ static void __get_system_dir(struct ftrace_subsystem_dir *dir)
> __get_system(dir->subsystem);
> }
>
> -static int ftrace_event_call_get(struct ftrace_event_call *call)
> +static int __ftrace_event_call_get(struct ftrace_event_call *call)
> {
> int ret = 0;
>
> - mutex_lock(&event_mutex);
> if ((call->flags & TRACE_EVENT_FL_REF_MASK) == TRACE_EVENT_FL_REF_MAX - 1)
> ret = -EBUSY;
> else
> call->flags++;
> +
> + return ret;
> +}
> +
> +static int ftrace_event_call_get(struct ftrace_event_call *call)
> +{
> + int ret = 0;
> +
> + mutex_lock(&event_mutex);
> + ret = __ftrace_event_call_get(call);
> mutex_unlock(&event_mutex);
>
> return ret;
> @@ -413,6 +422,35 @@ static void ftrace_event_call_put(struct ftrace_event_call *call)
> mutex_unlock(&event_mutex);
> }
>
> +static int ftrace_event_file_get(struct ftrace_event_file *this_file)
> +{
> + struct ftrace_event_file *file;
> + struct trace_array *tr;
> + int ret = -ENODEV;
> +
> + mutex_lock(&event_mutex);
> + do_for_each_event_file(tr, file) {
> + if (file == this_file) {
> + ret = __ftrace_event_call_get(file->event_call);
> + if (!ret)
> + tr->ref++;
> + goto out_unlock;
> + }
> + } while_for_each_event_file();
> + out_unlock:
> + mutex_unlock(&event_mutex);
> +
> + return ret;
> +}
> +
> +static void ftrace_event_file_put(struct ftrace_event_file *file)
> +{
> + struct trace_array *tr = file->tr;
> +
> + ftrace_event_call_put(file->event_call);
> + trace_array_put(tr);
> +}
> +
> static void __put_system_dir(struct ftrace_subsystem_dir *dir)
> {
> WARN_ON_ONCE(dir->ref_count == 0);
> @@ -438,33 +476,27 @@ static void put_system(struct ftrace_subsystem_dir *dir)
> static int tracing_open_generic_file(struct inode *inode, struct file *filp)
> {
> struct ftrace_event_file *file = inode->i_private;
> - struct trace_array *tr = file->tr;
> int ret;
>
> - if (trace_array_get(tr) < 0)
> - return -ENODEV;
> -
> - ret = tracing_open_generic(inode, filp);
> + ret = ftrace_event_file_get(file);
> if (ret < 0)
> - goto fail;
> + return ret;
>
> - ret = ftrace_event_call_get(file->event_call);
> + ret = tracing_open_generic(inode, filp);
> if (ret < 0)
> goto fail;
>
> return 0;
> fail:
> - trace_array_put(tr);
> + ftrace_event_file_put(file);
> return ret;
> }
>
> static int tracing_release_generic_file(struct inode *inode, struct file *filp)
> {
> struct ftrace_event_file *file = inode->i_private;
> - struct trace_array *tr = file->tr;
>
> - ftrace_event_call_put(file->event_call);
> - trace_array_put(tr);
> + ftrace_event_file_put(file);
>
> return 0;
> }
--
Masami HIRAMATSU
IT Management Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com
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