linux-kernel - Re: [RFC PATCH] tracing: Atomically get refcounts of event_call and trace

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Date:	Fri, 5 Jul 2013 02:32:23 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Frederic Weisbecker <fweisbec@...il.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

On 07/04, 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.

Yes, but the same can happen with event_call, so it seems that
this patch is not enough too.

Say, open(id) can take event_mutex when the caller of
trace_remove_event_call() has already freed ftrace_event_call.

Or I missed something...

Perhaps we can rely on d_unlinked() ? IOW, the caller of
__ftrace_event_call_get() should take event_mutex, check
d_unhashed(f_dentry) and only then do _get().

Nasty, I agree.

Oleg.

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