linux-kernel - Re: [RFC PATCH V2] tracing: Check f_dentry before accessing event_file/call in inode->i

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Date:	Thu, 18 Jul 2013 16:51:37 +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 V2] tracing: Check f_dentry before accessing
	event_file/call in inode->i_private

On 07/18, Masami Hiramatsu wrote:
>
> (2013/07/17 23:51), Oleg Nesterov wrote:
> > Well, perhaps you are right... But this TRACE_EVENT_FL_REF_MASK code
> > is new too, it is not that we only need a small fixlets to finish it.
>
> Would you mean that TRACE_EVENT_FL_REF_MASK may also have some problems?

It was you who initially pointed that it does have problems ;)

And, _afaics_ your patch which tries to fix this problem is not
exactly correct.

It removes trace_array_get/put from tracing_open_generic_file() and
tracing_release_generic_file(). This assumes that "call->flags++" is
enough, but it is not.

Yes, the next patch adds the "flags & TRACE_EVENT_FL_REF_MASK" check
into trace_remove_event_call() path. But this is still racy wrt
instance_delete() unless I missed something.

IOW, I believe that either .open() should do trace_array_get(), or
__trace_remove_event_dirs() needs another for-each-file loop which
checks file->call->flags & TRACE_EVENT_FL_REF_MASK.


> > So I think that it makes sense to discuss the alternatives before we
> > decide what exactly we should do.
>
> Your approach is also interesting for me, indeed. However, it is so
> different from current one. I think you should clarify what bug you
> would like to solve and how.

The same bugs which Steven's 1/4 tries to solve ;)

Oleg.

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