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

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Date:	Fri, 19 Jul 2013 15:33:27 +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: Re: [RFC PATCH V2] tracing: Check f_dentry before accessing
	event_file/call in inode->i_private

On 07/19, Masami Hiramatsu wrote:
>
> (2013/07/18 23:51), Oleg Nesterov wrote:
> > 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.
>
> No, it replaces trace_array_get/put with ftrace_event_file_get/put
> which calls trace_array_get/put inside.

Ah, I didn't notice your patch adds "file->tr->ref++" into
ftrace_event_file_get...

So I was wrong in any case, thanks for correcting me.

But,

> (Just one point, previous ftrace_event_file_get has a racy point
> when it does tr->ref++, it should be fixed.)

Not sure what do you mean, but unless I missed something again this
"tr->ref++" above still looks racy. instance_delete() checks tr->ref
first, then it takes event_mutex and removes/kfrees event_files.

But this doesn't really matter even if I am right, surely this can
be fixed. My only point, imho this is more complex than necessary.

In particular,

> > 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.
>
> Agreed :)

Yes ;) and this makes the ref-counting even more complex, we use
different methods to avoid the races with rmdir and event_remove().

> > The same bugs which Steven's 1/4 tries to solve ;)
>
> OK, let me confirm that, would you mean we still need 2/4 - 4/4?

Yes, yes.

Oleg.

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