linux-kernel - Re: [PATCH 1/3] [BUGFIX] tracing: Fix double free of event_trigger

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Date:   Tue, 24 Jul 2018 16:49:59 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Shuah Khan <shuah@...nel.org>,
        Tom Zanussi <tom.zanussi@...ux.intel.com>,
        Hiraku Toyooka <hiraku.toyooka@...ertrust.co.jp>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 1/3] [BUGFIX] tracing: Fix double free of
 event_trigger_data

On Wed, 25 Jul 2018 00:09:09 +0900
Masami Hiramatsu <mhiramat@...nel.org> wrote:

> Hmm, your patch seems to leak a memory since event_trigger_init() will
> be called twice on same trigger_data (Note that event_trigger_init()
> does not init ref counter, but increment it.) So we should decrement
> it when we find it is succeeded. Moreover, if register_trigger()

Good catch, and easily fixed.

> fails before calling data->ops->init() (see -EEXIST case), the ref
> counter will be 0 (-1 +1). But if it fails after data->ops->init(),
> the ref counter will be 1 (-1 +1 +1). It still be unstable.
> (Ah, that means we may have another trouble...)

I'm not sure there's a problem here. I now have:

 out_reg:
	/* Up the trigger_data count to make sure reg doesn't free it on failuer */
	event_trigger_init(trigger_ops, trigger_data);
	ret = cmd_ops->reg(glob, trigger_ops, trigger_data, file);
	/*
	 * The above returns on success the # of functions enabled,
	 * but if it didn't find any functions it returns zero.
	 * Consider no functions a failure too.
	 */
	if (!ret) {
		ret = -ENOENT;
	} else if (ret > 0)
		ret = 0;

	/* Down the counter of trigger_data or free it if not used anymore */
	event_trigger_free(trigger_ops, trigger_data);
 out:
	return ret;

Thus we increment trigger_data before calling reg, and free it
afterward. But if reg() did an init too, then the event_trigger_free()
just decs the ref counter.

As for register_trigger()


> 
> > 
> > P.S. This brings up another minor bug. The failure should return ENOMEM
> > not ENOENT.  
> 
> Hmm it seems we should review the register_trigger() implementation.
> It should return the return value of trace_event_trigger_enable_disable(),
> shouldn't it?
>

Yeah, that's not done well. I'll fix it up.

Thanks for pointing it out.

-- Steve