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

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Message-Id: <20180725100520.9370df20d29d529236f4a368@kernel.org>
Date:   Wed, 25 Jul 2018 10:05:20 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.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 Tue, 24 Jul 2018 16:49:59 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> 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;

Can we mixed up ret == 0 and ret > 0? It seems cmd_ops->reg() == 0
is a failure case.

> 
> 	/* 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. 

To avoid confusion, I would like to suggest to rename those pair to
event_trigger_data_get/put(). :)

> 
> 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!
> 
> Thanks for pointing it out.
> 
> -- Steve


-- 
Masami Hiramatsu <mhiramat@...nel.org>