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

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Message-ID: <20180724173008.454cdf10@gandalf.local.home>
Date:   Tue, 24 Jul 2018 17:30:08 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Tom Zanussi <tom.zanussi@...ux.intel.com>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Shuah Khan <shuah@...nel.org>,
        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:

> > 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.

Tom,

register_trigger() is messed up. I should have caught this when it was
first submitted, but I'm totally confused. The comments don't match the
code.

First we have this:

	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.
	 */

Which looks to be total BS.

As we have this:

/**
 * register_trigger - Generic event_command @reg implementation
 * @glob: The raw string used to register the trigger
 * @ops: The trigger ops associated with the trigger
 * @data: Trigger-specific data to associate with the trigger
 * @file: The trace_event_file associated with the event
 *
 * Common implementation for event trigger registration.
 *
 * Usually used directly as the @reg method in event command
 * implementations.
 *
 * Return: 0 on success, errno otherwise
 */
static int register_trigger(char *glob, struct event_trigger_ops *ops,
			    struct event_trigger_data *data,
			    struct trace_event_file *file)
{
	struct event_trigger_data *test;
	int ret = 0;

	list_for_each_entry_rcu(test, &file->triggers, list) {
		if (test->cmd_ops->trigger_type == data->cmd_ops->trigger_type) {
			ret = -EEXIST;
			goto out;
		}
	}

	if (data->ops->init) {
		ret = data->ops->init(data->ops, data);
		if (ret < 0)
			goto out;
	}

	list_add_rcu(&data->list, &file->triggers);
	ret++;

	update_cond_flag(file);
	if (trace_event_trigger_enable_disable(file, 1) < 0) {
		list_del_rcu(&data->list);
		update_cond_flag(file);
		ret--;
	}
out:
	return ret;
}

Where the comment is total wrong. It doesn't return 0 on success, it
returns 1. And if trace_event_trigger_enable_disable() fails it returns
zero.

And that can fail with the call->class->reg() return value, which could
fail for various strange reasons. I don't know why we would want to
return 0 when it fails?

I don't see where ->reg() would return anything but 1 on success. Maybe
I'm missing something. I'll look some more, but I'm thinking of changing
->reg() to return zero on all success, and negative on all errors and
just check those results.

-- Steve