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

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Date:   Wed, 25 Jul 2018 12:09:30 -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 Wed, 25 Jul 2018 11:01:22 -0500
Tom Zanussi <tom.zanussi@...ux.intel.com> wrote:

> > 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.  
> 
> Yes, it is BS in the case of event triggers.  This was taken from the 
> ftrace function trigger code, where it does make sense.  I think I left 
> it in thinking the code would at some point later converge.

OK, that makes a little more sense.

> 
> > 
> > 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  
> 
> And this is how it should work.
> 
> >   */
> > 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.
> >   
> 
> Right, in the case of event triggers, we only register one at a time, 
> whereas with the trace function triggers, with globbing multiple 
> functions can register triggers at the same time, so it makes sense 
> there to have reg() return a count and the more convoluted error handling.

OK, reg in function probes will be handled differently.

> 
> So I agree, simplifying things here by using the standard error handling 
> would be an improvement.

I'll start working on something for 4.19 to simplify it.

Thanks for confirming!

-- Steve