Re: [PATCH 1/3] [BUGFIX] tracing: Fix double free of event_trigger_data

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Hi Steve,

On 7/24/2018 4:30 PM, Steven Rostedt wrote:
On Tue, 24 Jul 2018 16:49:59 -0400
Steven Rostedt <rostedt@xxxxxxxxxxx> 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.

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.


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.

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

Tom

-- Steve




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