Date:	Mon, 3 Dec 2012 10:01:45 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Aristeu Rozanski <aris@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Serge Hallyn <serge.hallyn@...onical.com>,
	cgroups@...r.kernel.org
Subject: Re: [PATCH 5/5] device_cgroup: propagate local changes down the
 hierarchy

Hello, Aristeu.

On Tue, Nov 27, 2012 at 02:35:06PM -0500, Aristeu Rozanski wrote:
> This patch makes all changes propagate down in hierarchy respecting when
> possible local configurations.
> 
> Behavior changes will clean up exceptions in all the children except when the
> parent changes the behavior from allow to deny and the child's behavior was
> already deny, in which case the local exceptions will be reused. The inverse
> is not possible: you can't have a parent with behavior deny and a child with
> behavior accept.
> 
> New exceptions allowing additional access to devices won't be propagated, but
> it'll be possible to add an exception to access all of part of the newly
> allowed device(s).
> 
> New exceptions disallowing access to devices will be propagated down and the
> local group's exceptions will be revalidated for the new situation.

I think the inheritance policy needs to be documented in detail
listing the possible cases and the implemented behavior preferably
with rationale.  Can you please do that?

> +/**
> + * __revalidate_exceptions - walks through the exception list and revalidates
> + *			     the exceptions based on parents' behavior and
> + *			     exceptions. Called with devcgroup_mutex held.

new line

> + * @devcg: cgroup which exceptions will be checked
> + * returns: 0 in success, -ENOMEM in case of out of memory
> + */
> +static int __revalidate_exceptions(struct dev_cgroup *devcg)

Why __?

> +{
> +	struct dev_exception_item *ex;
> +	struct list_head *this, *tmp;
> +
> +	list_for_each_safe(this, tmp, &devcg->local.exceptions) {
> +		ex = container_of(this, struct dev_exception_item, list);
> +		if (parent_has_perm(devcg, ex)) {
> +			if (dev_exception_copy(&devcg->exceptions, ex))
> +				goto error;
> +		}
> +	}
> +	return 0;
> +
> +error:
> +	__dev_exception_clean(&devcg->exceptions);

Ditto.

> +	return -ENOMEM;
> +}
> +
> +/**
> + * propagate_behavior - propagates a change in the behavior to the children
> + * @devcg: device cgroup that changed behavior
> + *
> + * returns: 0 in case of success, != 0 in case of error
> + */
> +static int propagate_behavior(struct dev_cgroup *devcg)
> +{
> +	struct cgroup *root = devcg->css.cgroup, *pos, *saved = NULL,
> +			*prev = NULL;
> +	struct dev_cgroup *parent;
> +	int rc = 0;
> +
> +	while (1) {
> +		rcu_read_lock();
> +		cgroup_for_each_descendant_pre(pos, root) {
> +			if (saved && prev != saved) {
> +				prev = pos;
> +				continue;
> +			}
> +			break;
> +		}
> +		rcu_read_unlock();

Hmmm... this can race with new cgroup creation and a new child can
escape propagation.  devcg currently inherits from css_alloc() at
which it isn't visible to cgroup_for_each_*() iteration.  The
inheriting step should be moved to css_online() with explicit online
marking.  Please take a look at the recently posted cpuset for an
example.

> +
> +		if (!pos)
> +			break;
> +
> +		devcg = cgroup_to_devcgroup(pos);
> +		parent = cgroup_to_devcgroup(pos->parent);
> +
> +		/* first copy parent's state */
> +		devcg->behavior = parent->behavior;
> +		__dev_exception_clean(&devcg->exceptions);
> +		rc = dev_exceptions_copy(&devcg->exceptions, &parent->exceptions);
> +		if (rc) {
> +			devcg->behavior = DEVCG_DEFAULT_DENY;
> +			break;
> +		}
> +
> +		if (devcg->local.behavior == DEVCG_DEFAULT_DENY &&
> +		    devcg->behavior == DEVCG_DEFAULT_ALLOW) {
> +			devcg->behavior = DEVCG_DEFAULT_DENY;
> +		}
> +		if (devcg->local.behavior == devcg->behavior)
> +			rc = __revalidate_exceptions(devcg);
> +		if (rc)
> +			break;
> +		saved = pos;
> +	}
> +
> +	return rc;
> +}
> +
> +/**
> + * propagate_exception - propagates a new exception to the children
> + * @devcg: device cgroup that added a new exception
> + *
> + * returns: 0 in case of success, != 0 in case of error
> + */
> +static int propagate_exception(struct dev_cgroup *devcg)
> +{
> +	struct cgroup *root = devcg->css.cgroup, *pos, *saved = NULL,
> +			*prev = NULL;
> +	struct dev_cgroup *parent;
> +	int rc = 0;
> +
> +	while(1) {
> +		rcu_read_lock();
> +		cgroup_for_each_descendant_pre(pos, root) {
> +			if (saved && prev != saved) {
> +				prev = pos;
> +				continue;
> +			}
> +			break;
> +		}
> +		rcu_read_unlock();

Ditto.  Racy.

> +		if (!pos)
> +			break;
> +
> +		devcg = cgroup_to_devcgroup(pos);
> +		parent = cgroup_to_devcgroup(pos->parent);
> +
> +		__dev_exception_clean(&devcg->exceptions);
> +		if (devcg->behavior == parent->behavior) {
> +			rc = dev_exceptions_copy(&devcg->exceptions, &parent->exceptions);
> +			if (rc) {
> +				devcg->behavior = DEVCG_DEFAULT_DENY;
> +				break;
> +			}
> +			rc = __revalidate_exceptions(devcg);
> +			if (rc)
> +				break;
> +		} else {
> +			/* we never give more permissions to the child */
> +			WARN_ONCE(devcg->behavior == DEVCG_DEFAULT_ALLOW,
> +				  "devcg: parent/child behavior is inconsistent");
> +			rc = __revalidate_exceptions(devcg);
> +			if (rc)
> +				break;
> +		}
> +		saved = pos;
> +	}
> +	return rc;
> +}

Maybe I'm misunderstanding something but the behavior seems a bit
inconsistent.  So, you can't add an exception which isn't allowed by
your parent, right?  But, if your parent disallows an existing
exception, you get to keep it?  I think it would be more consistent to
go either

* Allow all settings but apply only as allowed by the parent.

* Deny settings disallowed by the parent.  If parent's config changes,
  delete configs which fall outside the new config.

It seems the implemented behavior is something inbetween which can be
quite confusing.

Thanks.

-- 
tejun
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