netdev - Re: [PATCH v1 3/5] cgroup: Protect access to task_cls

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Date:	Mon, 20 Aug 2012 13:29:38 +0200
From:	Daniel Wagner <wagi@...om.org>
To:	Neil Horman <nhorman@...driver.com>
Cc:	netdev@...r.kernel.org, cgroups@...r.kernel.org,
	Daniel Wagner <daniel.wagner@...-carit.de>,
	"David S. Miller" <davem@...emloft.net>,
	Gao feng <gaofeng@...fujitsu.com>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	John Fastabend <john.r.fastabend@...el.com>,
	Li Zefan <lizefan@...wei.com>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v1 3/5] cgroup: Protect access to task_cls_classid() when
 built as module

On Fri, Aug 17, 2012 at 02:28:55PM -0400, Neil Horman wrote:
> On Fri, Aug 17, 2012 at 04:58:12PM +0200, Daniel Wagner wrote:
> > From: Daniel Wagner <daniel.wagner@...-carit.de>
> > 
> > The module version of task_cls_classid() checks if net_cls_sbusys_id
> > is valid to indentify when it is okay to access the controller.
> > 
> > Instead relying on the subusys_id to be set, make it explicit
> > with a jump label.
> > 
> > Signed-off-by: Daniel Wagner <daniel.wagner@...-carit.de>
> > Cc: "David S. Miller" <davem@...emloft.net>
> > Cc: Gao feng <gaofeng@...fujitsu.com>
> > Cc: Jamal Hadi Salim <jhs@...atatu.com>
> > Cc: John Fastabend <john.r.fastabend@...el.com>
> > Cc: Li Zefan <lizefan@...wei.com>
> > Cc: Neil Horman <nhorman@...driver.com>
> > Cc: Tejun Heo <tj@...nel.org>
> > Cc: netdev@...r.kernel.org
> > Cc: cgroups@...r.kernel.org
> > ---
> >  include/net/cls_cgroup.h | 5 ++++-
> >  net/core/sock.c          | 5 +++++
> >  net/sched/cls_cgroup.c   | 9 +++++++++
> >  3 files changed, 18 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/net/cls_cgroup.h b/include/net/cls_cgroup.h
> > index 401672c..bbbd957 100644
> > --- a/include/net/cls_cgroup.h
> > +++ b/include/net/cls_cgroup.h
> > @@ -16,6 +16,7 @@
> >  #include <linux/cgroup.h>
> >  #include <linux/hardirq.h>
> >  #include <linux/rcupdate.h>
> > +#include <linux/jump_label.h>
> >  
> >  #ifdef CONFIG_CGROUPS
> >  struct cgroup_cls_state
> > @@ -44,6 +45,8 @@ static inline u32 task_cls_classid(struct task_struct *p)
> >  }
> >  
> >  #elif IS_MODULE(CONFIG_NET_CLS_CGROUP)
> > +extern struct static_key cgroup_cls_enabled;
> > +#define clscg_enabled static_key_false(&cgroup_cls_enabled)
> >  
> >  extern int net_cls_subsys_id;
> >  
> > @@ -52,7 +55,7 @@ static inline u32 task_cls_classid(struct task_struct *p)
> >  	int id;
> >  	u32 classid = 0;
> >  
> > -	if (in_interrupt())
> > +	if (!clscg_enabled || in_interrupt())
> >  		return 0;
> >  
> >  	rcu_read_lock();
> > diff --git a/net/core/sock.c b/net/core/sock.c
> > index 8f67ced..8106e77 100644
> > --- a/net/core/sock.c
> > +++ b/net/core/sock.c
> > @@ -327,6 +327,11 @@ int __sk_backlog_rcv(struct sock *sk, struct sk_buff *skb)
> >  EXPORT_SYMBOL(__sk_backlog_rcv);
> >  
> >  #if defined(CONFIG_CGROUPS)
> > +#if IS_MODULE(CONFIG_NET_CLS_CGROUP)
> > +struct static_key cgroup_cls_enabled = STATIC_KEY_INIT_FALSE;
> > +EXPORT_SYMBOL_GPL(cgroup_cls_enabled);
> > +#endif
> > +
> >  #if !defined(CONFIG_NET_CLS_CGROUP)
> >  int net_cls_subsys_id = -1;
> >  EXPORT_SYMBOL_GPL(net_cls_subsys_id);
> > diff --git a/net/sched/cls_cgroup.c b/net/sched/cls_cgroup.c
> > index 7743ea8..0635894 100644
> > --- a/net/sched/cls_cgroup.c
> > +++ b/net/sched/cls_cgroup.c
> > @@ -44,12 +44,21 @@ static struct cgroup_subsys_state *cgrp_create(struct cgroup *cgrp)
> >  
> >  	if (cgrp->parent)
> >  		cs->classid = cgrp_cls_state(cgrp->parent)->classid;
> > +#if IS_MODULE(CONFIG_NET_CLS_CGROUP)
> > +	else if (!clscg_enabled)
> > +                static_key_slow_inc(&cgroup_cls_enabled);
> This is racy I think.  The read of the static key is atomic with other reads,
> but the entire conditional is not atomic.  If two cpus were creating cgroups in
> parallel, it would be possible for both to read the static key as being zero
> (the second cpu would read the key before the first cpu could increment it).

D'oh, That is racy.

> > +#endif
> >  
> >  	return &cs->css;
> >  }
> >  
> >  static void cgrp_destroy(struct cgroup *cgrp)
> >  {
> > +#if IS_MODULE(CONFIG_NET_CLS_CGROUP)
> > +	if (!cgrp->parent && clscg_enabled)
> > +                static_key_slow_dec(&cgroup_cls_enabled);
> Ditto here with the race above.  I think what you want is one of:
> 
> 1) Use static_key_slow_[inc|dec] unconditionally 

While the static_key_slow_inc() case will work, I am not so sure about
the static_key_slow_dec(), e.g. we could still access inside
task_cls_classid() a destroyed container.

> 2) Keep a separate internal counter to track the number of cgroup instances
> so that you only inc the static key on the first create and dec it on the last
> delete.

If I got you right, than this would not be different then direclty using
static_key_slow_[inc|dec].

> I would think (1) would be sufficent.  It looks like static_key_slow_inc uses
> atomic_inc_not_zero to just do an inc anyway in the event that multiple inc
> events are made.

Would something like this work?

static inline u32 task_cls_classid(struct task_struct *p)
{
	u32 classid;
	struct cgroup_cls_state *css;

	if (!clscg_enabled || in_interrupt())
		return 0;

	rcu_read_lock();
	css = container_of(task_subsys_state(p, net_cls_subsys_id),
			   struct cgroup_cls_state, css);
	if (!css)
		classid = css->classid;
	else
		classid = 0;
	rcu_read_unlock();

	return classid;
}

Daniel
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