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Message-ID: <20120820170342.GC1734@hmsreliant.think-freely.org>
Date: Mon, 20 Aug 2012 13:03:42 -0400
From: Neil Horman <nhorman@...driver.com>
To: Daniel Wagner <wagi@...om.org>
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 Mon, Aug 20, 2012 at 01:29:38PM +0200, Daniel Wagner wrote:
> 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.
>
Possibly, yes, I think.
> > 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].
>
As long as a cgroup subsystems ->destroy method is only called when the
subsystem is being removed, then I think thats correct. I'm not 100% sure thats
the case though.
> > 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?
>
I think so yes, assuming that you also make the slow_inc|dec changes
> 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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