netdev - Re: [Patch net] cgroup: fix cgroup_sk_alloc() for sk_clone

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Message-ID: <20200620155751.GJ237539@carbon.dhcp.thefacebook.com>
Date:   Sat, 20 Jun 2020 08:57:51 -0700
From:   Roman Gushchin <guro@...com>
To:     Cong Wang <xiyou.wangcong@...il.com>
CC:     Zefan Li <lizefan@...wei.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Cameron Berkenpas <cam@...-zeon.de>,
        Peter Geis <pgwipeout@...il.com>,
        Lu Fengqi <lufq.fnst@...fujitsu.com>,
        Daniël Sonck <dsonck92@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [Patch net] cgroup: fix cgroup_sk_alloc() for sk_clone_lock()

On Fri, Jun 19, 2020 at 08:00:41PM -0700, Cong Wang wrote:
> On Fri, Jun 19, 2020 at 6:14 PM Roman Gushchin <guro@...com> wrote:
> >
> > On Sat, Jun 20, 2020 at 09:00:40AM +0800, Zefan Li wrote:
> > > I think so, though I'm not familiar with the bfp cgroup code.
> > >
> > > > If so, we might wanna fix it in a different way,
> > > > just checking if (!(css->flags & CSS_NO_REF)) in cgroup_bpf_put()
> > > > like in cgroup_put(). It feels more reliable to me.
> > > >
> > >
> > > Yeah I also have this idea in my mind.
> >
> > I wonder if the following patch will fix the issue?
> 
> Interesting, AFAIU, this refcnt is for bpf programs attached
> to the cgroup. By this suggestion, do you mean the root
> cgroup does not need to refcnt the bpf programs attached
> to it? This seems odd, as I don't see how root is different
> from others in terms of bpf programs which can be attached
> and detached in the same way.
> 
> I certainly understand the root cgroup is never gone, but this
> does not mean the bpf programs attached to it too.
> 
> What am I missing?

It's different because the root cgroup can't be deleted.

All this reference counting is required to automatically detach bpf programs
from a _deleted_ cgroup (look at cgroup_bpf_offline()). It's required
because a cgroup can be in dying state for a long time being pinned by a
pagecache page, for example. Only a user can detach a bpf program from
an existing cgroup.

Thanks!