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

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Date:   Tue, 23 Jun 2020 10:56:13 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     "Zhang,Qiang" <qiang.zhang@...driver.com>
Cc:     Roman Gushchin <guro@...com>, Cong Wang <xiyou.wangcong@...il.com>,
        Peter Geis <pgwipeout@...il.com>,
        Li Zefan <lizefan@...wei.com>,
        Cameron Berkenpas <cam@...-zeon.de>,
        Daniël Sonck <dsonck92@...il.com>,
        Lu Fengqi <lufq.fnst@...fujitsu.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [Patch net] cgroup: fix cgroup_sk_alloc() for sk_clone_lock()

On Tue, Jun 23, 2020 at 1:45 AM Zhang,Qiang <qiang.zhang@...driver.com> wrote:
>
> There are some message in kernelv5.4, I don't know if it will help.
>
> demsg:
>
> cgroup: cgroup: disabling cgroup2 socket matching due to net_prio or
> net_cls activation
...
> -----------[ cut here ]-----------
> percpu ref (cgroup_bpf_release_fn) <= 0 (-12) after switching to atomic
> WARNING: CPU: 1 PID: 0 at lib/percpu-refcount.c:161
> percpu_ref_switch_to_atomic_rcu+0x12a/0x140

Yes, this proves we have the refcnt bug which my patch tries to fix.
The negative refcnt is exactly a consequence of the bug, as without
my patch we just put the refcnt without holding it first.

If you can reproduce it, please test my patch:
https://patchwork.ozlabs.org/project/netdev/patch/20200616180352.18602-1-xiyou.wangcong@gmail.com/

But, so far I still don't have a good explanation to the NULL
pointer deref. I think that one is an older bug, and we need to check
for NULL even after we fix the refcnt bug, but I do not know how it is
just exposed recently with Zefan's patch. I am still trying to find an
explanation.

Thanks!