netdev - Re: Re:Re: Re:Re: Re: [PATCH net] ppp: Fix a scheduling-while-atomic bug in del

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Date:   Wed, 9 Aug 2017 14:00:19 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Gao Feng <gfree.wind@....163.com>
Cc:     xeb@...l.ru, David Miller <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: Re:Re: Re:Re: Re: [PATCH net] ppp: Fix a scheduling-while-atomic
 bug in del_chan

On Wed, Aug 9, 2017 at 12:17 AM, Gao Feng <gfree.wind@....163.com> wrote:
> Hi Cong,
>
> Actually I have one question about the SOCK_RCU_FREE.
> I don't think it could resolve the issue you raised even though it exists really.
>
> I checked the SOCK_RCU_FREE, it just defer the __sk_destruct after one rcu period.
> But when it performs, someone still could find this sock by callid during the del_chan period and it may still deference the sock which may freed soon.
>
> The right flow should be following.
> del_chan()
> wait a rcu period
> sock_put() ------------ It is safe that someone gets the sock because it already hold sock refcnt.
>
> When using SOCK_RCU_FREE, its flow would be following.
> wait a rcu period
> del_chan()
> free the sock directly -------- no sock refcnt check again.
> Because the del_chan happens after rcu wait, not before, so it isn't helpful with SOCK_RCU_FREE.


Yes, good point! With SOCK_RCU_FREE the sock_hold() should
not be needed. For RCU, unpublish should indeed happen before
grace period.


>
> I don't know if I misunderstand the SOCK_RCU_FREE usage.
>
> But it is a good news that the del_chan is only invoked in pptp_release actually and it would wait a rcu period before sock_put.
>

Looking at the code again, the reader lookup_chan() is actually
invoked in BH context, but neither add_chan() nor del_chan()
actually disables BH...