netdev - Re:Re: [PATCH net-next 1/1] driver: pptp: Remove unnecessary statements in pptp_sock

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Date:   Thu, 10 Aug 2017 11:54:50 +0800 (CST)
From:   "Gao Feng" <gfree.wind@....163.com>
To:     "Cong Wang" <xiyou.wangcong@...il.com>
Cc:     xeb@...l.ru, "David Miller" <davem@...emloft.net>,
        "Linux Kernel Network Developers" <netdev@...r.kernel.org>
Subject: Re:Re: [PATCH net-next 1/1] driver: pptp: Remove unnecessary
 statements in pptp_sock_destruct


At 2017-08-10 02:08:30, "Cong Wang" <xiyou.wangcong@...il.com> wrote:
>On Wed, Aug 9, 2017 at 8:57 AM,  <gfree.wind@....163.com> wrote:
>> From: Gao Feng <gfree.wind@....163.com>
>>
>> In the commit ddab82821fa6 ("ppp: Fix a scheduling-while-atomic bug in
>> del_chan"), I moved the synchronize_rcu() from del_chan() to pptp_release
>> after del_chan() to avoid one scheduling-while-atomic bug.
>>
>> Actually the del_chan() and pppox_unbind_sock are unneccessary in the
>> pptp_sock_destruct. Because the pptp sock refcnt wouldn't reach zero until
>> sk_state is set as PPPOX_DEAD in pptp_release. By that time, the del_chan()
>> and pppox_unbind_sock() have been invoked already and the condition check
>> "!(sk->sk_state & PPPOX_DEAD)" of this sock must be false in pptp_sock_destruct.
>
>I am not sure. The check for sock->sk in the beginning of pptp_release()
>indicates there could be a case we could skip del_chan() in pptp_release(),
>although I can't figure out how.
>
>Also there is a suspicious sock_put() in pptp_release().

Hi Cong, 

Thank you.  I also failed to find the case which causes the sock->sk is null in release().

There is a suspicious case in __sock_create following.
        err = pf->create(net, sock, protocol, kern);
        if (err < 0)
                goto out_module_put;
        .......
out_module_put:
        sock->ops = NULL;
        module_put(pf->owner);
out_sock_release:
        sock_release(sock);
        return err;
In the beginning, I thought when create is failed and the sock->sk is null, then the sock_release is invoked.
It could cause the sk is null in the release().
But I find  it has already reset the sock->ops as NULL before sock_release later, so the release() wouldn't be invoked actually.

Best Regards
Feng