netdev - 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 13:13:53 +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: Re:Re: Re: [PATCH net] ppp: Fix a scheduling-while-atomic
 bug in del_chan

At 2017-08-09 03:45:53, "Cong Wang" <xiyou.wangcong@...il.com> wrote:
>On Mon, Aug 7, 2017 at 6:10 PM, Gao Feng <gfree.wind@....163.com> wrote:
>>
>> Sorry, I don't get you clearly. Why the sock_hold() isn't helpful?
>
>I already told you, the dereference happends before sock_hold().
>
>        sock = rcu_dereference(callid_sock[call_id]);
>        if (sock) {
>                opt = &sock->proto.pptp;
>                if (opt->dst_addr.sin_addr.s_addr != s_addr) <=== HERE
>                        sock = NULL;
>                else
>                        sock_hold(sk_pppox(sock));
>        }
>
>If we don't wait for readers properly, sock could be freed at the
>same time when deference it.

Maybe I didn't show my explanation clearly.
I think it won't happen as I mentioned in the last email.
Because the pptp_release invokes the synchronize_rcu to make sure it, and actually there is no one which would invoke del_chan except pptp_release.
It is guaranteed by that the pptp_release doesn't put the sock refcnt until complete all cleanup include marking sk_state as PPPOX_DEAD. 

In other words, even though the pptp_release is not the last user of this sock, the other one wouldn't invoke del_chan in pptp_sock_destruct.
Because the condition "!(sk->sk_state & PPPOX_DEAD)" must be false. 

As summary, the del_chan and pppox_unbind_sock in pptp_sock_destruct are unnecessary. 
And it even brings confusing.

Best Regards
Feng

>
>> The pptp_release invokes synchronize_rcu after del_chan, it could make sure the others has increased the sock refcnt if necessary
>> and the lookup is over.
>> There is no one could get the sock after synchronize_rcu in pptp_release.
>
>
>If this were true, then this code in pptp_sock_destruct()
>would be unneeded:
>
>        if (!(sk->sk_state & PPPOX_DEAD)) {
>                del_chan(pppox_sk(sk));
>                pppox_unbind_sock(sk);
>        }
>
>
>>
>>
>> But I think about another problem.
>> It seems the pptp_sock_destruct should not invoke del_chan and pppox_unbind_sock.
>> Because when the sock refcnt is 0, the pptp_release must have be invoked already.
>>
>
>
>I don't know. Looks like sock_orphan() is only called
>in pptp_release(), but I am not sure if there is a case
>we call sock destructor before release.
>
>Also note, this socket is very special, it doesn't support
>poll(), sendmsg() or recvmsg()..