[PATCH net-next 1/1] driver: pptp: Remove unnecessary statements in pptp_sock_destruct

[PATCH net-next 1/1] driver: pptp: Remove unnecessary statements in pptp_sock_destruct

[gfree.wind]

From: Gao Feng <gfree.wind@vip.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.

So we could remove these statements in pptp_sock_destruct, and then restore
the synchronize_rcu() into del_chan.

Signed-off-by: Gao Feng <gfree.wind@vip.163.com>
---
 drivers/net/ppp/pptp.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/drivers/net/ppp/pptp.c b/drivers/net/ppp/pptp.c
index 6dde9a0..8632e1a 100644
--- a/drivers/net/ppp/pptp.c
+++ b/drivers/net/ppp/pptp.c
@@ -131,6 +131,7 @@ static void del_chan(struct pppox_sock *sock)
 	clear_bit(sock->proto.pptp.src_addr.call_id, callid_bitmap);
 	RCU_INIT_POINTER(callid_sock[sock->proto.pptp.src_addr.call_id], NULL);
 	spin_unlock(&chan_lock);
+	synchronize_rcu();
 }
 
 static int pptp_xmit(struct ppp_channel *chan, struct sk_buff *skb)
@@ -519,7 +520,6 @@ static int pptp_release(struct socket *sock)
 
 	po = pppox_sk(sk);
 	del_chan(po);
-	synchronize_rcu();
 
 	pppox_unbind_sock(sk);
 	sk->sk_state = PPPOX_DEAD;
@@ -535,10 +535,6 @@ static int pptp_release(struct socket *sock)
 
 static void pptp_sock_destruct(struct sock *sk)
 {
-	if (!(sk->sk_state & PPPOX_DEAD)) {
-		del_chan(pppox_sk(sk));
-		pppox_unbind_sock(sk);
-	}
 	skb_queue_purge(&sk->sk_receive_queue);
 }
 
-- 
1.9.1

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

[Cong Wang]

On Wed, Aug 9, 2017 at 8:57 AM,  <gfree.wind@vip.163.com> wrote:
> From: Gao Feng <gfree.wind@vip.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().

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

[Gao Feng]

At 2017-08-10 02:08:30, "Cong Wang" <xiyou.wangcong@gmail.com> wrote:
>On Wed, Aug 9, 2017 at 8:57 AM,  <gfree.wind@vip.163.com> wrote:
>> From: Gao Feng <gfree.wind@vip.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