Re: [PATCH net v2] igmp: fix the problem when mc leave group

[David Miller <davem@davemloft.net>]

From: Ding Tianhong <dingtianhong@huawei.com>
Date: Wed, 2 Jul 2014 13:50:48 +0800

> The problem was triggered by these steps:
> 
> 1) create socket, bind and then setsockopt for add mc group.
>    mreq.imr_multiaddr.s_addr = inet_addr("255.0.0.37");
>    mreq.imr_interface.s_addr = inet_addr("192.168.1.2");
>    setsockopt(sockfd, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq));
> 
> 2) drop the mc group for this socket.
>    mreq.imr_multiaddr.s_addr = inet_addr("255.0.0.37");
>    mreq.imr_interface.s_addr = inet_addr("0.0.0.0");
>    setsockopt(sockfd, IPPROTO_IP, IP_DROP_MEMBERSHIP, &mreq, sizeof(mreq));
> 
> 3) and then drop the socket, I found the mc group was still used by the dev:
> 
>    netstat -g
> 
>    Interface       RefCnt Group
>    --------------- ------ ---------------------
>    eth2		   1	  255.0.0.37
> 
> Normally even though the IP_DROP_MEMBERSHIP return error, the mc group still need
> to be released for the netdev when drop the socket, but this process was broken when
> route default is NULL, the reason is that:
> 
> The ip_mc_leave_group() will choose the in_dev by the imr_interface.s_addr, if input addr
> is NULL, the default route dev will be chosen, then the ifindex is got from the dev,
> then polling the inet->mc_list and return -ENODEV, but if the default route dev is NULL,
> the in_dev and ifIndex is both NULL, when polling the inet->mc_list, the mc group will be
> released from the mc_list, but the dev didn't dec the refcnt for this mc group, so
> when dropping the socket, the mc_list is NULL and the dev still keep this group.
> 
> v1->v2: According Hideaki's suggestion, we should align with IPv6 (RFC3493) and BSDs,
> 	so I add the checking for the in_dev before polling the mc_list, make sure when
> 	we remove the mc group, dec the refcnt to the real dev which was using the mc address.
> 	The problem would never happened again.
> 
> Signed-off-by: Ding Tianhong <dingtianhong@huawei.com>

Applied, thank you.