netdev - Re: [PATCH net-next v4] bridge: export multicast database via netlink

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Message-ID: <20121207084833.GH16122@casper.infradead.org>
Date:	Fri, 7 Dec 2012 08:48:33 +0000
From:	Thomas Graf <tgraf@...g.ch>
To:	Cong Wang <amwang@...hat.com>
Cc:	netdev@...r.kernel.org, bridge@...ts.linux-foundation.org,
	Herbert Xu <herbert@...dor.hengli.com.au>,
	Stephen Hemminger <shemminger@...tta.com>,
	"David S. Miller" <davem@...emloft.net>,
	Jesper Dangaard Brouer <brouer@...hat.com>
Subject: Re: [PATCH net-next v4] bridge: export multicast database via netlink

On 12/07/12 at 11:23am, Cong Wang wrote:
> +static int br_mdb_dump(struct sk_buff *skb, struct netlink_callback *cb)
> +{
> +	struct net_device *dev;
> +	struct net *net = sock_net(skb->sk);
> +	struct nlmsghdr *nlh;

Set nlh = NULL

> +	int idx = 0, s_idx;
> +
> +	s_idx = cb->args[0];
> +
> +	rcu_read_lock();
> +
> +	for_each_netdev_rcu(net, dev) {
> +		if (dev->priv_flags & IFF_EBRIDGE) {
> +			struct br_port_msg *bpm;
> +
> +			if (idx < s_idx)
> +				goto skip;
> +
> +			nlh = nlmsg_put(skb, NETLINK_CB(cb->skb).portid,
> +					cb->nlh->nlmsg_seq, RTM_GETMDB,
> +					sizeof(*bpm), NLM_F_MULTI);
> +			if (nlh == NULL)
> +				break;
> +
> +			bpm = nlmsg_data(nlh);
> +			bpm->ifindex = dev->ifindex;
> +			if (br_mdb_fill_info(skb, cb, dev) < 0)
> +				goto out;
> +			if (br_rports_fill_info(skb, cb, dev) < 0)
> +				goto out;

You need to reset cb->args[1] to 0 here so that when you process the
next mdb it will not skip any entries.

> +
> +			nlmsg_end(skb, nlh);
> +		skip:
> +			idx++;
> +		}
> +	}
> +
> +out:

You need to call nlmsg_end(skb, nlh) here if nlh != NULL
because you need to finalize the message in case you come
from the "goto out" above. Otherwise your partial message
is corrupt.

> +	cb->seq = cb->args[2];

This can't possibly work if you have multiple bridges unless
all of them have an identical mdb->seq.

Maybe leave the consistent dumping problem out for now and just
set cb->seq = net->dev_base_seq so that you at least cover all
bridges.

We don't need to guarantee that no rehash has happened throughout
the dump, we only need to ensure that no rehash happnened if a
bridge required more than one netlink message. You could store
mdb->seq in cb->args[3] and compare it with the current mdb->seq
after br_rports_fill_info() finished, if they differ you could
just cb->seq++. I suggst you leave this out for now and work on this
in a follow-up patch to not complicate this any further.
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