Subject: Re: [PATCH net-next v2 4/8] net: ipv4: Convert inet_rtm_getroute to rcu versions of route lookup

From: John Fastabend <[email-masked]>
Date: Wed, 31 May 2017 13:11:31 -0700

On 05/25/2017 10:42 AM, Roopa Prabhu wrote:
> From: David Ahern <[email-masked]>
> 
> Convert inet_rtm_getroute to use ip_route_input_rcu and
> ip_route_output_key_hash_rcu passing the fib_result arg to both.
> The rcu lock is held through the creation of the response, so the
> rtable/dst does not need to be attached to the skb and is passed
> to rt_fill_info directly.
> 
> In converting from ip_route_output_key to ip_route_output_key_hash_rcu
> the xfrm_lookup_route in ip_route_output_flow is dropped since
> flowi4_proto is not set for a route get request.
> 
> Signed-off-by: David Ahern <[email-masked]>
> Signed-off-by: Roopa Prabhu <[email-masked]>
> ---

Hi Roopa, David,

I'm getting a usage count bug with this patch,

unregister_netdevice: waiting for lo to become free. Usage count = 1

see below,

>  net/ipv4/route.c | 21 +++++++++++++--------
>  1 file changed, 13 insertions(+), 8 deletions(-)
> 
> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> index d8fcecc..1fa9127 100644
> --- a/net/ipv4/route.c
> +++ b/net/ipv4/route.c
> @@ -2534,11 +2534,11 @@ struct rtable *ip_route_output_flow(struct net *net, struct flowi4 *flp4,
>  }
>  EXPORT_SYMBOL_GPL(ip_route_output_flow);
>  
> +/* called with rcu_read_lock held */
>  static int rt_fill_info(struct net *net,  __be32 dst, __be32 src, u32 table_id,
>  			struct flowi4 *fl4, struct sk_buff *skb, u32 portid,
> -			u32 seq)
> +			u32 seq, struct rtable *rt)
>  {
> -	struct rtable *rt = skb_rtable(skb);
>  	struct rtmsg *r;
>  	struct nlmsghdr *nlh;
>  	unsigned long expires = 0;
> @@ -2653,6 +2653,7 @@ static int inet_rtm_getroute(struct sk_buff *in_skb, struct nlmsghdr *nlh,
>  	struct net *net = sock_net(in_skb->sk);
>  	struct rtmsg *rtm;
>  	struct nlattr *tb[RTA_MAX+1];
> +	struct fib_result res = {};
>  	struct rtable *rt = NULL;
>  	struct flowi4 fl4;
>  	__be32 dst = 0;
> @@ -2709,10 +2710,12 @@ static int inet_rtm_getroute(struct sk_buff *in_skb, struct nlmsghdr *nlh,
>  	fl4.flowi4_mark = mark;
>  	fl4.flowi4_uid = uid;
>  
> +	rcu_read_lock();
> +
>  	if (iif) {
>  		struct net_device *dev;
>  
> -		dev = __dev_get_by_index(net, iif);
> +		dev = dev_get_by_index_rcu(net, iif);
>  		if (!dev) {
>  			err = -ENODEV;
>  			goto errout_free;
> @@ -2721,14 +2724,14 @@ static int inet_rtm_getroute(struct sk_buff *in_skb, struct nlmsghdr *nlh,
>  		skb->protocol	= htons(ETH_P_IP);
>  		skb->dev	= dev;
>  		skb->mark	= mark;
> -		err = ip_route_input(skb, dst, src, rtm->rtm_tos, dev);
> +		err = ip_route_input_rcu(skb, dst, src, rtm->rtm_tos,
> +					 dev, &res);
>  
>  		rt = skb_rtable(skb);
>  		if (err == 0 && rt->dst.error)
>  			err = -rt->dst.error;
>  	} else {
> -		rt = ip_route_output_key(net, &fl4);
> -
> +		rt = ip_route_output_key_hash_rcu(net, &fl4, &res, skb);
>  		err = 0;
>  		if (IS_ERR(rt))
>  			err = PTR_ERR(rt);
> @@ -2737,7 +2740,6 @@ static int inet_rtm_getroute(struct sk_buff *in_skb, struct nlmsghdr *nlh,
>  	if (err)
>  		goto errout_free;
>  
> -	skb_dst_set(skb, &rt->dst);

Why did you remove this? Neither ip_route_input() or ip_route_output_key()
seem to justify this with a quick scan on my side. Feel free to correct me
here.

The following fix resolves my issues,

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index f1f2e5a..8f373bd 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -2750,6 +2750,7 @@ static int inet_rtm_getroute(struct sk_buff *in_skb, struct nlmsghdr *nlh,
        if (err)
                goto errout_free;
 
+       skb_dst_set(skb, &rt->dst);
        if (rtm->rtm_flags & RTM_F_NOTIFY)
                rt->rt_flags |= RTCF_NOTIFY;

Thanks,
John 

>  	if (rtm->rtm_flags & RTM_F_NOTIFY)
>  		rt->rt_flags |= RTCF_NOTIFY;
>  
> @@ -2745,15 +2747,18 @@ static int inet_rtm_getroute(struct sk_buff *in_skb, struct nlmsghdr *nlh,
>  		table_id = rt->rt_table_id;
>  
>  	err = rt_fill_info(net, dst, src, table_id, &fl4, skb,
> -			   NETLINK_CB(in_skb).portid, nlh->nlmsg_seq);
> +			   NETLINK_CB(in_skb).portid, nlh->nlmsg_seq, rt);
>  	if (err < 0)
>  		goto errout_free;
>  
> +	rcu_read_unlock();
> +
>  	err = rtnl_unicast(skb, net, NETLINK_CB(in_skb).portid);
>  errout:
>  	return err;
>  
>  errout_free:
> +	rcu_read_unlock();
>  	kfree_skb(skb);
>  	goto errout;
>  }
>