Re: [PATCH 2/3] ipv4/icmp: l3mdev: Perform icmp error route lookup on source device routing table

[David Miller <davem@davemloft.net>]

From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Tue, 11 Aug 2020 15:50:02 -0400

> @@ -465,6 +465,7 @@ static struct rtable *icmp_route_lookup(struct net *net,
>  					int type, int code,
>  					struct icmp_bxm *param)
>  {
> +	struct net_device *route_lookup_dev = NULL;
>  	struct rtable *rt, *rt2;
>  	struct flowi4 fl4_dec;
>  	int err;
> @@ -479,7 +480,17 @@ static struct rtable *icmp_route_lookup(struct net *net,
>  	fl4->flowi4_proto = IPPROTO_ICMP;
>  	fl4->fl4_icmp_type = type;
>  	fl4->fl4_icmp_code = code;
> -	fl4->flowi4_oif = l3mdev_master_ifindex(skb_dst(skb_in)->dev);
> +	/*
> +	 * The device used for looking up which routing table to use is
> +	 * preferably the source whenever it is set, which should ensure
> +	 * the icmp error can be sent to the source host, else fallback
> +	 * on the destination device.
> +	 */
> +	if (skb_in->dev)
> +		route_lookup_dev = skb_in->dev;
> +	else if (skb_dst(skb_in))
> +		route_lookup_dev = skb_dst(skb_in)->dev;
> +	fl4->flowi4_oif = l3mdev_master_ifindex(route_lookup_dev);

The caller of icmp_route_lookup() uses the opposite prioritization of
devices for determining the network namespace to use:

	if (rt->dst.dev)
		net = dev_net(rt->dst.dev);
	else if (skb_in->dev)
		net = dev_net(skb_in->dev);
	else
		goto out;

Do we have to reverse the ordering there too?

And when I read fallback in your commit message description, I
imagined that you would have a two tiered lookup scheme.  First you
would be trying the skb_in->dev for a lookup (to accomodate the VRF
case), and if that failed you'd try again with skb_dst()->dev.