netdev - Re: [PATCH net] ipv4: Avoid caching dsts when lookup skipped nh oif check

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Message-ID: <7c395b52-d639-9001-c6fa-ccacec4ce0d9@cumulusnetworks.com>
Date:   Thu, 20 Apr 2017 16:18:23 -0600
From:   David Ahern <dsa@...ulusnetworks.com>
To:     Robert Shearman <rshearma@...cade.com>, davem@...emloft.net
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH net] ipv4: Avoid caching dsts when lookup skipped nh oif
 check

On 4/20/17 6:58 AM, Robert Shearman wrote:
> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> index acd69cfe2951..f667783ffd19 100644
> --- a/net/ipv4/route.c
> +++ b/net/ipv4/route.c
> @@ -2125,6 +2125,14 @@ static struct rtable *__mkroute_output(const struct fib_result *res,
>  		fi = NULL;
>  	}
>  
> +	/* If the flag to skip the nh oif check is set then the output
> +	 * device may not match the nh device, so cannot use or add to
> +	 * cache in that case.
> +	 */
> +	if (unlikely(fl4->flowi4_flags & FLOWI_FLAG_SKIP_NH_OIF &&
> +		     FIB_RES_NH(*res).nh_dev != dev_out))
> +		do_cache = false;
> +
>  	fnhe = NULL;
>  	do_cache &= fi != NULL;
>  	if (do_cache) {
> 

I believe this is a better fix:

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 5e1e60546fce..fb74a16958af 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -2407,7 +2407,7 @@ struct rtable *__ip_route_output_key_hash(struct
net *net, struct flowi4 *fl4,
                }

                /* L3 master device is the loopback for that domain */
-               dev_out = l3mdev_master_dev_rcu(dev_out) ? :
net->loopback_dev;
+               dev_out = l3mdev_master_dev_rcu(FIB_RES_DEV(res)) ? :
net->loopback_dev;
                fl4->flowi4_oif = dev_out->ifindex;
                flags |= RTCF_LOCAL;
                goto make_route;

Fixes: 5f02ce24c2696 ("net: l3mdev: Allow the l3mdev to be a loopback")

With your change above, references to vrf devices are still taken
(dev_out is the vrf device based on the flow struct) even though the
route's nexthop is in another domain. And the commit log should
reference the use case which is policy routing overriding the VRF rule.