Re: [PATCH v4 net-next 2/2] ipv4: ICMP packet inspection for multipath

ICMP packets are inspected to let them route together with the flow they
belong to, minimizing the chance that a problematic path will affect flows
on other paths, and so that anycast environments can work with ECMP.

Signed-off-by: Peter Nørlund <redacted>
---
 include/net/route.h |   11 +++++++++-
 net/ipv4/icmp.c     |   19 ++++++++++++++++-
 net/ipv4/route.c    |   57 +++++++++++++++++++++++++++++++++++++++++++++------
 3 files changed, 79 insertions(+), 8 deletions(-)

diff --git a/include/net/route.h b/include/net/route.h
index 10a7d21..0f7abdc 100644
--- a/include/net/route.h
+++ b/include/net/route.h

@@ -28,6 +28,7 @@
 #include <net/inetpeer.h>
 #include <net/flow.h>
 #include <net/inet_sock.h>
+#include <net/ip_fib.h>
 #include <linux/in_route.h>
 #include <linux/rtnetlink.h>
 #include <linux/rcupdate.h>

@@ -112,7 +113,15 @@ struct in_device;
 int ip_rt_init(void);
 void rt_cache_flush(struct net *net);
 void rt_flush_dev(struct net_device *dev);
-struct rtable *__ip_route_output_key(struct net *, struct flowi4 *flp);
+struct rtable *__ip_route_output_key_hash(struct net *, struct flowi4 *flp,
+                                         int mp_hash);
+
+static inline struct rtable *__ip_route_output_key(struct net *net,
+                                                  struct flowi4 *flp)
+{
+       return __ip_route_output_key_hash(net, flp, -1);
+}
+
 struct rtable *ip_route_output_flow(struct net *, struct flowi4 *flp,
                                    struct sock *sk);
 struct dst_entry *ipv4_blackhole_route(struct net *net,

diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
index 79fe05b..f81daca 100644
--- a/net/ipv4/icmp.c
+++ b/net/ipv4/icmp.c

@@ -440,6 +440,22 @@ out_unlock:
        icmp_xmit_unlock(sk);
 }

+#ifdef CONFIG_IP_ROUTE_MULTIPATH
+
+/* Source and destination is swapped. See ip_multipath_icmp_hash */
+static int icmp_multipath_hash_skb(const struct sk_buff *skb)
+{
+       const struct iphdr *iph = ip_hdr(skb);
+
+       return fib_multipath_hash(iph->daddr, iph->saddr);
+}
+
+#else
+
+#define icmp_multipath_hash_skb(skb) (-1)
+
+#endif
+
 static struct rtable *icmp_route_lookup(struct net *net,
                                        struct flowi4 *fl4,
                                        struct sk_buff *skb_in,

@@ -464,7 +480,8 @@ static struct rtable *icmp_route_lookup(struct net *net,
        fl4->flowi4_oif = vrf_master_ifindex(skb_in->dev) ? : skb_in->dev->ifindex;

        security_skb_classify_flow(skb_in, flowi4_to_flowi(fl4));
-       rt = __ip_route_output_key(net, fl4);
+       rt = __ip_route_output_key_hash(net, fl4,
+                                       icmp_multipath_hash_skb(skb_in));
        if (IS_ERR(rt))
                return rt;

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 3dc83b6..29048e1 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c

@@ -1652,6 +1652,48 @@ out:
        return err;
 }

+#ifdef CONFIG_IP_ROUTE_MULTIPATH
+
+/* To make ICMP packets follow the right flow, the multipath hash is
+ * calculated from the inner IP addresses in reverse order.
+ */
+static int ip_multipath_icmp_hash(struct sk_buff *skb)
+{

+       const struct iphdr *outer_iph = ip_hdr(skb);
+       struct icmphdr _icmph;
+       const struct icmphdr *icmph;
+       struct iphdr _inner_iph;
+       const struct iphdr *inner_iph;
+
+       if (unlikely((outer_iph->frag_off & htons(IP_OFFSET)) != 0))
+               goto standard_hash;
+
+       icmph = skb_header_pointer(skb, outer_iph->ihl * 4, sizeof(_icmph),
+                                  &_icmph);
+       if (!icmph)
+               goto standard_hash;
+
+       if (icmph->type != ICMP_DEST_UNREACH &&
+           icmph->type != ICMP_REDIRECT &&
+           icmph->type != ICMP_TIME_EXCEEDED &&
+           icmph->type != ICMP_PARAMETERPROB) {
+               goto standard_hash;
+       }
+
+       inner_iph = skb_header_pointer(skb,
+                                      outer_iph->ihl * 4 + sizeof(_icmph),
+                                      sizeof(_inner_iph), &_inner_iph);
+       if (!inner_iph)
+               goto standard_hash;
+
+       return fib_multipath_hash(inner_iph->daddr, inner_iph->saddr);
+
+standard_hash:
+       return fib_multipath_hash(outer_iph->saddr, outer_iph->daddr);
+}
+
+#endif /* CONFIG_IP_ROUTE_MULTIPATH */
+
 static int ip_mkroute_input(struct sk_buff *skb,
                            struct fib_result *res,
                            const struct flowi4 *fl4,

@@ -1662,7 +1704,10 @@ static int ip_mkroute_input(struct sk_buff *skb,
        if (res->fi && res->fi->fib_nhs > 1) {
                int h;

-               h = fib_multipath_hash(saddr, daddr);
+               if (unlikely(ip_hdr(skb)->protocol == IPPROTO_ICMP))
+                       h = ip_multipath_icmp_hash(skb);
+               else
+                       h = fib_multipath_hash(saddr, daddr);
                fib_select_multipath(res, h);
        }
 #endif

@@ -2032,7 +2077,8 @@ add:
  * Major route resolver routine.
  */

-struct rtable *__ip_route_output_key(struct net *net, struct flowi4 *fl4)
+struct rtable *__ip_route_output_key_hash(struct net *net, struct flowi4 *fl4,
+                                         int mp_hash)
 {
        struct net_device *dev_out = NULL;
        __u8 tos = RT_FL_TOS(fl4);

@@ -2195,10 +2241,9 @@ struct rtable *__ip_route_output_key(struct net *net, struct flowi4 *fl4)

 #ifdef CONFIG_IP_ROUTE_MULTIPATH
        if (res.fi->fib_nhs > 1 && fl4->flowi4_oif == 0) {
-               int h;
-
-               h = fib_multipath_hash(fl4->saddr, fl4->daddr);
-               fib_select_multipath(&res, h);
+               if (mp_hash < 0)
+                       mp_hash = fib_multipath_hash(fl4->saddr, fl4->daddr);
+               fib_select_multipath(&res, mp_hash);
        }
        else
 #endif
--

1.7.10.4

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