net: neighbour: Add mcast_resolicit to configure the number of multicast resolicitations in PROBE state.

We send unicast neighbor (ARP or NDP) solicitations ucast_probes
times in PROBE state. Zhu Yanjun reported that some implementation
does not reply against them and the entry will become FAILED, which
is undesirable.

We had been dealt with such nodes by sending multicast probes mcast_
solicit times after unicast probes in PROBE state. In 2003, I made
a change not to send them to improve compatibility with IPv6 NDP.

Let's introduce per-protocol per-interface sysctl knob "mcast_
reprobe" to configure the number of multicast (re)solicitation for
reconfirmation in PROBE state. The default is 0, since we have
been doing so for 10+ years.

Reported-by: Zhu Yanjun <[e-mail]>
CC: Ulf Samuelsson <[e-mail]>
Signed-off-by: YOSHIFUJI Hideaki <[e-mail]>
Signed-off-by: YOSHIFUJI Hideaki <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/neighbour.h

···
 	NEIGH_VAR_MCAST_PROBES,
 	NEIGH_VAR_UCAST_PROBES,
 	NEIGH_VAR_APP_PROBES,
+	NEIGH_VAR_MCAST_REPROBES,
 	NEIGH_VAR_RETRANS_TIME,
 	NEIGH_VAR_BASE_REACHABLE_TIME,
 	NEIGH_VAR_DELAY_PROBE_TIME,

include/uapi/linux/neighbour.h

···
 	NDTPA_PROXY_QLEN,		/* u32 */
 	NDTPA_LOCKTIME,			/* u64, msecs */
 	NDTPA_QUEUE_LENBYTES,		/* u32 */
+	NDTPA_MCAST_REPROBES,		/* u32 */
 	__NDTPA_MAX
 };
 #define NDTPA_MAX (__NDTPA_MAX - 1)

net/core/neighbour.c

···
 static __inline__ int neigh_max_probes(struct neighbour *n)
 {
 	struct neigh_parms *p = n->parms;
-	int max_probes = NEIGH_VAR(p, UCAST_PROBES) + NEIGH_VAR(p, APP_PROBES);
-	if (!(n->nud_state & NUD_PROBE))
-		max_probes += NEIGH_VAR(p, MCAST_PROBES);
-	return max_probes;
+	return NEIGH_VAR(p, UCAST_PROBES) + NEIGH_VAR(p, APP_PROBES) +
+	       (n->nud_state & NUD_PROBE ? NEIGH_VAR(p, MCAST_REPROBES) :
+	        NEIGH_VAR(p, MCAST_PROBES));
 }
 
 static void neigh_invalidate(struct neighbour *neigh)
···
 			NEIGH_VAR(parms, UCAST_PROBES)) ||
 	    nla_put_u32(skb, NDTPA_MCAST_PROBES,
 			NEIGH_VAR(parms, MCAST_PROBES)) ||
+	    nla_put_u32(skb, NDTPA_MCAST_REPROBES,
+			NEIGH_VAR(parms, MCAST_REPROBES)) ||
 	    nla_put_msecs(skb, NDTPA_REACHABLE_TIME, parms->reachable_time) ||
 	    nla_put_msecs(skb, NDTPA_BASE_REACHABLE_TIME,
 			  NEIGH_VAR(parms, BASE_REACHABLE_TIME)) ||
···
 	[NDTPA_APP_PROBES]		= { .type = NLA_U32 },
 	[NDTPA_UCAST_PROBES]		= { .type = NLA_U32 },
 	[NDTPA_MCAST_PROBES]		= { .type = NLA_U32 },
+	[NDTPA_MCAST_REPROBES]		= { .type = NLA_U32 },
 	[NDTPA_BASE_REACHABLE_TIME]	= { .type = NLA_U64 },
 	[NDTPA_GC_STALETIME]		= { .type = NLA_U64 },
 	[NDTPA_DELAY_PROBE_TIME]	= { .type = NLA_U64 },
···
 				break;
 			case NDTPA_MCAST_PROBES:
 				NEIGH_VAR_SET(p, MCAST_PROBES,
+					      nla_get_u32(tbp[i]));
+				break;
+			case NDTPA_MCAST_REPROBES:
+				NEIGH_VAR_SET(p, MCAST_REPROBES,
 					      nla_get_u32(tbp[i]));
 				break;
 			case NDTPA_BASE_REACHABLE_TIME:
···
 		NEIGH_SYSCTL_ZERO_INTMAX_ENTRY(MCAST_PROBES, "mcast_solicit"),
 		NEIGH_SYSCTL_ZERO_INTMAX_ENTRY(UCAST_PROBES, "ucast_solicit"),
 		NEIGH_SYSCTL_ZERO_INTMAX_ENTRY(APP_PROBES, "app_solicit"),
+		NEIGH_SYSCTL_ZERO_INTMAX_ENTRY(MCAST_REPROBES, "mcast_resolicit"),
 		NEIGH_SYSCTL_USERHZ_JIFFIES_ENTRY(RETRANS_TIME, "retrans_time"),
 		NEIGH_SYSCTL_JIFFIES_ENTRY(BASE_REACHABLE_TIME, "base_reachable_time"),
 		NEIGH_SYSCTL_JIFFIES_ENTRY(DELAY_PROBE_TIME, "delay_first_probe_time"),