ipv6: add sysctl_mld_qrv to configure query robustness variable

This patch adds a new sysctl_mld_qrv knob to configure the mldv1/v2 query
robustness variable. It specifies how many retransmit of unsolicited mld
retransmit should happen. Admins might want to tune this on lossy links.

Also reset mld state on interface down/up, so we pick up new sysctl
settings during interface up event.

IPv6 certification requests this knob to be available.

I didn't make this knob netns specific, as it is mostly a setting in a
physical environment and should be per host.

Cc: Flavio Leitner <fbl@redhat.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Acked-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -1152,6 +1152,11 @@ anycast_src_echo_reply - BOOLEAN
 	FALSE: disabled
 	Default: FALSE
 
+mld_qrv - INTEGER
+	Controls the MLD query robustness variable (see RFC3810 9.1).
+	Default: 2 (as specified by RFC3810 9.1)
+	Minimum: 1 (as specified by RFC6636 4.5)
+
 IPv6 Fragmentation:
 
 ip6frag_high_thresh - INTEGER

include/net/ipv6.h

@@ -121,6 +121,7 @@ struct frag_hdr {
 
 /* sysctls */
 extern int sysctl_mld_max_msf;
+extern int sysctl_mld_qrv;
 
 #define _DEVINC(net, statname, modifier, idev, field)			\
 ({									\

net/ipv6/mcast.c

@@ -121,6 +121,7 @@ static int ip6_mc_leave_src(struct sock *sk, struct ipv6_mc_socklist *iml,
 #define IPV6_MLD_MAX_MSF	64
 
 int sysctl_mld_max_msf __read_mostly = IPV6_MLD_MAX_MSF;
+int sysctl_mld_qrv __read_mostly = MLD_QRV_DEFAULT;
 
 /*
  *	socket join on multicast group
@@ -1191,15 +1192,16 @@ static void mld_update_qrv(struct inet6_dev *idev,
 	 * and SHOULD NOT be one. Catch this here if we ever run
 	 * into such a case in future.
 	 */
+	const int min_qrv = min(MLD_QRV_DEFAULT, sysctl_mld_qrv);
 	WARN_ON(idev->mc_qrv == 0);
 
 	if (mlh2->mld2q_qrv > 0)
 		idev->mc_qrv = mlh2->mld2q_qrv;
 
-	if (unlikely(idev->mc_qrv < 2)) {
+	if (unlikely(idev->mc_qrv < min_qrv)) {
 		net_warn_ratelimited("IPv6: MLD: clamping QRV from %u to %u!\n",
-				     idev->mc_qrv, MLD_QRV_DEFAULT);
-		idev->mc_qrv = MLD_QRV_DEFAULT;
+				     idev->mc_qrv, min_qrv);
+		idev->mc_qrv = min_qrv;
 	}
 }
 
@@ -2478,6 +2480,14 @@ void ipv6_mc_down(struct inet6_dev *idev)
 	mld_clear_delrec(idev);
 }
 
+static void ipv6_mc_reset(struct inet6_dev *idev)
+{
+	idev->mc_qrv = sysctl_mld_qrv;
+	idev->mc_qi = MLD_QI_DEFAULT;
+	idev->mc_qri = MLD_QRI_DEFAULT;
+	idev->mc_v1_seen = 0;
+	idev->mc_maxdelay = unsolicited_report_interval(idev);
+}
 
 /* Device going up */
 
@@ -2488,6 +2498,7 @@ void ipv6_mc_up(struct inet6_dev *idev)
 	/* Install multicast list, except for all-nodes (already installed) */
 
 	read_lock_bh(&idev->lock);
+	ipv6_mc_reset(idev);
 	for (i = idev->mc_list; i; i = i->next)
 		igmp6_group_added(i);
 	read_unlock_bh(&idev->lock);
@@ -2508,13 +2519,7 @@ void ipv6_mc_init_dev(struct inet6_dev *idev)
 			(unsigned long)idev);
 	setup_timer(&idev->mc_dad_timer, mld_dad_timer_expire,
 		    (unsigned long)idev);
-
-	idev->mc_qrv = MLD_QRV_DEFAULT;
-	idev->mc_qi = MLD_QI_DEFAULT;
-	idev->mc_qri = MLD_QRI_DEFAULT;
-
-	idev->mc_maxdelay = unsolicited_report_interval(idev);
-	idev->mc_v1_seen = 0;
+	ipv6_mc_reset(idev);
 	write_unlock_bh(&idev->lock);
 }
 

net/ipv6/sysctl_net_ipv6.c

@@ -16,6 +16,8 @@
 #include <net/addrconf.h>
 #include <net/inet_frag.h>
 
+static int one = 1;
+
 static struct ctl_table ipv6_table_template[] = {
 	{
 		.procname	= "bindv6only",
@@ -63,6 +65,14 @@ static struct ctl_table ipv6_rotable[] = {
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec
 	},
+	{
+		.procname	= "mld_qrv",
+		.data		= &sysctl_mld_qrv,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= &one
+	},
 	{ }
 };