[PATCH v4 5/7] ipv6 addrconf: implement RFC7559 router solicitation backoff

["Maciej Żenczykowski" <zenczykowski@gmail.com>]

From: Maciej Żenczykowski <maze@google.com>

This implements:
  https://tools.ietf.org/html/rfc7559

Backoff is performed according to RFC3315 section 14:
  https://tools.ietf.org/html/rfc3315#section-14

Signed-off-by: Maciej Żenczykowski <maze@google.com>
---
 include/net/if_inet6.h |  1 +
 net/ipv6/addrconf.c    | 31 +++++++++++++++++++++++++++----
 2 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/include/net/if_inet6.h b/include/net/if_inet6.h
index 1c8b6820b694..515352c6280a 100644
--- a/include/net/if_inet6.h
+++ b/include/net/if_inet6.h
@@ -201,6 +201,7 @@ struct inet6_dev {
 	struct ipv6_devstat	stats;
 
 	struct timer_list	rs_timer;
+	__s32			rs_interval;	/* in jiffies */
 	__u8			rs_probes;
 
 	__u8			addr_gen_mode;
diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 255be34cdbce..6384a1cde056 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -112,6 +112,24 @@ static inline u32 cstamp_delta(unsigned long cstamp)
 	return (cstamp - INITIAL_JIFFIES) * 100UL / HZ;
 }
 
+static inline s32 rfc3315_s14_backoff_init(s32 initial)
+{
+	u32 r = (9 << 20) / 10 + (prandom_u32() % ((2 << 20) / 10 + 1));
+	s32 v = initial * (u64)r >> 20;       /* ~ multiply by 0.9 .. 1.1 */
+	return v;
+}
+
+static inline s32 rfc3315_s14_backoff_update(s32 cur, s32 ceiling)
+{
+	u32 r = (19 << 20) / 10 + (prandom_u32() % ((2 << 20) / 10 + 1));
+	s32 v = cur * (u64)r >> 20;           /* ~ multiply by 1.9 .. 2.1 */
+	if (v > ceiling) {
+		r -= 1 << 20;
+		v = ceiling * (u64)r >> 20;   /* ~ multiply by 0.9 .. 1.1 */
+	}
+	return v;
+}
+
 #ifdef CONFIG_SYSCTL
 static int addrconf_sysctl_register(struct inet6_dev *idev);
 static void addrconf_sysctl_unregister(struct inet6_dev *idev);
@@ -3698,11 +3716,13 @@ static void addrconf_rs_timer(unsigned long data)
 			goto put;
 
 		write_lock(&idev->lock);
+		idev->rs_interval = rfc3315_s14_backoff_update(
+			idev->rs_interval, idev->cnf.rtr_solicit_max_interval);
 		/* The wait after the last probe can be shorter */
 		addrconf_mod_rs_timer(idev, (idev->rs_probes ==
 					     idev->cnf.rtr_solicits) ?
 				      idev->cnf.rtr_solicit_delay :
-				      idev->cnf.rtr_solicit_interval);
+				      idev->rs_interval);
 	} else {
 		/*
 		 * Note: we do not support deprecated "all on-link"
@@ -3973,10 +3993,11 @@ static void addrconf_dad_completed(struct inet6_ifaddr *ifp)
 
 		write_lock_bh(&ifp->idev->lock);
 		spin_lock(&ifp->lock);
+		ifp->idev->rs_interval = rfc3315_s14_backoff_init(
+			ifp->idev->cnf.rtr_solicit_interval);
 		ifp->idev->rs_probes = 1;
 		ifp->idev->if_flags |= IF_RS_SENT;
-		addrconf_mod_rs_timer(ifp->idev,
-				      ifp->idev->cnf.rtr_solicit_interval);
+		addrconf_mod_rs_timer(ifp->idev, ifp->idev->rs_interval);
 		spin_unlock(&ifp->lock);
 		write_unlock_bh(&ifp->idev->lock);
 	}
@@ -5132,8 +5153,10 @@ update_lft:
 
 	if (update_rs) {
 		idev->if_flags |= IF_RS_SENT;
+		idev->rs_interval = rfc3315_s14_backoff_init(
+			idev->cnf.rtr_solicit_interval);
 		idev->rs_probes = 1;
-		addrconf_mod_rs_timer(idev, idev->cnf.rtr_solicit_interval);
+		addrconf_mod_rs_timer(idev, idev->rs_interval);
 	}
 
 	/* Well, that's kinda nasty ... */
-- 
2.8.0.rc3.226.g39d4020