[PATCH v2 net 3/5] ipv6: mcast: fix delay calculation in igmp6_join_group()

[Eric Dumazet <edumazet@google.com>]

When joining a multicast group, if a report work is already pending
(e.g. scheduled by a query or a previous join), igmp6_join_group()
cancels the delayed work and recalculates the delay:

	if (cancel_delayed_work(&ma->mca_work)) {
		refcount_dec(&ma->mca_refcnt);
		delay = ma->mca_work.timer.expires - jiffies;
	}

Unlike igmp6_group_queried(), igmp6_join_group() did not check
if delay >= interval. This leads to two issues:

1. If the timer has already expired (timer.expires <= jiffies), the
   stale expiry is reused by mod_delayed_work(), causing the second
   unsolicited report to fire on the very next tick without a
   randomized delay.
2. If the timer was originally armed by a query with a large
   maximum response delay, delay could exceed
   unsolicited_report_interval(ma->idev).

Fix this by initializing delay to unsolicited_report_interval(ma->idev)
and re-randomizing it with get_random_u32_below(interval) when
delay >= interval, mirroring the logic in igmp6_group_queried().

Fixes: 2d9a93b4902b ("mld: convert from timer to delayed work")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Taehee Yoo <ap420073@gmail.com>
---
v2: refined the changelog

 net/ipv6/mcast.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/ipv6/mcast.c b/net/ipv6/mcast.c
index 66f5858e5fea68e27da1f857b5596808632618a3..4423b90dc9abffff9661da22caa4b9e2f618def7 100644
--- a/net/ipv6/mcast.c
+++ b/net/ipv6/mcast.c
@@ -2639,7 +2639,7 @@ static void ip6_mc_clear_src(struct ifmcaddr6 *pmc)
 
 static void igmp6_join_group(struct ifmcaddr6 *ma)
 {
-	unsigned long delay;
+	unsigned long delay, interval;
 
 	mc_assert_locked(ma->idev);
 
@@ -2648,13 +2648,17 @@ static void igmp6_join_group(struct ifmcaddr6 *ma)
 
 	igmp6_send(&ma->mca_addr, ma->idev->dev, ICMPV6_MGM_REPORT);
 
-	delay = get_random_u32_below(unsolicited_report_interval(ma->idev));
+	interval = unsolicited_report_interval(ma->idev);
+	delay = interval;
 
 	if (cancel_delayed_work(&ma->mca_work)) {
 		refcount_dec(&ma->mca_refcnt);
 		delay = ma->mca_work.timer.expires - jiffies;
 	}
 
+	if (delay >= interval)
+		delay = get_random_u32_below(interval);
+
 	if (!mod_delayed_work(mld_wq, &ma->mca_work, delay))
 		refcount_inc(&ma->mca_refcnt);
 	WRITE_ONCE(ma->mca_flags, ma->mca_flags |
-- 
2.55.0.897.gb25b4bd76c-goog