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

On Fri, Aug 28, 2026 at 08:45:29AM +0000, Eric Dumazet wrote:

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")

Looks like it was broken from the start:

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Taehee Yoo <ap420073@gmail.com>

Reviewed-by: Ido Schimmel <idosch@nvidia.com>