[PATCH v2 net 0/5] ipv6: mcast: RCU and timer fixes

This series addresses several RCU synchronization and timer calculation
issues identified in IPv6 multicast (MLD) handling within net/ipv6/mcast.c
while I was working on fixing a syzbot report in net/ipv4/icmp.c.

Patch 1 fixes an RCU reader diversion in ip6_mc_del1_src() where mutating
psf->sf_next to insert an unlinked source node into the tombstone list
diverted concurrent lockless readers (e.g. ipv6_chk_mcast_addr()) into
pmc->mca_tomb, causing them to miss remaining active sources.

Patch 2 converts ip6_mc_source() to use copy-on-write RCU updates.
Previously, source additions and deletions modified the socket's
psl->sl_addr array in-place, causing concurrent lockless readers in
inet6_mc_check() (UDP/RAW receive path) to observe torn 16-byte IPv6
addresses or duplicated/missed sources.

Patch 3 fixes delay calculation in igmp6_join_group() when canceling an
existing delayed work, preventing unsigned jiffies underflows when the
timer has already expired and clamping the delay to the unsolicited report
interval.

Patch 4 ensures rcu_assign_pointer() is consistently used for __rcu list
updates in __ipv6_dev_mc_dec(), ipv6_sock_mc_drop(), __ipv6_sock_mc_close(),
and related helpers.

Patch 5 switches igmp6_mc_seq_show() to use jiffies_delta_to_clock_t()
with a signed long delta, preventing underflows in /proc/net/igmp6 timer
duration reporting.

v2: addressed AI review.
v1: https://lore.kernel.org/netdev/20260826103711.3302915-1-edumazet@google.com/ (local)

Eric Dumazet (5):
  ipv6: mcast: fix RCU list diversion in ip6_mc_del1_src()
  ipv6: mcast: use copy-on-write RCU updates in ip6_mc_source()
  ipv6: mcast: fix delay calculation in igmp6_join_group()
  ipv6: mcast: use rcu_assign_pointer() for __rcu list updates
  ipv6: mcast: use jiffies_delta_to_clock_t() in igmp6_mc_seq_show()

 include/net/if_inet6.h |   2 -
 net/ipv6/mcast.c       | 148 ++++++++++++++++++++++++-----------------
 2 files changed, 88 insertions(+), 62 deletions(-)

-- 
2.55.0.897.gb25b4bd76c-goog