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ipv4: igmp: annotate data-races around in_dev->mc_count

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/proc/net/igmp walks the multicast list for IPv4 interfaces locklessly
under RCU and prints state->in_dev->mc_count. Concurrently, device
init/destruction and multicast join/leave paths update the count
under the RTNL lock. Fix this intentional lockless snapshot by
annotating the read with READ_ONCE() and the updates with WRITE_ONCE().

Signed-off-by: Yuyang Huang <yuyanghuang@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260605014318.3890804-2-yuyanghuang@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
This commit is contained in:
Yuyang Huang 2026-06-05 10:43:17 +09:00 committed by Paolo Abeni
parent 193b76eb10
commit 1719841cab
1 changed files with 7 additions and 4 deletions
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11
net/ipv4/igmp.c
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@ -1566,7 +1566,7 @@ static void ____ip_mc_inc_group(struct in_device *in_dev, __be32 addr,
#endif
im->next_rcu = in_dev->mc_list;
in_dev->mc_count++;
WRITE_ONCE(in_dev->mc_count, in_dev->mc_count + 1);
rcu_assign_pointer(in_dev->mc_list, im);
ip_mc_hash_add(in_dev, im);
@ -1790,7 +1790,8 @@ void __ip_mc_dec_group(struct in_device *in_dev, __be32 addr, gfp_t gfp)
if (new_users == 0) {
ip_mc_hash_remove(in_dev, i);
*ip = i->next_rcu;
in_dev->mc_count--;
WRITE_ONCE(in_dev->mc_count,
in_dev->mc_count - 1);
__igmp_group_dropped(i, gfp);
inet_ifmcaddr_notify(in_dev->dev, i,
RTM_DELMULTICAST);
@ -1922,7 +1923,7 @@ void ip_mc_destroy_dev(struct in_device *in_dev)
while ((i = rtnl_dereference(in_dev->mc_list)) != NULL) {
in_dev->mc_list = i->next_rcu;
in_dev->mc_count--;
WRITE_ONCE(in_dev->mc_count, in_dev->mc_count - 1);
ip_mc_clear_src(i);
ip_ma_put(i);
}
@ -2974,7 +2975,9 @@ static int igmp_mc_seq_show(struct seq_file *seq, void *v)
if (rcu_access_pointer(state->in_dev->mc_list) == im) {
seq_printf(seq, "%d\t%-10s: %5d %7s\n",
state->dev->ifindex, state->dev->name, state->in_dev->mc_count, querier);
state->dev->ifindex, state->dev->name,
READ_ONCE(state->in_dev->mc_count),
querier);
}
delta = im->timer.expires - jiffies;