netdev - Re: [PATCH v2 net-next 08/19] ipmr: do not acquire mrt_lock while calling ip_mr

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Date:   Sat, 23 Jul 2022 00:10:05 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH v2 net-next 08/19] ipmr: do not acquire mrt_lock while
 calling ip_mr_forward()

On Fri, Jul 22, 2022 at 10:37:24PM +0200, Eric Dumazet wrote:
> Thanks for the report.
> 
> I guess there are multiple ways to solve this issue, one being:
> 
> diff --git a/net/ipv4/ipmr.c b/net/ipv4/ipmr.c
> index 73651d17e51f31c8755da6ac3c1c2763a99b1117..1c288a7b60132365c072874d1f811b70679a2bcb
> 100644
> --- a/net/ipv4/ipmr.c
> +++ b/net/ipv4/ipmr.c
> @@ -1004,7 +1004,9 @@ static void ipmr_cache_resolve(struct net *net,
> struct mr_table *mrt,
> 
>                         rtnl_unicast(skb, net, NETLINK_CB(skb).portid);
>                 } else {
> +                       rcu_read_lock();
>                         ip_mr_forward(net, mrt, skb->dev, skb, c, 0);
> +                       rcu_read_unlock();
>                 }
>         }
>  }
> @@ -1933,7 +1935,7 @@ static int ipmr_find_vif(const struct mr_table
> *mrt, struct net_device *dev)
>  }
> 
>  /* "local" means that we should preserve one skb (for local delivery) */
> -/* Called uner rcu_read_lock() */
> +/* Called under rcu_read_lock() */
>  static void ip_mr_forward(struct net *net, struct mr_table *mrt,
>                           struct net_device *dev, struct sk_buff *skb,
>                           struct mfc_cache *c, int local)

It sure makes lockdep stop complaining...

I just noticed that we appear to have the same problem with the
equivalent call path for ipv6: ip6mr_mfc_add -> ip6mr_cache_resolve ->
ip6_mr_forward, although I don't have smcroute or the kernel configured
for any IPv6 multicast routes right now, so I can't say for sure.