ip6mr_rtm_dumproute() calls mr_table_dump() or mr_rtm_dumproute(),
and mr_rtm_dumproute() finally calls mr_table_dump().

mr_table_dump() calls the passed function, _ip6mr_fill_mroute().

_ip6mr_fill_mroute() is a wrapper for ip6mr_fill_mroute() to cast
struct mr_mfc * to struct mfc6_cache *.

ip6mr_fill_mroute() can already be called safely under RCU.

Let's convert ip6mr_rtm_dumproute() to RCU.

Now there is no user of the rtnl_held field in struct
fib_dump_filter, and the next patch will remove it.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 net/ipv6/ip6mr.c | 29 ++++++++++++++++++++---------
 1 file changed, 20 insertions(+), 9 deletions(-)

diff --git a/net/ipv6/ip6mr.c b/net/ipv6/ip6mr.c
index 389471e740bc..cd28bea8c11a 100644
--- a/net/ipv6/ip6mr.c
+++ b/net/ipv6/ip6mr.c
@@ -1391,7 +1391,7 @@ static const struct rtnl_msg_handler ip6mr_rtnl_msg_handlers[] __initconst_or_mo
 	{.owner = THIS_MODULE, .protocol = RTNL_FAMILY_IP6MR,
 	 .msgtype = RTM_GETROUTE,
 	 .doit = ip6mr_rtm_getroute, .dumpit = ip6mr_rtm_dumproute,
-	 .flags = RTNL_FLAG_DOIT_UNLOCKED},
+	 .flags = RTNL_FLAG_DOIT_UNLOCKED | RTNL_FLAG_DUMP_UNLOCKED},
 };
 
 int __init ip6_mr_init(void)
@@ -2751,15 +2751,17 @@ static int ip6mr_rtm_dumproute(struct sk_buff *skb, struct netlink_callback *cb)
 {
 	const struct nlmsghdr *nlh = cb->nlh;
 	struct fib_dump_filter filter = {
-		.rtnl_held = true,
+		.rtnl_held = false,
 	};
 	int err;
 
+	rcu_read_lock();
+
 	if (cb->strict_check) {
 		err = ip_valid_fib_dump_req(sock_net(skb->sk), nlh,
 					    &filter, cb);
 		if (err < 0)
-			return err;
+			goto unlock;
 	}
 
 	if (filter.table_id) {
@@ -2767,17 +2769,26 @@ static int ip6mr_rtm_dumproute(struct sk_buff *skb, struct netlink_callback *cb)
 
 		mrt = __ip6mr_get_table(sock_net(skb->sk), filter.table_id);
 		if (!mrt) {
-			if (rtnl_msg_family(cb->nlh) != RTNL_FAMILY_IP6MR)
-				return skb->len;
+			if (rtnl_msg_family(cb->nlh) != RTNL_FAMILY_IP6MR) {
+				err = skb->len;
+				goto unlock;
+			}
 
 			NL_SET_ERR_MSG_MOD(cb->extack, "MR table does not exist");
-			return -ENOENT;
+			err = -ENOENT;
+			goto unlock;
 		}
+
 		err = mr_table_dump(mrt, skb, cb, _ip6mr_fill_mroute,
 				    &mfc_unres_lock, &filter);
-		return skb->len ? : err;
+		err = skb->len ? : err;
+		goto unlock;
 	}
 
-	return mr_rtm_dumproute(skb, cb, ip6mr_mr_table_iter,
-				_ip6mr_fill_mroute, &mfc_unres_lock, &filter);
+	err = mr_rtm_dumproute(skb, cb, ip6mr_mr_table_iter,
+			       _ip6mr_fill_mroute, &mfc_unres_lock, &filter);
+unlock:
+	rcu_read_unlock();
+
+	return err;
 }
-- 
2.53.0.1213.gd9a14994de-goog