[PATCH v1 net] mpls: Set rt->rt_nhn just before returning from mpls_nh_build_multi().

[Kuniyuki Iwashima <kuniyu@google.com>]

Commit f0914b8436c5 ("mpls: Hold dev refcnt for mpls_nh.") added
change_nexthops() loop to call netdev_put() for the nexthop devices
before freeing mpls_route.

Then, mpls_nh_build_multi() was also changed to avoid iterating
uninitialised nexthops in mpls_rt_free_rcu().

However, setting rt->rt_nhn to 0 at the entry of mpls_nh_build_multi()
makes the following change_nexthops() no-op.

Let's set rt->rt_nhn just before returning from mpls_nh_build_multi().

Fixes: f0914b8436c5 ("mpls: Hold dev refcnt for mpls_nh.")
Reported-by: Anthony Doeraene <anthony.doeraene@uclouvain.be>
Closes: https://lore.kernel.org/netdev/036a0c95-f5d4-46ab-88e7-1eab567d7a84@uclouvain.be/
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 net/mpls/af_mpls.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/net/mpls/af_mpls.c b/net/mpls/af_mpls.c
index ca504d9626cf..4a81514e919a 100644
--- a/net/mpls/af_mpls.c
+++ b/net/mpls/af_mpls.c
@@ -922,8 +922,7 @@ static int mpls_nh_build_multi(struct mpls_route_config *cfg,
 	struct nlattr *nla_via, *nla_newdst;
 	int remaining = cfg->rc_mp_len;
 	int err = 0;
-
-	rt->rt_nhn = 0;
+	u8 nhs = 0;
 
 	change_nexthops(rt) {
 		int attrlen;
@@ -959,12 +958,15 @@ static int mpls_nh_build_multi(struct mpls_route_config *cfg,
 			rt->rt_nhn_alive--;
 
 		rtnh = rtnh_next(rtnh, &remaining);
-		rt->rt_nhn++;
+		nhs++;
 	} endfor_nexthops(rt);
 
+	rt->rt_nhn = nhs;
+
 	return 0;
 
 errout:
+	rt->rt_nhn = nhs;
 	return err;
 }
 
-- 
2.55.0.229.g6434b31f56-goog