[PATCH net-next v2 1/3] ipv4: Don't raise a cached pmtu from another path's Packet Too Big

Since commit 7d3f3b4367f3 ("net: ipv4: Cache pmtu for all packet paths
if multipath enabled") __ip_rt_update_pmtu() updates the exception of
every path of a multipath route, but update_or_create_fnhe() overwrites
whatever pmtu a path already has. A Packet Too Big from a wider path
thus raises the pmtu a narrower path has learned, and flows on that path
blackhole until it gets its own Packet Too Big again.

The early return in __ip_rt_update_pmtu() already keeps a pmtu from going
up on the path the Packet Too Big was looked up on. Do the same for the
other paths: leave an unexpired exception with a lower pmtu alone.

Signed-off-by: Vladimir Vdovin <redacted>
---
 net/ipv4/route.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index d0a5f6d77094..596af782c1fb 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c

@@ -676,6 +676,14 @@ static void update_or_create_fnhe(struct fib_nh_common *nhc, __be32 daddr,
 	}
 
 	if (fnhe) {
+		/* __ip_rt_update_pmtu() caches a pmtu on every path of a
+		 * multipath route, don't let a Packet Too Big from one path
+		 * raise the pmtu another path already learned.
+		 */
+		if (pmtu && fnhe->fnhe_genid == genid && fnhe->fnhe_pmtu &&
+		    pmtu > fnhe->fnhe_pmtu &&
+		    time_before(jiffies, fnhe->fnhe_expires))
+			goto out_unlock;
 		if (fnhe->fnhe_genid != genid)
 			fnhe->fnhe_genid = genid;
 		if (gw)

-- 
2.56.0