ipv4: lock mtu in fnhe when received PMTU < net.ipv4.route.min_pmtu

Prior to the rework of PMTU information storage in commit
2c8cec5c10bc ("ipv4: Cache learned PMTU information in inetpeer."),
when a PMTU event advertising a PMTU smaller than
net.ipv4.route.min_pmtu was received, we would disable setting the DF
flag on packets by locking the MTU metric, and set the PMTU to
net.ipv4.route.min_pmtu.

Since then, we don't disable DF, and set PMTU to
net.ipv4.route.min_pmtu, so the intermediate router that has this link
with a small MTU will have to drop the packets.

This patch reestablishes pre-2.6.39 behavior by splitting
rtable->rt_pmtu into a bitfield with rt_mtu_locked and rt_pmtu.
rt_mtu_locked indicates that we shouldn't set the DF bit on that path,
and is checked in ip_dont_fragment().

One possible workaround is to set net.ipv4.route.min_pmtu to a value low
enough to accommodate the lowest MTU encountered.

Fixes: 2c8cec5c10bc ("ipv4: Cache learned PMTU information in inetpeer.")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 9 insertions, 2 deletions

diff --git a/include/net/ip.h b/include/net/ip.h
index 746abff9ce51..f49b3a576bec 100644
--- a/include/net/ip.h
+++ b/include/net/ip.h
@@ -328,6 +328,13 @@ int ip_decrease_ttl(struct iphdr *iph)
 	return --iph->ttl;
 }
 
+static inline int ip_mtu_locked(const struct dst_entry *dst)
+{
+	const struct rtable *rt = (const struct rtable *)dst;
+
+	return rt->rt_mtu_locked || dst_metric_locked(dst, RTAX_MTU);
+}
+
 static inline
 int ip_dont_fragment(const struct sock *sk, const struct dst_entry *dst)
 {
@@ -335,7 +342,7 @@ int ip_dont_fragment(const struct sock *sk, const struct dst_entry *dst)
 
 	return  pmtudisc == IP_PMTUDISC_DO ||
 		(pmtudisc == IP_PMTUDISC_WANT &&
-		 !(dst_metric_locked(dst, RTAX_MTU)));
+		 !ip_mtu_locked(dst));
 }
 
 static inline bool ip_sk_accept_pmtu(const struct sock *sk)
@@ -361,7 +368,7 @@ static inline unsigned int ip_dst_mtu_maybe_forward(const struct dst_entry *dst,
 	struct net *net = dev_net(dst->dev);
 
 	if (net->ipv4.sysctl_ip_fwd_use_pmtu ||
-	    dst_metric_locked(dst, RTAX_MTU) ||
+	    ip_mtu_locked(dst) ||
 	    !forwarding)
 		return dst_mtu(dst);