netdev - [PATCH] bridge:fragmented packets dropped by bridge

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Message-Id: <20190730122534.30687-1-rdong.ge@gmail.com>
Date:   Tue, 30 Jul 2019 12:25:34 +0000
From:   Rundong Ge <rdong.ge@...il.com>
To:     davem@...emloft.net
Cc:     kuznet@....inr.ac.ru, yoshfuji@...ux-ipv6.org,
        netdev@...r.kernel.org, pablo@...filter.org, kadlec@...filter.org,
        fw@...len.de, roopa@...ulusnetworks.com,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        bridge@...ts.linux-foundation.org, nikolay@...ulusnetworks.com,
        linux-kernel@...r.kernel.org, rdong.ge@...il.com
Subject: [PATCH] bridge:fragmented packets dropped by bridge

Given following setup:
-modprobe br_netfilter
-echo '1' > /proc/sys/net/bridge/bridge-nf-call-iptables
-brctl addbr br0
-brctl addif br0 enp2s0
-brctl addif br0 enp3s0
-brctl addif br0 enp6s0
-ifconfig enp2s0 mtu 1300
-ifconfig enp3s0 mtu 1500
-ifconfig enp6s0 mtu 1500
-ifconfig br0 up

                 multi-port
mtu1500 - mtu1500|bridge|1500 - mtu1500
  A                  |            B
                   mtu1300

With netfilter defragmentation/conntrack enabled, fragmented
packets from A will be defragmented in prerouting, and refragmented
at postrouting.
But in this scenario the bridge found the frag_max_size(1500) is
larger than the dst mtu stored in the fake_rtable whitch is
always equal to the bridge's mtu 1300, then packets will be dopped.

This modifies ip_skb_dst_mtu to use the out dev's mtu instead
of bridge's mtu in bridge refragment.

Signed-off-by: Rundong Ge <rdong.ge@...il.com>
---
 include/net/ip.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/include/net/ip.h b/include/net/ip.h
index 29d89de..0512de3 100644
--- a/include/net/ip.h
+++ b/include/net/ip.h
@@ -450,6 +450,8 @@ static inline unsigned int ip_dst_mtu_maybe_forward(const struct dst_entry *dst,
 static inline unsigned int ip_skb_dst_mtu(struct sock *sk,
 					  const struct sk_buff *skb)
 {
+	if ((skb_dst(skb)->flags & DST_FAKE_RTABLE) && skb->dev)
+		return min(skb->dev->mtu, IP_MAX_MTU);
 	if (!sk || !sk_fullsock(sk) || ip_sk_use_pmtu(sk)) {
 		bool forwarding = IPCB(skb)->flags & IPSKB_FORWARDED;
 
-- 
1.8.3.1