Subject: Re: macvtap performs IP defragmentation, causing MTU problems for virtual machines

From: Florian Westphal <[email-masked]>
Date: Mon, 2 Oct 2023 11:20:10 +0200

Henrik Lindström <[email-masked]> wrote:
> I found this old thread describing why macvlan does this:
> https://lore.kernel.org/netdev/[email-masked]/
> Interestingly, the problem described in that thread seems to be more
> general than macvlans, and i can still reproduce it by simply having
> multiple physical interfaces.
> So it looks like macvlans are being special-cased right now, as a
> workaround for a more general defragmentation problem?

Looks like it, maybe Eric remembers details here.

AFAIU however this issue isn't specific to macvlan, looks like some people
insist that receiving a fragmented multicast packet on n devices means we
should pass n defragmented packets up to the stack (we don't; ip defrag
will discard "duplicates").

There is a vif identifier for l3mdev sake (that did not exist back then),
we could use that as a discriminator for mcast case.

Something like this (totally untested):

diff --git a/net/ipv4/ip_fragment.c b/net/ipv4/ip_fragment.c
--- a/net/ipv4/ip_fragment.c
+++ b/net/ipv4/ip_fragment.c
@@ -479,11 +479,29 @@ static int ip_frag_reasm(struct ipq *qp, struct sk_buff *skb,
 	return err;
 }
 
+static int ip_defrag_vif(const struct sk_buff *skb, const struct net_device *dev)
+{
+	int vif = l3mdev_master_ifindex_rcu(dev);
+
+	if (vif)
+		return vif;
+
+	/* some folks insist that receiving a fragmented mcast dgram on n devices shall
+	 * result in n defragmented packets.
+	 */
+	if (skb->pkt_type == PACKET_BROADCAST || skb->pkt_type == PACKET_MULTICAST) {
+		if (dev)
+			vif = dev->ifindex;
+	}
+
+	return 0;
+}
+
 /* Process an incoming IP datagram fragment. */
 int ip_defrag(struct net *net, struct sk_buff *skb, u32 user)
 {
 	struct net_device *dev = skb->dev ? : skb_dst(skb)->dev;
-	int vif = l3mdev_master_ifindex_rcu(dev);
+	int vif = ip_defrag_vif(skb, dev);
 	struct ipq *qp;
 
 	__IP_INC_STATS(net, IPSTATS_MIB_REASMREQDS);

... which should allow to remove the macvlan defrag step.