[PATCH net v2 3/3] ipmr: do not drop GSO packets that fit the MTU after segmentation

ipmr_prepare_xmit() drops a packet with DF set when skb->len exceeds the
route MTU, without checking whether it is a GSO packet. A UDP multicast
stream received with fraglist GRO or UDP GRO forwarding enabled is
aggregated into GSO packets, and every aggregate is dropped with
FragFails, while ip_forward() lets the same packets pass by checking
skb_gso_validate_network_len(). Do the same for multicast routing,
leaving room for the outer header on tunnel vifs. ip6mr has no such
check and leaves GSO packets to ip6_output().

On an IPQ8074 router with fraglist GRO on, a 10 Mbit/s stream of
1316-byte DF datagrams routed by igmpproxy loses 14.5 to 15.5% (n=3)
and 0% with this change.

Fixes: 9fd1ff5d2ac7 ("udp: Support UDP fraglist GRO/GSO.")
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Julius Bairaktaris <redacted>
---
 net/ipv4/ipmr.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/ipmr.c b/net/ipv4/ipmr.c
index f0666749b14a7..983565c0711fa 100644
--- a/net/ipv4/ipmr.c
+++ b/net/ipv4/ipmr.c

@@ -1914,6 +1914,7 @@ static int ipmr_prepare_xmit(struct net *net, struct mr_table *mrt,
 	struct rtable *rt;
 	struct flowi4 fl4;
 	int    encap = 0;
+	unsigned int mtu;
 
 	vif_dev = vif_dev_read(vif);
 	if (!vif_dev)

@@ -1946,7 +1947,10 @@ static int ipmr_prepare_xmit(struct net *net, struct mr_table *mrt,
 			return -1;
 	}
 
-	if (skb->len+encap > dst4_mtu(&rt->dst) && (ntohs(iph->frag_off) & IP_DF)) {
+	mtu = dst4_mtu(&rt->dst);
+	if (skb->len + encap > mtu && (ntohs(iph->frag_off) & IP_DF) &&
+	    !(skb_is_gso(skb) && mtu > encap &&
+	      skb_gso_validate_network_len(skb, mtu - encap))) {
 		/* Do not fragment multicasts. Alas, IPv4 does not
 		 * allow to send ICMP, so that packets will disappear
 		 * to blackhole.

-- 
2.53.0