This patch fixes an issue in skb_gso_network_seglen where the calculated
segment length includes the HBH headers of BIG TCP jumbograms despite these
headers being removed before segmentation. These headers are added by GRO
or by ip6_xmit for BIG TCP packets and are later removed by GSO. This bug
causes MTU validation of BIG TCP jumbograms to fail.

Signed-off-by: Mariusz Klimek <maklimek97@gmail.com>
---
 net/core/gso.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/core/gso.c b/net/core/gso.c
index bcd156372f4d..251a49181031 100644
--- a/net/core/gso.c
+++ b/net/core/gso.c
@@ -180,6 +180,10 @@ static unsigned int skb_gso_network_seglen(const struct sk_buff *skb)
 	unsigned int hdr_len = skb_transport_header(skb) -
 			       skb_network_header(skb);
 
+	/* Jumbogram HBH header is removed upon segmentation. */
+	if (skb->protocol == htons(ETH_P_IPV6) && skb->len > IPV6_MAXPLEN)
+		hdr_len -= sizeof(struct hop_jumbo_hdr);
+
 	return hdr_len + skb_gso_transport_seglen(skb);
 }
 
-- 
2.47.3

This patch removes the IP6SKB_FAKEJUMBO flag that is used as a work-around
to bypass MTU validation of BIG TCP jumbograms due to a bug in
skb_gso_network_seglen. This work-around is no longer required now that the
bug is fixed.

Signed-off-by: Mariusz Klimek <maklimek97@gmail.com>
---
 include/linux/ipv6.h  | 1 -
 net/ipv6/ip6_output.c | 4 +---
 2 files changed, 1 insertion(+), 4 deletions(-)

diff --git a/include/linux/ipv6.h b/include/linux/ipv6.h
index 7294e4e89b79..9f076171106e 100644
--- a/include/linux/ipv6.h
+++ b/include/linux/ipv6.h
@@ -155,7 +155,6 @@ struct inet6_skb_parm {
 #define IP6SKB_L3SLAVE         64
 #define IP6SKB_JUMBOGRAM      128
 #define IP6SKB_SEG6	      256
-#define IP6SKB_FAKEJUMBO      512
 #define IP6SKB_MULTIPATH      1024
 #define IP6SKB_MCROUTE        2048
 };
diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index f904739e99b9..9af9ec6bdb8c 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -179,8 +179,7 @@ ip6_finish_output_gso_slowpath_drop(struct net *net, struct sock *sk,
 static int ip6_finish_output_gso(struct net *net, struct sock *sk,
 				 struct sk_buff *skb, unsigned int mtu)
 {
-	if (!(IP6CB(skb)->flags & IP6SKB_FAKEJUMBO) &&
-	    !skb_gso_validate_network_len(skb, mtu))
+	if (!skb_gso_validate_network_len(skb, mtu))
 		return ip6_finish_output_gso_slowpath_drop(net, sk, skb, mtu);
 
 	return ip6_finish_output2(net, sk, skb);
@@ -323,7 +322,6 @@ int ip6_xmit(const struct sock *sk, struct sk_buff *skb, struct flowi6 *fl6,
 
 		proto = IPPROTO_HOPOPTS;
 		seg_len = 0;
-		IP6CB(skb)->flags |= IP6SKB_FAKEJUMBO;
 	}
 
 	skb_push(skb, sizeof(struct ipv6hdr));
-- 
2.47.3

This patch removes the manual lowering of the client MTU in big_tcp.sh. The
MTU lowering was previously required as a work-around due to a bug in the
MTU validation of BIG TCP jumbograms. The MTU was lowered to 1442, but note
that 1492 (1500 - 8) would of worked just as well. Now that the bug has
been fixed, the manual client MTU modification can be removed entirely.

Signed-off-by: Mariusz Klimek <maklimek97@gmail.com>
---
 tools/testing/selftests/net/big_tcp.sh | 1 -
 1 file changed, 1 deletion(-)

diff --git a/tools/testing/selftests/net/big_tcp.sh b/tools/testing/selftests/net/big_tcp.sh
index 2db9d15cd45f..b5d9145296d3 100755
--- a/tools/testing/selftests/net/big_tcp.sh
+++ b/tools/testing/selftests/net/big_tcp.sh
@@ -32,7 +32,6 @@ setup() {
 	ip -net $ROUTER_NS link add link2 type veth peer name link3 netns $SERVER_NS
 
 	ip -net $CLIENT_NS link set link0 up
-	ip -net $CLIENT_NS link set link0 mtu 1442
 	ip -net $CLIENT_NS addr add $CLIENT_IP4/24 dev link0
 	ip -net $CLIENT_NS addr add $CLIENT_IP6/64 dev link0 nodad
 	ip -net $CLIENT_NS route add $SERVER_IP4 dev link0 via $CLIENT_GW4
-- 
2.47.3