[PATCH net v3 2/4] ip_tunnel: move the TCP fixed-ID flag to the inner header

[Julius Bairaktaris]

SKB_GSO_TCP_FIXEDID describes the outer header of an encapsulated GSO
packet and SKB_GSO_TCP_FIXEDID_INNER the inner one. TCP GRO sets
SKB_GSO_TCP_FIXEDID for the header it merged. When such a packet is
forwarded into a tunnel, iptunnel_handle_offloads() makes that header the
inner one but leaves the flag, so software segmentation gives every
segment the outer ID of the first and increments the inner IDs.

Move the flag to SKB_GSO_TCP_FIXEDID_INNER when the packet becomes
encapsulated.

Fixes: 3271f19bf7b9 ("net: gso: restore ids of outer ip headers correctly")
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Julius Bairaktaris <julius@xxxxxxxxxxxxxx>
---
 net/ipv4/ip_tunnel_core.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/ip_tunnel_core.c b/net/ipv4/ip_tunnel_core.c
index bab42b9e277f7..752ddba1298b6 100644
--- a/net/ipv4/ip_tunnel_core.c
+++ b/net/ipv4/ip_tunnel_core.c
@@ -174,18 +174,29 @@ EXPORT_SYMBOL_GPL(iptunnel_metadata_reply);
 int iptunnel_handle_offloads(struct sk_buff *skb,
 			     int gso_type_mask)
 {
+	bool inner = false;
 	int err;
 
 	if (likely(!skb->encapsulation)) {
 		skb_reset_inner_headers(skb);
 		skb->encapsulation = 1;
+		inner = true;
 	}
 
 	if (skb_is_gso(skb)) {
+		struct skb_shared_info *shinfo;
+
 		err = skb_header_unclone(skb, GFP_ATOMIC);
 		if (unlikely(err))
 			return err;
-		skb_shinfo(skb)->gso_type |= gso_type_mask;
+
+		shinfo = skb_shinfo(skb);
+		/* The header SKB_GSO_TCP_FIXEDID described is now the inner one */
+		if (inner && (shinfo->gso_type & SKB_GSO_TCP_FIXEDID)) {
+			shinfo->gso_type &= ~SKB_GSO_TCP_FIXEDID;
+			shinfo->gso_type |= SKB_GSO_TCP_FIXEDID_INNER;
+		}
+		shinfo->gso_type |= gso_type_mask;
 		return 0;
 	}
 
-- 
2.53.0