Subject: [PATCH net 1/4] netfilter: nf_flow_table_ip: Introduce nf_flow_vlan_push()

From: Florian Westphal <[email-masked]>
Date: Wed, 4 Mar 2026 18:29:37 +0100

From: Eric Woudstra <[email-masked]>

With double vlan tagged packets in the fastpath, getting the error:

skb_vlan_push got skb with skb->data not at mac header (offset 18)

Introduce nf_flow_vlan_push(), that can correctly push the inner vlan
in the fastpath. It is closedly modelled on existing nf_flow_pppoe_push()

Fixes: c653d5a78f34 ("netfilter: flowtable: inline vlan encapsulation in xmit path")
Signed-off-by: Eric Woudstra <[email-masked]>
Signed-off-by: Florian Westphal <[email-masked]>
---
 net/netfilter/nf_flow_table_ip.c | 25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/net/netfilter/nf_flow_table_ip.c b/net/netfilter/nf_flow_table_ip.c
index 3fdb10d9bf7f..e65c8148688e 100644
--- a/net/netfilter/nf_flow_table_ip.c
+++ b/net/netfilter/nf_flow_table_ip.c
@@ -544,6 +544,27 @@ static int nf_flow_offload_forward(struct nf_flowtable_ctx *ctx,
 	return 1;
 }
 
+static int nf_flow_vlan_push(struct sk_buff *skb, __be16 proto, u16 id)
+{
+	if (skb_vlan_tag_present(skb)) {
+		struct vlan_hdr *vhdr;
+
+		if (skb_cow_head(skb, VLAN_HLEN))
+			return -1;
+
+		__skb_push(skb, VLAN_HLEN);
+		skb_reset_network_header(skb);
+
+		vhdr = (struct vlan_hdr *)(skb->data);
+		vhdr->h_vlan_TCI = htons(id);
+		vhdr->h_vlan_encapsulated_proto = skb->protocol;
+		skb->protocol = proto;
+	} else {
+		__vlan_hwaccel_put_tag(skb, proto, id);
+	}
+	return 0;
+}
+
 static int nf_flow_pppoe_push(struct sk_buff *skb, u16 id)
 {
 	int data_len = skb->len + sizeof(__be16);
@@ -738,8 +759,8 @@ static int nf_flow_encap_push(struct sk_buff *skb,
 		switch (tuple->encap[i].proto) {
 		case htons(ETH_P_8021Q):
 		case htons(ETH_P_8021AD):
-			if (skb_vlan_push(skb, tuple->encap[i].proto,
-					  tuple->encap[i].id) < 0)
+			if (nf_flow_vlan_push(skb, tuple->encap[i].proto,
+					      tuple->encap[i].id) < 0)
 				return -1;
 			break;
 		case htons(ETH_P_PPP_SES):
-- 
2.52.0