bpf: Make bpf_skb_vlan_push helper metadata-safe

Use the metadata-aware helper to move packet bytes after skb_push(),
ensuring metadata remains valid after calling the BPF helper.

Also, take care to reserve sufficient headroom for metadata to fit.

Signed-off-by: Jakub Sitnicki <[email protected]>

Author: jsitnicki

include/linux/if_vlan.h

@@ -355,16 +355,17 @@ static inline int __vlan_insert_inner_tag(struct sk_buff *skb,
 					  __be16 vlan_proto, u16 vlan_tci,
 					  unsigned int mac_len)
 {
+	const u8 meta_len = mac_len > ETH_TLEN ? skb_metadata_len(skb) : 0;
 	struct vlan_ethhdr *veth;
 
-	if (skb_cow_head(skb, VLAN_HLEN) < 0)
+	if (skb_cow_head(skb, meta_len + VLAN_HLEN) < 0)
 		return -ENOMEM;
 
 	skb_push(skb, VLAN_HLEN);
 
 	/* Move the mac header sans proto to the beginning of the new header. */
 	if (likely(mac_len > ETH_TLEN))
-		memmove(skb->data, skb->data + VLAN_HLEN, mac_len - ETH_TLEN);
+		skb_postpush_data_move(skb, VLAN_HLEN, mac_len - ETH_TLEN);
 	if (skb_mac_header_was_set(skb))
 		skb->mac_header -= VLAN_HLEN;