Subject: Re: [PATCH RFC bpf-next 5/9] bpf: Make bpf_skb_vlan_push helper metadata-safe

From: Jakub Sitnicki <[email-masked]>
Date: Fri, 03 Oct 2025 14:03:48 +0200

On Mon, Sep 29, 2025 at 04:09 PM +02, Jakub Sitnicki wrote:
> 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-masked]>
> ---
>  include/linux/if_vlan.h | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/if_vlan.h b/include/linux/if_vlan.h
> index 4ecc2509b0d4..b0e1f57d51aa 100644
> --- a/include/linux/if_vlan.h
> +++ b/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_HLEN ? skb_metadata_len(skb) : 0;

This is a typo. Should be:

+       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;