Subject: Re: [PATCH bpf-next v4 5/6] bpf: clear decap tunnel GSO state in skb_adjust_room

From: Willem de Bruijn <[email-masked]>
Date: Thu, 16 Apr 2026 08:32:13 -0400

Nick Hudson wrote:
> On shrink in bpf_skb_adjust_room(), clear tunnel-specific GSO flags
> according to the decapsulation flags:
> 
> - BPF_F_ADJ_ROOM_DECAP_L4_UDP clears SKB_GSO_UDP_TUNNEL{,_CSUM}
> - BPF_F_ADJ_ROOM_DECAP_L4_GRE clears SKB_GSO_GRE{,_CSUM}
> - BPF_F_ADJ_ROOM_DECAP_IPXIP4 clears SKB_GSO_IPXIP4
> - BPF_F_ADJ_ROOM_DECAP_IPXIP6 clears SKB_GSO_IPXIP6
> 
> When all tunnel-related GSO bits are cleared, also clear
> skb->encapsulation.
> 
> Handle the ESP inside a UDP tunnel case where encapsulation should remain
> set.
> 
> If UDP decap is performed, clear encap_hdr_csum and remcsum_offload.
> 
> Co-developed-by: Max Tottenham <[email-masked]>
> Signed-off-by: Max Tottenham <[email-masked]>
> Co-developed-by: Anna Glasgall <[email-masked]>
> Signed-off-by: Anna Glasgall <[email-masked]>
> Signed-off-by: Nick Hudson <[email-masked]>
> ---
>  net/core/filter.c | 38 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 38 insertions(+)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 7f8d43420afb..e113ae2f3f14 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -3667,6 +3667,44 @@ static int bpf_skb_net_shrink(struct sk_buff *skb, u32 off, u32 len_diff,
>  		if (!(flags & BPF_F_ADJ_ROOM_FIXED_GSO))
>  			skb_increase_gso_size(shinfo, len_diff);
>  
> +		/* Selective GSO flag clearing based on decap type.
> +		 * Only clear the flags for the tunnel layer being removed.
> +		 */
> +		if ((flags & BPF_F_ADJ_ROOM_DECAP_L4_UDP) &&
> +		    (shinfo->gso_type & (SKB_GSO_UDP_TUNNEL |
> +					 SKB_GSO_UDP_TUNNEL_CSUM)))
> +			shinfo->gso_type &= ~(SKB_GSO_UDP_TUNNEL |
> +					      SKB_GSO_UDP_TUNNEL_CSUM);
> +		if ((flags & BPF_F_ADJ_ROOM_DECAP_L4_GRE) &&
> +		    (shinfo->gso_type & (SKB_GSO_GRE | SKB_GSO_GRE_CSUM)))
> +			shinfo->gso_type &= ~(SKB_GSO_GRE |
> +					      SKB_GSO_GRE_CSUM);
> +		if ((flags & BPF_F_ADJ_ROOM_DECAP_IPXIP4) &&
> +		    (shinfo->gso_type & SKB_GSO_IPXIP4))
> +			shinfo->gso_type &= ~SKB_GSO_IPXIP4;
> +		if ((flags & BPF_F_ADJ_ROOM_DECAP_IPXIP6) &&
> +		    (shinfo->gso_type & SKB_GSO_IPXIP6))
> +			shinfo->gso_type &= ~SKB_GSO_IPXIP6;
> +
> +		/* Clear encapsulation flag only when no tunnel GSO flags remain */
> +		if (flags & (BPF_F_ADJ_ROOM_DECAP_L4_MASK |
> +			     BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK)) {
> +			if (!(shinfo->gso_type & (SKB_GSO_UDP_TUNNEL |
> +						  SKB_GSO_UDP_TUNNEL_CSUM |
> +						  SKB_GSO_GRE |
> +						  SKB_GSO_GRE_CSUM |
> +						  SKB_GSO_IPXIP4 |
> +						  SKB_GSO_IPXIP6 |
> +						  SKB_GSO_ESP)))
> +				if (skb->encapsulation)
> +					skb->encapsulation = 0;
> +
> +			if (flags & BPF_F_ADJ_ROOM_DECAP_L4_UDP) {
> +				skb->encap_hdr_csum = 0;

This field is not used with UDP_L4.

Similar to remcsum, I'd ignore it entirely in this series.

> +				skb->remcsum_offload = 0;

Why still include remote checksum handling?

> +			}
> +		}
> +
>  		/* Header must be checked, and gso_segs recomputed. */
>  		shinfo->gso_type |= SKB_GSO_DODGY;
>  		shinfo->gso_segs = 0;
> -- 
> 2.34.1
>