Subject: Re: [RFC PATCH 1/1] bpf: Add tunnel decapsulation and GSO state updates per new flags

From: Willem de Bruijn <[email-masked]>
Date: Wed, 25 Feb 2026 10:45:09 -0500

Hudson, Nick wrote:
> 
> 
> > On 20 Feb 2026, at 21:08, Willem de Bruijn <[email-masked]> wrote:
> > 
> > !-------------------------------------------------------------------|
> >  This Message Is From an External Sender
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> > 
> > Nick Hudson wrote:
> >> Enable BPF programs to properly handle GSO state when decapsulating
> >> tunneled packets by adding selective GSO flag clearing and a trusted
> >> mode for GSO handling.
> >> 
> >> New decapsulation flags:
> >> 
> >> - BPF_F_ADJ_ROOM_DECAP_L4_UDP: Clear UDP tunnel GSO flags
> >>  (SKB_GSO_UDP_TUNNEL, SKB_GSO_UDP_TUNNEL_CSUM)
> >> - BPF_F_ADJ_ROOM_DECAP_L4_GRE: Clear GRE tunnel GSO flags
> >>  (SKB_GSO_GRE, SKB_GSO_GRE_CSUM)
> >> - BPF_F_ADJ_ROOM_DECAP_IPXIP4: Clear SKB_GSO_IPXIP4 flag for
> >>  IPv4-in-IPv4 (IPIP) and IPv6-in-IPv4 (SIT) tunnels
> >> - BPF_F_ADJ_ROOM_DECAP_IPXIP6: Clear SKB_GSO_IPXIP6 flag for
> >>  IPv6-in-IPv6 and IPv4-in-IPv6 tunnels
> >> - BPF_F_ADJ_ROOM_NO_DODGY: Preserve gso_segs and don't set
> >>  SKB_GSO_DODGY when the BPF program is trusted and modifications
> >>  are known to be valid
> >> 
> >> The existing anonymous enum for BPF_FUNC_skb_adjust_room flags is
> >> renamed to enum bpf_adj_room_flags to enable CO-RE (Compile Once -
> >> Run Everywhere) lookups in BPF programs.
> >> 
> >> By default, bpf_skb_adjust_room sets SKB_GSO_DODGY and resets
> >> gso_segs to 0, forcing revalidation. The NO_DODGY flag bypasses this
> >> for trusted programs that guarantee GSO correctness.
> >> 
> >> Usage example (decapsulating UDP tunnel with IPv4 inner packet):
> >>  bpf_skb_adjust_room(skb, -hdr_len, BPF_ADJ_ROOM_NET,
> >>                      BPF_F_ADJ_ROOM_DECAP_L3_IPV4 |
> >>                      BPF_F_ADJ_ROOM_DECAP_L4_UDP);
> > 
> > This patch is doing to much in one patch.
> 
> Sure, I’ll split it up.
> 
> > 
> > Also not convinced of the need for the NO_DODGY flag.
> 
> The reason for NO_DODGY is that, without it, the egress interface will see the
> SKB_GSO_DODGY flag. In our use case, we want to avoid marking the egress tap as
> NETIF_F_GSO_ROBUST, so the skb will fail skb_gso_ok() with SKB_GSO_DODGY set.
> When skb_gso_ok() fails, validate_xmit_skb() calls skb_gso_segment().

I understand why you might want it. But the dodgy check has long been
there for a reason: becauses these transformations are not blindly
accepted by the kernel. This use case does not change that.