Re: [PATCH v5 5/6] bpf: clear decap tunnel GSO state in skb_adjust_room

bot+bpf-ci
2026-04-20T11:19:32Z

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> diff --git a/net/core/filter.c b/net/core/filter.c
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -3666,6 +3666,39 @@ 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.
> + */

This isn't a bug, but the opening /* should be on its own line following BPF
subsystem convention for multi-line comments.

> + 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;
> + }

All of this new GSO flag clearing logic is added inside the if (skb_is_gso(skb))
block in bpf_skb_net_shrink(). Does this mean non-GSO packets bypass the
decapsulation state clearing?

BPF programs can call bpf_skb_adjust_room() to decapsulate non-GSO packets
as well. If a non-GSO packet undergoes decapsulation with the tunnel headers
stripped, but skb->encapsulation remains set, could this cause downstream
network stack components to misinterpret the packet as still being
encapsulated?

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

---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/24662733093