Re: [PATCH bpf-next v7 5/6] bpf: clear decap state on skb_adjust_room shrink path

On shrink in bpf_skb_adjust_room(), apply decapsulation state updates
according to BPF_F_ADJ_ROOM_DECAP_* flags.

For GSO skbs, clear only the tunnel gso_type bits that correspond to the
requested decap layer:
- DECAP_L4_UDP: SKB_GSO_UDP_TUNNEL{,_CSUM}
- DECAP_L4_GRE: SKB_GSO_GRE{,_CSUM}
- DECAP_IPXIP4: SKB_GSO_IPXIP4
- DECAP_IPXIP6: SKB_GSO_IPXIP6

Then clear skb->encapsulation only if no tunnel GSO bits remain, keeping
encapsulation set for cases such as ESP-in-UDP where tunnel state remains.

For non-GSO skbs, there are no tunnel GSO bits to consult, so clear
skb->encapsulation directly when DECAP_L4_* or DECAP_IPXIP_* flags are set.

This keeps decap state handling consistent between GSO and non-GSO packets.

This looks like a bug fix for the DECAP flags added in commit 0e08a219f910.
That commit added the BPF_F_ADJ_ROOM_DECAP_* flags and documented that they
would clear GSO state and encapsulation flags, but the behavior was never
implemented in the code. This commit implements that documented behavior.

Should this include:

  Fixes: 0e08a219f910 ("bpf: add BPF_F_ADJ_ROOM_DECAP_* flags for tunnel decapsulation")


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