Re: [PATCH bpf-next v3 5/6] bpf: clear decap tunnel GSO state in skb_adjust_room

[Hudson, Nick]

> On Apr 8, 2026, at 4:10 PM, Willem de Bruijn <willemdebruijn.kernel@xxxxxxxxx> wrote:
> 
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> 
> 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}, and
>>                                     SKB_GSO_TUNNEL_REMCSUM
>> - 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 and GSO state removed then reset encap_hdr_csum, and
>> remcsum_offload.
>> 
>> Co-developed-by: Max Tottenham <mtottenh@xxxxxxxxxx>
>> Signed-off-by: Max Tottenham <mtottenh@xxxxxxxxxx>
>> Co-developed-by: Anna Glasgall <aglasgal@xxxxxxxxxx>
>> Signed-off-by: Anna Glasgall <aglasgal@xxxxxxxxxx>
>> Signed-off-by: Nick Hudson <nhudson@xxxxxxxxxx>
>> ---
>> net/core/filter.c | 40 ++++++++++++++++++++++++++++++++++++++++
>> 1 file changed, 40 insertions(+)
>> 
>> diff --git a/net/core/filter.c b/net/core/filter.c
>> index 7f8d43420afb..04059d07d368 100644
>> --- a/net/core/filter.c
>> +++ b/net/core/filter.c
>> @@ -3667,6 +3667,46 @@ 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 |
>> + SKB_GSO_TUNNEL_REMCSUM)))
>> + shinfo->gso_type &= ~(SKB_GSO_UDP_TUNNEL |
>> +      SKB_GSO_UDP_TUNNEL_CSUM |
>> +      SKB_GSO_TUNNEL_REMCSUM);
> 
> REMCSUM was previously not included in the series.

Yeah, I was trying to address

https://sashiko.dev/#/patchset/20260318134242.2725749-1-nhudson%40akamai.com?part=5


> 
> It is a non-obvious and rare enough feature that I would exclude it,
> or move it to a separate patch.

I’m happy to drop it.


> 
>> + 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 = !!(shinfo->gso_type & SKB_GSO_UDP_TUNNEL_CSUM);
> 
> Since the flag is never set, only possibly cleared: just clear this field when clearing the flag?
> 
> It appears that this is only used for deprecated UFO anyway.
> 
>> + skb->remcsum_offload = !!(shinfo->gso_type & SKB_GSO_TUNNEL_REMCSUM);
> 
> Always zero?

Yeah, I’ll fix these up in v4

Thanks.

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