On 09/10, Tobias Böhm wrote: > Hi, > > when decapsulating VXLAN packets with bpf_skb_adjust_room and redirecting to > a tap device I observed unexpected segmentation. > > In my setup there is a sched_cls program attached at the ingress path of a > physical NIC with GRO enabled. Packets are redirected either directly for > plain traffic, or decapsulated beforehand in case of VXLAN. Decapsulation is > done by bpf_skb_adjust_room with BPF_F_ADJ_ROOM_DECAP_L3_IPV4. > > For both kinds of traffic GRO on the physical NIC works as expected > resulting in merged packets. > > Large non-decapsulated packets are transmitted directly on the tap interface > as expected. But surprisingly, decapsulated packets are being segmented > again before transmission. > > When analyzing and comparing the call chains I observed that > netif_skb_features returns different values for the different kind of > traffic. > > The tap devices have the following features set: > > dev->features = 0x1558c9 > dev->hw_enc_features = 0x10000001 > > For the non-decapsulated traffic netif_skb_features returns 0x1558c9 but for > the decapsulated traffic it returns 0x1. This is same value as the result of > "dev->features & dev->hw_enc_features". > > In netif_skb_features this operation effectively happens in case > skb->encapsulation is set. Inspecting the skb in both cases showed that in > case of decapsulation the skb->encapsulation flag was indeed still set. > > I wonder if there is a reason that the skb->encapsulation flag is not unset > in bpf_skb_net_shrink when BPF_F_ADJ_ROOM_DECAP_* flags are present? Since > skb->encapsulation is set in bpf_skb_net_grow when adding space for > encapsulation my expectation would be that the flag is also unset when doing > the opposite operation. + Willem and netdev for visibility.
