Re: [PATCH] bpf: in bpf_skb_adjust_room correct inner protocol for vxlan

Appreciate for your reply Willem!

The original intention of this commit is that when we use bpf_skb_adjust_room  to encapsulate 
Vxlan packets, we find some powerful device features disabled. 

Setting the inner_protocol directly as skb->protocol is the root cause.

I understand that it’s not easy to handle all tunnel protocol in one bpf helper function. But for my
immature idea, when pushing Ethernet header, setting the inner_protocol as ETH_P_TEB may
be better.

Now the flag BPF_F_ADJ_ROOM_ENCAP_L4_UDP includes many udp tunnel types( e.g. 
udp+mpls, geneve, vxlan). Adding an independent flag to represents Vxlan looks a little 
reduplicative. What’s your suggestion?

Thanks again for your reply!

2021年2月8日 下午9:06，Willem de Bruijn [off-list ref] 写道：

On Mon, Feb 8, 2021 at 7:16 AM huangxuesen [off-list ref] wrote:

quoted

From: huangxuesen <redacted>

When pushing vxlan tunnel header, set inner protocol as ETH_P_TEB in skb
to avoid HW device disabling udp tunnel segmentation offload, just like
vxlan_build_skb does.

Drivers for NIC may invoke vxlan_features_check to check the
inner_protocol in skb for vxlan packets to decide whether to disable
NETIF_F_GSO_MASK. Currently it sets inner_protocol as the original
skb->protocol, that will make mlx5_core disable TSO and lead to huge
performance degradation.

Signed-off-by: huangxuesen <redacted>
Signed-off-by: chengzhiyong <redacted>
Signed-off-by: wangli <redacted>
---
net/core/filter.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 255aeee72402..f8d3ba3fe10f 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c

@@ -3466,7 +3466,12 @@ static int bpf_skb_net_grow(struct sk_buff *skb, u32 off, u32 len_diff,
               skb->inner_mac_header = inner_net - inner_mac_len;
               skb->inner_network_header = inner_net;
               skb->inner_transport_header = inner_trans;
-               skb_set_inner_protocol(skb, skb->protocol);
+
+               if (flags & BPF_F_ADJ_ROOM_ENCAP_L4_UDP &&
+                   inner_mac_len == ETH_HLEN)
+                       skb_set_inner_protocol(skb, htons(ETH_P_TEB));

This may be used by vxlan, but it does not imply it.

Adding ETH_HLEN bytes likely means pushing an Ethernet header, but same point.

Conversely, pushing an Ethernet header is not limited to UDP encap.

This probably needs a new explicit BPF_F_ADJ_ROOM_.. flag, rather than
trying to infer from imprecise heuristics.