XDP and sk_buff

Question

I started coding in ebpf and XDP. I am using python bcc to load the XDP program to the NICs. I am trying to work with __sk_buff structure, but when I am trying to access any filed of skb the verifier failed to load the program.

int xdp_test(struct sk_buff *skb)
{
    void *data = (void*)(long)skb->data;
    void *data_end = (void*)(long)skb->data_end;

    if (data + sizeof(struct ethhdr) + sizeof(struct iphdr) < data_end)
    {
        struct iphdr * ip = ip_hdr(skb);
        // according to my checks, it failed because of this line. I cant access to ip->protocol (or any other fileds)
        if (ip->protocol == IPPROTO_TCP)
        {
               return XDP_PASS;
        }
    }
    ...
    return XDP_PASS
}

I just want to calculates layer 4 checksum on my program using bpf_l4_csum_replace Which takes skb as the first argument.

Why is that happening? Can I even use __sk_buff structure in XDP? Or I have to use the xdp_md struct?

UPDATE: Thanks to Qeole, I understood that I cannot use sk_buff using XDP. There is a way to calculate TCP checksum using xdp_md?

Answer 1

Indeed you cannot use the struct __sk_buff in XDP programs. You have to use the struct xdp_md instead. XDP performance is due for a great part to the kernel calling the eBPF program before the allocation and initialisation of the socket buffer (struct sk_buff in the kernel), which saves time and resources, but also means you don't have access to that structure in your XDP program.