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    SubjectRe: [PATCH v2] xdp: Sample xdp program implementing ip forward
    On Tue, 10 Oct 2017 12:58:52 +0530
    Christina Jacob <christina.jacob.koikara@gmail.com> wrote:

    > +SEC("xdp3")
    > +int xdp_prog3(struct xdp_md *ctx)
    > +{
    > + void *data_end = (void *)(long)ctx->data_end;
    > + void *data = (void *)(long)ctx->data;
    > + struct ethhdr *eth = data;
    > + int rc = XDP_DROP, forward_to;
    > + long *value;
    > + struct trie_value *prefix_value;
    > + long *dest_mac = NULL, *src_mac = NULL;
    > + u16 h_proto;
    > + u64 nh_off;
    > + u32 ipproto;
    > + union key_4 key4;

    Reverse-xmas tree rule: Prefer ordering declarations longest to shortest.


    [...]
    > + if (h_proto == htons(ETH_P_ARP)) {
    > + return XDP_PASS;
    > + } else if (h_proto == htons(ETH_P_IP)) {
    > + int src_ip = 0, dest_ip = 0;
    > + struct direct_map *direct_entry;
    > +
    > + ipproto = parse_ipv4(data, nh_off, data_end, &src_ip, &dest_ip);
    > + direct_entry = (struct direct_map *)bpf_map_lookup_elem
    > + (&exact_match, &dest_ip);
    > + /*check for exact match, this would give a faster lookup*/
    > + if (direct_entry && direct_entry->mac && direct_entry->arp.mac) {
    > + src_mac = &direct_entry->mac;
    > + dest_mac = &direct_entry->arp.mac;
    > + forward_to = direct_entry->ifindex;
    > + } else {
    > + /*Look up in the trie for lpm*/
    > + key4.b32[0] = 32;
    > + key4.b8[4] = dest_ip % 0x100;
    > + key4.b8[5] = (dest_ip >> 8) % 0x100;
    > + key4.b8[6] = (dest_ip >> 16) % 0x100;
    > + key4.b8[7] = (dest_ip >> 24) % 0x100;
    > + prefix_value = ((struct trie_value *)bpf_map_lookup_elem
    > + (&lpm_map, &key4));
    > + if (!prefix_value) {
    > + return XDP_DROP;
    > + } else {
    > + src_mac = &prefix_value->value;
    > + if (src_mac) {
    > + dest_mac = (long *)bpf_map_lookup_elem
    > + (&arp_table, &dest_ip);
    > + if (!dest_mac) {
    > + if (prefix_value->gw) {
    > + dest_ip = *(__be32 *)&prefix_value->gw;
    > + dest_mac = (long *)bpf_map_lookup_elem(&arp_table, &dest_ip);
    > + } else {
    > + return XDP_DROP;
    > + }
    > + }
    > + forward_to = prefix_value->ifindex;
    > + } else {
    > + return XDP_DROP;
    > + }
    > + }
    > + }
    > + } else {
    > + ipproto = 0;
    > + }

    The nesting in this function is getting annoying to read.
    Kernel code often use "early return" style coding to solve this.

    A quick search turns up this guide, look at section "Early return"
    https://en.wikibooks.org/wiki/Computer_Programming/Coding_Style/Minimize_nesting

    --
    Best regards,
    Jesper Dangaard Brouer
    MSc.CS, Principal Kernel Engineer at Red Hat
    LinkedIn: http://www.linkedin.com/in/brouer

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