bpf_for() example

[unikzforce]

can you please add a bpf_for() example in the documentation?

i tried to write down one ( kernel 6.7.10 ) using latest libbpf 1.3 and clium ebpf 0.14 , but the program is creating compile error in bpf2go generation step,

my program:

#include <linux/bpf.h>
#include <linux/if_ether.h>
#include <bpf/bpf_helpers.h>
#include <linux/pkt_cls.h>

char LICENSE[] SEC("license") = "Dual BSD/GPL";

#define MAX_INTERFACES 20

struct {
    __uint(type, BPF_MAP_TYPE_ARRAY);
    __type(key, __u32);
    __type(value, __u32);
    __uint(max_entries, MAX_INTERFACES);
} interfaces_array SEC(".maps");

struct {
    __uint(type, BPF_MAP_TYPE_ARRAY);
    __type(key, __u32);
    __type(value, __u32);
    __uint(max_entries, 1);
} interfaces_array_length SEC(".maps");

static int redirect_packet(struct __sk_buff *skb, __u32 interface_index) {
    bpf_printk("///////////// id = %llx, multicast: redirection to %d \n",
               bpf_ktime_get_ns(), interface_index);
    return bpf_clone_redirect(skb, interface_index, 0);
}

static int process_interface(struct __sk_buff *skb, __u32 key, __u32 interface_index) {
    if (interface_index != 0 && interface_index != skb->ingress_ifindex) {
        return redirect_packet(skb, interface_index);
    }
    return 0; // Continue iterating
}

SEC("tc")
int switch_agent_unknown_unicast_flooding(struct __sk_buff *skb)
{
    bpf_printk(
            "///////////////////////////////////////////////////////////////////////////////////////////////////");
    // we can use current_time as something like a unique identifier for packet
    __u64 current_time = bpf_ktime_get_ns();
    struct ethhdr *eth = (void *)(long)skb->data;

    if ((void *)(eth + 1) > (void *)(long)skb->data_end)
        return BPF_DROP;

    bpf_printk(
            "///////////// id = %llx, interface = %d, Packet received, source MAC: %02x:%02x:%02x:%02x:%02x:%02x\n",
            current_time, skb->ingress_ifindex, eth->h_source[0], eth->h_source[1],
            eth->h_source[2], eth->h_source[3], eth->h_source[4], eth->h_source[5]);

    bpf_printk(
            "///////////// id = %llx, interface = %d, Packet received, dest MAC: %02x:%02x:%02x:%02x:%02x:%02x\n",
            current_time, skb->ingress_ifindex, eth->h_dest[0], eth->h_dest[1], eth->h_dest[2],
            eth->h_dest[3], eth->h_dest[4], eth->h_dest[5]);

    int ingress_ifindex = skb->ingress_ifindex;


    int zero = 0; // Key for the first element
    __u32* number_of_interfaces_ptr = bpf_map_lookup_elem(&interfaces_array_length, &zero);
    if (!number_of_interfaces_ptr || *number_of_interfaces_ptr == 0) {
        return TC_ACT_OK;
    }
    __u32 number_of_interfaces = *number_of_interfaces_ptr;

    bpf_printk("///////////// id = %llx, interface = %d, start to multicast\n", current_time, skb->ingress_ifindex);

    if (number_of_interfaces>20)
    {
        number_of_interfaces = 20;
    }


    int i;

    bpf_for (i, 0, 20) {

        __u32* interface_index_ptr = bpf_map_lookup_elem(&interfaces_array, &i);

        if (interface_index_ptr && i<number_of_interfaces) {
            __u32 interface_index = *interface_index_ptr;

            if (interface_index != ingress_ifindex) {
                bpf_clone_redirect(skb, interface_index, 0);
                bpf_printk("///////////// id = %llx, multicast: redirection to %d \n",
                           current_time, interface_index);
            }
        }
    }

    return TC_ACT_OK;
}

and this is the error:

Compiled /IdeaProjects/wormhole/ebpf/switch_agent/switchagentxdp_bpfel.o
Stripped /IdeaProjects/wormhole/ebpf/switch_agent/switchagentxdp_bpfel.o
Wrote /IdeaProjects/wormhole/ebpf/switch_agent/switchagentxdp_bpfel.go
Compiled /IdeaProjects/wormhole/ebpf/switch_agent/switchagentxdp_bpfeb.o
Stripped /IdeaProjects/wormhole/ebpf/switch_agent/switchagentxdp_bpfeb.o
Wrote /IdeaProjects/wormhole/ebpf/switch_agent/switchagentxdp_bpfeb.go
/IdeaProjects/wormhole/ebpf/switch_agent/bpf/switch_agent_unknown_unicast_flooding.bpf.c:79:2: error: variable has incomplete type 'struct bpf_iter_num'
        bpf_for (i, 0, 20) {
        ^
/usr/local/include/bpf/bpf_helpers.h:372:22: note: expanded from macro 'bpf_for'
        struct bpf_iter_num ___it __attribute__((aligned(8), /* enforce, just in case */        \
                            ^
/usr/local/include/bpf/bpf_helpers.h:307:8: note: forward declaration of 'struct bpf_iter_num'
struct bpf_iter_num;
       ^
1 error generated.
Error: can't execute clang: exit status 1
exit status 1
ebpf/switch_agent/gen.go:4: running "go": exit status 1

[lmb]

Your problem isn't with bpf2go, instead you have a bug in your code as pointed out by the compiler. Since this isn't an issue with the library per-se I can't help you, sorry.

[unikzforce]

aaah got it, but regardless of my problem above, could you please kindly consider adding a working bpf_for() example in the documentation?