Af Xdp

NIC → XDP program (BPF) → XDP_REDIRECT → AF_XDP socket → userspace
                ↓
           XDP_DROP/PASS/TX

#include <bpf/xsk.h>
#include <bpf/libbpf.h>
#include <xdp/xsk.h>

#define NUM_FRAMES     4096
#define FRAME_SIZE     XSK_UMEM__DEFAULT_FRAME_SIZE
#define RX_BATCH_SIZE  64

struct xsk_umem_info {
    struct xsk_ring_prod fill;
    struct xsk_ring_cons comp;
    struct xsk_umem *umem;
    void *buffer;
};

struct xsk_socket_info {
    struct xsk_ring_cons rx;
    struct xsk_ring_prod tx;
    struct xsk_socket *xsk;
};

int xsk_setup(struct xsk_umem_info *umem_info,
              struct xsk_socket_info *xsk_info,
              int ifindex, int queue_id, int xsk_flags)
{
    umem_info->buffer = mmap(NULL, NUM_FRAMES * FRAME_SIZE,
        PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);

    struct xsk_umem_config umem_cfg = {
        .fill_size = XSK_RING_PROD__DEFAULT_NUM_DESCS,
        .comp_size = XSK_RING_CONS__DEFAULT_NUM_DESCS,
        .frame_size = FRAME_SIZE,
        .frame_headroom = XSK_UMEM__DEFAULT_FRAME_HEADROOM,
        .flags = 0,
    };

    int ret = xsk_umem__create(&umem_info->umem, umem_info->buffer,
        NUM_FRAMES * FRAME_SIZE, &umem_info->fill, &umem_info->comp,
        &umem_cfg);
    if (ret)
        return ret;

    struct xsk_socket_config xsk_cfg = {
        .rx_size = XSK_RING_CONS__DEFAULT_NUM_DESCS,
        .tx_size = XSK_RING_PROD__DEFAULT_NUM_DESCS,
        .libbpf_flags = XSK_LIBBPF_FLAGS__INHIBIT_PROG_LOAD,
        .xdp_flags = XDP_FLAGS_UPDATE_IF_NOEXIST,
        .bind_flags = xsk_flags,  // XDP_ZEROCOPY or XDP_COPY
    };

    return xsk_socket__create(&xsk_info->xsk, "eth0", queue_id,
        umem_info->umem, &xsk_info->rx, &xsk_info->tx, &xsk_cfg);
}

void populate_fill_ring(struct xsk_umem_info *umem) {
    uint32_t idx;
    uint32_t ret = xsk_ring_prod__reserve(&umem->fill, RX_BATCH_SIZE, &idx);
    for (uint32_t i = 0; i < ret; i++)
        *xsk_ring_prod__fill_addr(&umem->fill, idx + i) = i * FRAME_SIZE;
    xsk_ring_prod__submit(&umem->fill, ret);
}

// xdp_redirect.c
#include <linux/bpf.h>
#include <bpf/bpf_helpers.h>

struct {
    __uint(type, BPF_MAP_TYPE_XSKMAP);
    __uint(max_entries, 64);
    __type(key, int);
    __type(value, int);
} xsks_map SEC(".maps");

SEC("xdp")
int xdp_redirect_prog(struct xdp_md *ctx)
{
    int index = ctx->rx_queue_index;
    return bpf_redirect_map(&xsks_map, index, 0);
}

# Load and attach
bpftool prog load xdp_redirect.o /sys/fs/bpf/xdp_redirect
ip link set dev eth0 xdp obj xdp_redirect.o sec xdp

# Pin xsks_map and update with socket fd

while (running) {
    uint32_t idx_rx = 0, rcvd;
    rcvd = xsk_ring_cons__peek(&xsk_info->rx, RX_BATCH_SIZE, &idx_rx);
    if (!rcvd)
        continue;

    for (uint32_t i = 0; i < rcvd; i++) {
        const struct xdp_desc *desc = xsk_ring_cons__rx_desc(&xsk_info->rx, idx_rx + i);
        uint64_t addr = desc->addr;
        uint32_t len = desc->len;
        uint8_t *pkt = (uint8_t *)xsk_umem__get_data(umem_info->buffer, addr);
        process_packet(pkt, len);
    }
    xsk_ring_cons__release(&xsk_info->rx, rcvd);

    // Return frames to fill ring
    refill_fill_ring(umem_info, rcvd);
}

# Check driver ZC support
ethtool -i eth0
# Kernel log on bind:
dmesg | grep xsk
# "Zero-copy enabled" or "Copy mode"

# Modern libbpf includes xsk API
pkg-config --libs libbpf
# -lbpf -lxdp (if separate libxdp installed)