limen/src/ipv4.cpp

#include "ipv4.h"
#include <array>
#include "net.h"
#include "parse_buffer.h"
#include "debug_log.h"

namespace ipv4 {

namespace {

constexpr ip4_addr IP_BROADCAST_ALL = {255, 255, 255, 255};

std::array<addr_filter, 4> addr_filters;
size_t addr_filter_count = 0;

struct protocol_entry {
    uint8_t protocol;
    protocol_handler fn;
};
std::array<protocol_entry, 8> protocol_handlers;
size_t protocol_handler_count = 0;

bool default_addr_filter(const ip4_addr& dst) {
    const auto& ns = net::get_state();
    return dst == ns.ip || dst == IP_BROADCAST_ALL || dst == SUBNET_BROADCAST;
}

} // namespace

uint16_t checksum(const void* data, size_t len) {
    auto p = static_cast<const uint8_t*>(data);
    uint32_t sum = 0;
    for (size_t i = 0; i < len - 1; i += 2) {
        sum += (p[i] << 8) | p[i + 1];
    }
    if (len & 1) {
        sum += p[len - 1] << 8;
    }
    while (sum >> 16) {
        sum = (sum & 0xFFFF) + (sum >> 16);
    }
    return __builtin_bswap16(~sum);
}

void register_addr_filter(addr_filter fn) {
    if (addr_filter_count >= addr_filters.size()) {
        dlog("ipv4::register_addr_filter overflow: filter dropped");
        return;
    }
    addr_filters[addr_filter_count++] = fn;
}

bool addressed_to_us(ip4_addr dst) {
    for (size_t i = 0; i < addr_filter_count; i++) {
        if (addr_filters[i](dst)) {
            return true;
        }
    }
    return false;
}

void register_protocol(uint8_t protocol, protocol_handler fn) {
    if (protocol_handler_count >= protocol_handlers.size()) {
        dlogf("ipv4::register_protocol overflow: protocol=%u dropped", protocol);
        return;
    }
    protocol_handlers[protocol_handler_count++] = {protocol, fn};
}

void handle(std::span<const uint8_t> frame, span_writer& tx) {
    parse_buffer pb(frame);
    pb.consume<eth::header>();
    auto* ip = pb.consume<header>();
    if (!ip) {
        return;
    }
    if ((ip->ver_ihl >> 4) != 4) {
        return;
    }

    size_t options_len = ip->header_len() - sizeof(header);
    if (options_len > 0 && !pb.skip(options_len)) {
        return;
    }

    for (size_t i = 0; i < protocol_handler_count; i++) {
        if (protocol_handlers[i].protocol == ip->protocol) {
            protocol_handlers[i].fn(frame, tx);
            break;
        }
    }
}

__attribute__((constructor))
static void register_self() {
    net::register_ethertype(eth::ETH_IPV4, handle);
    register_addr_filter(default_addr_filter);
}

} // namespace ipv4