ethtool ioctl returns unpopulated ethtool_link_settings

Question

I'm trying to use the Ethtool ioctl API to retrieve linkspeed data from my NICs, but I just just get zeroes back in the ethtool_link_settings instance. Using the ethtool command line tool returns the expected values, and my NIC driver supports the newer ETHTOOL_GLINKSETTINGS API.

#include <iostream>
#include <cstring>

#include <linux/ethtool.h>
#include <linux/sockios.h>
#include <netinet/in.h>
#include <net/if.h>
#include <sys/ioctl.h>
#include <unistd.h>

int main()
{
    auto ifn = if_nameindex();
    auto fd = socket(AF_INET, SOCK_DGRAM, IPPROTO_IP);

    for (auto i = ifn; i->if_name; ++i) {
        // Skip the loopback
        if (i->if_index == 1) {
            continue;
        }

        std::cout << "Testing: " << i->if_name << std::endl;

        auto ifr = ifreq{};
        std::strncpy(ifr.ifr_name, i->if_name, IF_NAMESIZE);

        auto msg = ethtool_link_settings{};
        msg.cmd = ETHTOOL_GLINKSETTINGS;
        ifr.ifr_data = reinterpret_cast<char*>(&msg);

        if (ioctl(fd, SIOCETHTOOL, &ifr) == -1) {
            std::cerr << "ioctl fail: " << strerror(errno) << std::endl;
        }

        std::cout << "\tSpeed: " << msg.speed
                  << "\n\tDuplex: " << static_cast<int>(msg.duplex)
                  << "\n\tPort: " << static_cast<int>(msg.port)
                  << std::endl;
    }

    close(fd);
    if_freenameindex(ifn);
    return EXIT_SUCCESS;
}

Results in:

Testing: enp0s3
    Speed: 0
    Duplex: 0
    Port: 0
Testing: enp0s8
    Speed: 0
    Duplex: 0
    Port: 0
Testing: enp0s9
    Speed: 0
    Duplex: 0
    Port: 0
Testing: enp0s10
    Speed: 0
    Duplex: 0
    Port: 0

I'm sure I'm doing something stupid, but I can't see it.

Answer 1

Buried in the comments of the link_mode_masks_nwords field of struct ethtool_link_settings in /usr/include/linux/ethtool.h are some admirable cryptic comments that ETHTOOL_GLINKSETTINGS does a bit of handshaking, so you'll need to call it more than once.

I've adjusted your code by using the code of the ethtool command to perform the ETHTOOL_GLINKSETTINGS command:

#include <iostream>
#include <cstring>

#include <linux/ethtool.h>
#include <linux/sockios.h>
#include <linux/netlink.h>
#include <netinet/in.h>
#include <net/if.h>
#include <sys/ioctl.h>
#include <unistd.h>

int main()
{
    auto ifn = if_nameindex();
    auto fd = socket(AF_INET, SOCK_DGRAM, IPPROTO_IP);

    for (auto i = ifn; i->if_name; ++i) {
        struct {
                struct ethtool_link_settings req;
                __u32 link_mode_data[3 * 127];
            } ecmd;

        // Skip the loopback
        if (i->if_index == 1) {
            continue;
        }

        std::cout << "Testing: " << i->if_name << std::endl;

        auto ifr = ifreq{};
        std::strncpy(ifr.ifr_name, i->if_name, IF_NAMESIZE);

        ecmd.req.cmd = ETHTOOL_GLINKSETTINGS;
        ifr.ifr_data = reinterpret_cast<char*>(&ecmd);

        if (ioctl(fd, SIOCETHTOOL, &ifr) == -1) {
            std::cerr << "ioctl fail: " << strerror(errno) << std::endl;
            return 1;
        }

        if (ecmd.req.link_mode_masks_nwords >= 0 || ecmd.req.cmd != ETHTOOL_GLINKSETTINGS)
            return 1;

        ecmd.req.link_mode_masks_nwords = -ecmd.req.link_mode_masks_nwords;

        if (ioctl(fd, SIOCETHTOOL, &ifr) == -1) {
            std::cerr << "ioctl fail: " << strerror(errno) << std::endl;
            return 1;
        }

        std::cout << "\tSpeed: " << ecmd.req.speed
                  << "\n\tDuplex: " << static_cast<int>(ecmd.req.duplex)
                  << "\n\tPort: " << static_cast<int>(ecmd.req.port)
                  << std::endl;
    }

    close(fd);
    if_freenameindex(ifn);
    return EXIT_SUCCESS;
}