creator-utils.cc

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/*
 * Copyright (c) University of Washington
 *
 * SPDX-License-Identifier: GPL-2.0-only
 */
 
#include "creator-utils.h"
 
#include "encode-decode.h"
 
#include <arpa/inet.h>
#include <cstring>
#include <errno.h>
#include <iomanip>
#include <iostream>
#include <net/ethernet.h>
#include <net/if.h>
#include <netinet/in.h>
#include <sstream>
#include <stdlib.h>
#include <string>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <unistd.h>
 
namespace ns3
{
 
/// Flag to enable / disable verbose log mode
bool gVerbose = false;
 
/**
 * \brief Send the file descriptor back to the code that invoked the creation.
 *
 * \param path The socket address information from the Unix socket we use
 * to send the created socket back to.
 * \param fd The file descriptor we're going to send.
 * \param magic_number A verification number to verify the caller is talking to the
 * right process.
 */
void
SendSocket(const char* path, int fd, const int magic_number)
{
    //
    // Open a Unix (local interprocess) socket to call back to the emu net
    // device.
    //
    LOG("Create Unix socket");
    int sock = socket(PF_UNIX, SOCK_DGRAM, 0);
    ABORT_IF(sock == -1, "Unable to open socket", 1);
 
    //
    // We have this string called path, which is really a hex representation
    // of the endpoint that the net device created.  It used a forward encoding
    // method (BufferToString) to take the sockaddr_un it made and passed
    // the resulting string to us.  So we need to take the inverse method
    // (StringToBuffer) and build the same sockaddr_un over here.
    //
    socklen_t clientAddrLen;
    struct sockaddr_un clientAddr;
 
    LOG("Decode address " << path);
    bool rc = ns3::StringToBuffer(path, (uint8_t*)&clientAddr, &clientAddrLen);
    ABORT_IF(rc == false, "Unable to decode path", 0);
 
    LOG("Connect");
    int status = connect(sock, (struct sockaddr*)&clientAddr, clientAddrLen);
    ABORT_IF(status == -1, "Unable to connect to emu device", 1);
 
    LOG("Connected");
 
    //
    // This is arcane enough that a few words are worthwhile to explain what's
    // going on here.
    //
    // The interesting information (the socket FD) is going to go back to the
    // fd net device as an integer of ancillary data.  Ancillary data is bits
    // that are not a part a socket payload (out-of-band data).  We're also
    // going to send one integer back.  It's just initialized to a magic number
    // we use to make sure that the fd device is talking to the emu socket
    // creator and not some other creator process.
    //
    // The struct iovec below is part of a scatter-gather list.  It describes a
    // buffer.  In this case, it describes a buffer (an integer) containing the
    // data that we're going to send back to the emu net device (that magic
    // number).
    //
    struct iovec iov;
    uint32_t magic = magic_number;
    iov.iov_base = &magic;
    iov.iov_len = sizeof(magic);
 
    //
    // The CMSG macros you'll see below are used to create and access control
    // messages (which is another name for ancillary data).  The ancillary
    // data is made up of pairs of struct cmsghdr structures and associated
    // data arrays.
    //
    // First, we're going to allocate a buffer on the stack to contain our
    // data array (that contains the socket).  Sometimes you'll see this called
    // an "ancillary element" but the msghdr uses the control message termimology
    // so we call it "control."
    //
    constexpr size_t msg_size = sizeof(int);
    char control[CMSG_SPACE(msg_size)];
 
    //
    // There is a msghdr that is used to minimize the number of parameters
    // passed to sendmsg (which we will use to send our ancillary data).  This
    // structure uses terminology corresponding to control messages, so you'll
    // see msg_control, which is the pointer to the ancillary data and controllen
    // which is the size of the ancillary data array.
    //
    // So, initialize the message header that describes our ancillary/control data
    // and point it to the control message/ancillary data we just allocated space
    // for.
    //
    struct msghdr msg;
    msg.msg_name = nullptr;
    msg.msg_namelen = 0;
    msg.msg_iov = &iov;
    msg.msg_iovlen = 1;
    msg.msg_control = control;
    msg.msg_controllen = sizeof(control);
    msg.msg_flags = 0;
 
    //
    // A cmsghdr contains a length field that is the length of the header and
    // the data.  It has a cmsg_level field corresponding to the originating
    // protocol.  This takes values which are legal levels for getsockopt and
    // setsockopt (here SOL_SOCKET).  We're going to use the SCM_RIGHTS type of
    // cmsg, that indicates that the ancillary data array contains access rights
    // that we are sending back to the emu net device.
    //
    // We have to put together the first (and only) cmsghdr that will describe
    // the whole package we're sending.
    //
    struct cmsghdr* cmsg;
    cmsg = CMSG_FIRSTHDR(&msg);
    cmsg->cmsg_level = SOL_SOCKET;
    cmsg->cmsg_type = SCM_RIGHTS;
    cmsg->cmsg_len = CMSG_LEN(msg_size);
    //
    // We also have to update the controllen in case other stuff is actually
    // in there we may not be aware of (due to macros).
    //
    msg.msg_controllen = cmsg->cmsg_len;
 
    //
    // Finally, we get a pointer to the start of the ancillary data array and
    // put our file descriptor in.
    //
    int* fdptr = (int*)(CMSG_DATA(cmsg));
    *fdptr = fd; //
 
    //
    // Actually send the file descriptor back to the emulated net device.
    //
    ssize_t len = sendmsg(sock, &msg, 0);
    ABORT_IF(len == -1, "Could not send socket back to emu net device", 1);
 
    LOG("sendmsg complete");
}
 
} // namespace ns3