scm_functions.c

/*************************************************************************\
*                  Copyright (C) Michael Kerrisk, 2019.                   *
*                                                                         *
* This program is free software. You may use, modify, and redistribute it *
* under the terms of the GNU Lesser General Public License as published   *
* by the Free Software Foundation, either version 3 or (at your option)   *
* any later version. This program is distributed without any warranty.    *
* See the files COPYING.lgpl-v3 and COPYING.gpl-v3 for details.           *
\*************************************************************************/

/* scm_functions.c

   Functions to exchange ancillary data over UNIX domain sockets.
   These functions are simplistic, in that they ignore the "real" data
   content on the assumption that the sockets are being used only for
   the purposes of exchanging ancillary data. In many real-world
   applications, the application makes use of both the "real" data
   channel and the ancillary data, with some kind of protocol that
   determines how the "real" and ancillary data are used together.
*/
#include "scm_functions.h"

/* Send the file descriptor 'fd' over the connected UNIX domain socket
   'sockfd'. Returns 0 on success, or -1 on error. */

int
sendfd(int sockfd, int fd)
{
    struct msghdr msgh;
    struct iovec iov;
    int data;
    struct cmsghdr *cmsgp;

    /* Allocate a char array of suitable size to hold the ancillary data.
       However, since this buffer is in reality a 'struct cmsghdr', use a
       union to ensure that it is aligned as required for that structure.
       Alternatively, we could allocate the buffer using malloc(), which
       returns a buffer that satisfies the strictest alignment requirements
       of any type. However, if we employ that approach, we must ensure
       that we free() the buffer on all return paths from this function. */
    union {
        char   buf[CMSG_SPACE(sizeof(int))];
                        /* Space large enough to hold an 'int' */
        struct cmsghdr align;
    } controlMsg;

    /* The 'msg_name' field can be used to specify the address of the
       destination socket when sending a datagram. However, we do not
       need to use this field because we presume that 'sockfd' is a
       connected socket. */

    msgh.msg_name = NULL;
    msgh.msg_namelen = 0;

    /* On Linux, we must transmit at least one byte of real data in
       order to send ancillary data. We transmit an arbitrary integer
       whose value is ignored by recvfd(). */

    msgh.msg_iov = &iov;
    msgh.msg_iovlen = 1;
    iov.iov_base = &data;
    iov.iov_len = sizeof(int);
    data = 12345;

    /* Set 'msghdr' fields that describe ancillary data */

    msgh.msg_control = controlMsg.buf;
    msgh.msg_controllen = sizeof(controlMsg.buf);

    /* Set up ancillary data describing file descriptor to send */

    cmsgp = CMSG_FIRSTHDR(&msgh);
    cmsgp->cmsg_level = SOL_SOCKET;
    cmsgp->cmsg_type = SCM_RIGHTS;
    cmsgp->cmsg_len = CMSG_LEN(sizeof(int));
    *((int *) CMSG_DATA(cmsgp)) = fd;

    /* Send real plus ancillary data */

    if (sendmsg(sockfd, &msgh, 0) == -1)
        return -1;

    return 0;
}

/* Receive a file descriptor on a connected UNIX domain socket. Returns
   the received file descriptor on success, or -1 on error. */

int
recvfd(int sockfd)
{
    struct msghdr msgh;
    struct iovec iov;
    int data;
    ssize_t nr;

    /* Allocate a char buffer for the ancillary data. See the comments
       in sendfd() */
    union {
        char   buf[CMSG_SPACE(sizeof(int))];
        struct cmsghdr align;
    } controlMsg;
    struct cmsghdr *cmsgp;

    /* The 'msg_name' field can be used to obtain the address of the
       sending socket. However, we do not need this information. */

    msgh.msg_name = NULL;
    msgh.msg_namelen = 0;

    /* Specify buffer for receiving real data */

    msgh.msg_iov = &iov;
    msgh.msg_iovlen = 1;
    iov.iov_base = &data;       /* Real data is an 'int' */
    iov.iov_len = sizeof(int);

    /* Set 'msghdr' fields that describe ancillary data */

    msgh.msg_control = controlMsg.buf;
    msgh.msg_controllen = sizeof(controlMsg.buf);

    /* Receive real plus ancillary data; content of real data is ignored */

    nr = recvmsg(sockfd, &msgh, 0);
    if (nr == -1)
        return -1;

    cmsgp = CMSG_FIRSTHDR(&msgh);

    /* Check the validity of the 'cmsghdr' */

    if (cmsgp == NULL ||
            cmsgp->cmsg_len != CMSG_LEN(sizeof(int)) ||
            cmsgp->cmsg_level != SOL_SOCKET ||
            cmsgp->cmsg_type != SCM_RIGHTS) {
        errno = EINVAL;
        return -1;
    }

    /* Return the received file descriptor to our caller */

    return *((int *) CMSG_DATA(cmsgp));
}