sending fds over unix domain sockets

catfd.c

/*
 * Opens a Unix domain socket and cats any fds passed over it.
 */

#include <err.h>
#include <stdbool.h>
#include <string.h>
#include <sysexits.h>
#include <unistd.h>

#include <sys/socket.h>
#include <sys/types.h>
#include <sys/un.h>
#include <sys/wait.h>


void cat(int fd)
{
    char buf[4096];

    while (true) {
        ssize_t n = read(fd, buf, sizeof(buf));
        if (n == 0) {
            close(fd);
            return;
        }
        if (n < 0)
            err(EX_IOERR, "read(%d)", fd);
        if (n != write(1, buf, n))
            err(EX_IOERR, "write");
    }
    __builtin_unreachable();
}


int main(int argc, char **argv)
{
    if (2 != argc)
        errx(EX_USAGE, "takes path to UDS to create");

    int sockfd = socket(AF_UNIX, SOCK_DGRAM, 0);
    if (sockfd < 0)
        err(EX_OSERR, "socket");

    struct sockaddr_un name = { .sun_family = AF_UNIX };
    char *path = argv[1];
    if (strlen(path) > sizeof(name.sun_path)-1)
        errx(EX_USAGE, "UDS pathnames can't be longer than %zu (yours is %zu)",
             sizeof(name.sun_path)-1, strlen(path));

    strncpy(name.sun_path, path, sizeof(name.sun_path)-1);
    if (bind(sockfd, (const struct sockaddr *)&name, sizeof(name)))
        err(EX_OSERR, "bind(%s)", path);

    pid_t child = fork();
    if (child < 0)
        err(EX_OSERR, "fork");
    if (child) {
        sigaction(SIGINT,
                  &(struct sigaction){ .sa_handler = SIG_IGN,
                                       .sa_flags = SA_RESETHAND },
                  NULL);
        do { waitpid(child, NULL, 0); } while (!kill(child, 0));
        unlink(path);
        return 0;
    }

    while (true) {
        char buf[CMSG_LEN(sizeof(int))]
                __attribute__((aligned(__alignof__(struct cmsghdr))));
        struct msghdr msg = {
            // NB: can't receive any data
            .msg_control = buf,
            .msg_controllen = sizeof(buf),
        };
        if (recvmsg(sockfd, &msg, 0) < 0)
            err(EX_IOERR, "recvmsg");

        struct cmsghdr *c;
        for (c = CMSG_FIRSTHDR(&msg); c; c = CMSG_NXTHDR(&msg, c)) {
            if (c->cmsg_level == SOL_SOCKET &&
                c->cmsg_type == SCM_RIGHTS)
                cat(*(int *)CMSG_DATA(c));
        }
    }

    return 0;
}

sendfd.c

/*
 * Send stdin on a Unix domain socket.
 *
 * You can use the shell to redirect arbitrary fds into this program.
 */

#include <assert.h>
#include <err.h>
#include <string.h>
#include <sysexits.h>
#include <unistd.h>

#include <sys/socket.h>
#include <sys/types.h>
#include <sys/un.h>


static void usage(void)
{
    errx(EX_USAGE, "takes path to Unix domain socket; sends fd 0");
}

int main(int argc, char **argv)
{
    if (2 != argc)
        usage();

    if (isatty(0))  /* NB: try removing this and see what happens */
        errx(EX_USAGE, "you probably don't want to send your tty.");

    int sockfd = socket(AF_UNIX, SOCK_DGRAM, 0);
    if (sockfd < 0)
        err(EX_OSERR, "socket");

    struct sockaddr_un name = { .sun_family = AF_UNIX };
    char *path = argv[1];
    if (strlen(path) > sizeof(name.sun_path)-1)
        errx(EX_USAGE, "UDS pathnames can't be longer than %zu (yours is %zu)",
             sizeof(name.sun_path)-1, strlen(path));

    strncpy(name.sun_path, path, sizeof(name.sun_path)-1);

    char buf[CMSG_LEN(sizeof(int))];
    struct msghdr msg = {
        .msg_name = &name,
        .msg_namelen = sizeof(name),
        .msg_control = buf,
        .msg_controllen = CMSG_LEN(sizeof(int)),
    };
    struct cmsghdr *cmsg;
    cmsg = CMSG_FIRSTHDR(&msg);
    assert(cmsg);
    cmsg->cmsg_level = SOL_SOCKET;
    cmsg->cmsg_type = SCM_RIGHTS;
    cmsg->cmsg_len = sizeof(buf);
    *(int *)CMSG_DATA(cmsg) = 0;

    if (sendmsg(sockfd, &msg, 0) < 0)
        err(EX_IOERR, "sendmsg");

    return 0;
}