fd_pusher.c

#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/wait.h>
#include <sys/fcntl.h>
#include <sys/un.h>
#include <stdbool.h>
#include <stdio.h>
#include <errno.h>
#include <stdint.h>
#include <unistd.h>

struct cmsgbuf
{
    struct cmsghdr hdr;
    int fd;
};

static int pull_handle(int socket)
{
    char empty;
    struct iovec nothing = {
        .iov_base = &empty,
        .iov_len = 1,
    };

    struct cmsgbuf msgbuf;
    struct msghdr msg = {
        .msg_name = NULL,
        .msg_namelen = 0,
        .msg_iov = &nothing,
        .msg_iovlen = 1,
        .msg_flags = 0,
        .msg_control = &msgbuf,
        .msg_controllen = sizeof(struct cmsghdr) + sizeof(int),
    };

    struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
    cmsg->cmsg_len = msg.msg_controllen;
    cmsg->cmsg_level = SOL_SOCKET;
    cmsg->cmsg_type = SCM_RIGHTS;
    int *dfd = (int*)CMSG_DATA(cmsg);
    *dfd = -1;

    fprintf(stderr, "Pulling handle ...\n");
    int ret = recvmsg(socket, &msg, MSG_NOSIGNAL);
    fprintf(stderr, "Pulled handle ... %d\n", msgbuf.fd);
    if (ret < 0)
        return -1;
    return msgbuf.fd;
}

int main(void)
{
    int fds[2] = { -1, -1 };
    int fd = -1;
    int ret = 0;

    if (socketpair(AF_UNIX, SOCK_STREAM, 0, fds) < 0)
        return 1;

    pid_t pid = fork();
    if (pid > 0)
    {
        fd = pull_handle(fds[0]);
        close(fds[1]);

        if (fd < 0)
        {
            ret = 1;
            goto end;
        }

        char buf[1024];
        ssize_t len = read(fd, buf, sizeof(buf) - 1);
        if (len < 0)
        {
            ret = 1;
            goto end;
        }
        buf[len] = '\0';
        fprintf(stderr, "%s\n", buf);

        int stat;
        waitpid(pid, &stat, 0);
    }
    else
    {
        close(0);
        dup(fds[1]);
        close(fds[1]);

        close(fds[0]);
        fds[0] = -1;
        fds[1] = -1;

        char *argv[] = { "/tmp/test", "/tmp/grab-me", NULL };
        int ret = execvp("/tmp/test", argv);
        if (ret < 0)
        {
            ret = 1;
            goto end;
        }
    }

end:
    if (fds[0] >= 0)
        close(fds[0]);
    if (fds[1] >= 0)
        close(fds[1]);
    if (fd >= 0)
        close(fd);

    return ret;
}