recv-creds.c

#define _GNU_SOURCE
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <sys/un.h>

static int recv_creds(int fd) {
    unsigned char buf[128], ctrl[128];
    struct iovec iov = {
        .iov_base = buf,
        .iov_len = sizeof(buf)
    };
    struct msghdr mh = {
        .msg_iov = &iov,
        .msg_iovlen = 1,
        .msg_control = ctrl,
        .msg_controllen = sizeof(ctrl)
    };
    struct ucred ucred_peer;
    socklen_t ucred_peer_len = sizeof(ucred_peer);
    struct cmsghdr *cmsg;
    int ret;

    ret = getsockopt(fd, SOL_SOCKET, SO_PEERCRED, &ucred_peer, &ucred_peer_len);
    if (ret < 0)
        return 0;

    printf("SO_PEERCRED on fd %d: pid %d uid %d gid %d\n",
           fd, ucred_peer.pid, ucred_peer.uid, ucred_peer.gid);

    ret = recvmsg(fd, &mh, 0);
    if (ret < 0)
        return ret;

    for (cmsg = CMSG_FIRSTHDR(&mh); cmsg; cmsg = CMSG_NXTHDR(&mh, cmsg)) {
        if (cmsg->cmsg_level == SOL_SOCKET &&
            cmsg->cmsg_type  == SCM_CREDENTIALS) {

            struct ucred *ucred = (struct ucred *) CMSG_DATA(cmsg);
            printf("SCM_CREDENTIALS on fd %d: pid %d uid %d gid %d\n",
                   fd, ucred->pid, ucred->uid, ucred->gid);
        }
    }

    return 0;
}

int main(void) {
    const int one = 1;
    int ret, fd[2];
    pid_t pid;

    ret = socketpair(AF_UNIX, SOCK_SEQPACKET, 0, fd);
    if (ret < 0)
        return ret;

    ret = setsockopt(fd[1], SOL_SOCKET, SO_PASSCRED, &one, sizeof(one));
    if (ret < 0)
        return ret;

    write(fd[0], &one, sizeof(one));

    pid = fork();
    if (pid < 0)
        return pid;

    if (pid == 0) {
        /* child */
        setresgid(1000, 1000, 1000);
        setresuid(1000, 1000, 1000);
        write(fd[0], &one, sizeof(one));
        exit(0);
    } else {
        int i;

        printf("Current PID: %d, child PID: %d\n", getpid(), pid);

        for (i = 0; i < 2; i++) {
            printf("---------------------------------------\n");
            ret = recv_creds(fd[1]);
            if (ret < 0)
                return ret;
        }
    }

    return 0;
}