unix_cred_pass.c

#define _GNU_SOURCE
#define _POSIX_C_SOURCE 200809L
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <unistd.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#if defined(__linux__)
#include <sys/uio.h>
#include <sys/prctl.h>
#elif defined(__APPLE__) || defined(__FreeBSD__) || defined(__OpenBSD__) || defined(__NetBSD__)
#include <sys/ucred.h>
#endif

static int make_server(const char *path)
{
    int fd = socket(AF_UNIX, SOCK_STREAM, 0);
    if (fd < 0) return -1;
    struct sockaddr_un addr;
    memset(&addr, 0, sizeof addr);
    addr.sun_family = AF_UNIX;
    strncpy(addr.sun_path, path, sizeof addr.sun_path - 1);
    unlink(path);
    if (bind(fd, (struct sockaddr *)&addr, sizeof addr) < 0) {
        close(fd);
        return -1;
    }
    if (listen(fd, 1) < 0) {
        close(fd);
        return -1;
    }
    return fd;
}

static int make_client(const char *path)
{
    int fd = socket(AF_UNIX, SOCK_STREAM, 0);
    if (fd < 0) return -1;
    struct sockaddr_un addr;
    memset(&addr, 0, sizeof addr);
    addr.sun_family = AF_UNIX;
    strncpy(addr.sun_path, path, sizeof addr.sun_path - 1);
    if (connect(fd, (struct sockaddr *)&addr, sizeof addr) < 0) {
        close(fd);
        return -1;
    }
    return fd;
}

int main(void)
{
    const char *path = "unix_cred.sock";
    int srv = make_server(path);
    if (srv < 0) {
        perror("make_server");
        return 1;
    }
    pid_t pid = fork();
    if (pid < 0) return 1;
    if (pid == 0) {
        int c = make_client(path);
        if (c < 0) return 1;
        const char *msg = "hello\n";
        write(c, msg, strlen(msg));
        close(c);
        return 0;
    } else {
        int cli;
        struct sockaddr_un addr;
        socklen_t alen = sizeof addr;
        cli = accept(srv, (struct sockaddr *)&addr, &alen);
        if (cli < 0) {
            perror("accept");
            return 1;
        }
#if defined(__linux__)
        int opt = 1;
        if (setsockopt(cli, SOL_SOCKET, SO_PASSCRED, &opt, sizeof opt) < 0) {
            perror("setsockopt");
        }
        char buf[128];
        struct iovec iov;
        iov.iov_base = buf;
        iov.iov_len = sizeof buf;
        char cmsgbuf[CMSG_SPACE(sizeof(struct ucred))];
        struct msghdr msg;
        memset(&msg, 0, sizeof msg);
        msg.msg_iov = &iov;
        msg.msg_iovlen = 1;
        msg.msg_control = cmsgbuf;
        msg.msg_controllen = sizeof cmsgbuf;
        ssize_t n = recvmsg(cli, &msg, 0);
        if (n > 0) {
            struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
            if (cmsg && cmsg->cmsg_level == SOL_SOCKET && cmsg->cmsg_type == SCM_CREDENTIALS) {
                struct ucred *uc = (struct ucred *)CMSG_DATA(cmsg);
                buf[n] = 0;
                printf("got from pid=%d uid=%d gid=%d: %s", uc->pid, uc->uid, uc->gid, buf);
            }
        }
#elif defined(__APPLE__) || defined(__FreeBSD__) || defined(__OpenBSD__) || defined(__NetBSD__)
        uid_t uid;
        gid_t gid;
        if (getpeereid(cli, &uid, &gid) == 0) {
            char buf[128];
            ssize_t n = read(cli, buf, sizeof buf - 1);
            if (n > 0) {
                buf[n] = 0;
                printf("got from uid=%d gid=%d: %s", (int)uid, (int)gid, buf);
            }
        }
#else
        char buf[128];
        ssize_t n = read(cli, buf, sizeof buf - 1);
        if (n > 0) {
            buf[n] = 0;
            printf("got: %s", buf);
        }
#endif
        close(cli);
        close(srv);
        unlink(path);
        wait(NULL);
        return 0;
    }
}