server.c

#include <sys/socket.h>
#include <sys/types.h>
#include <sys/un.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <errno.h>
#include <unistd.h>

struct ucred {
  pid_t pid;    /* process ID of the sending process */
  uid_t uid;    /* user ID of the sending process */
  gid_t gid;    /* group ID of the sending process */
};

void main(void) {
    int sock = socket(AF_UNIX, SOCK_STREAM, 0);
    if(!sock)
        exit(EXIT_FAILURE);

    struct sockaddr_un addr = {0};
    addr.sun_family = AF_UNIX;
    strcpy(addr.sun_path, "/tmp/socket_cred_poc.sock");
    unlink("/tmp/socket_cred_poc.sock");
    if(bind(sock, (struct sockaddr *) &addr, sizeof(struct sockaddr_un)) == -1) {
        perror("bind");
        exit(EXIT_FAILURE);
    }

    if(listen(sock, 5) == -1) {
        fprintf(stderr, "Errno: %d, %d\n", errno, EINVAL);
        perror("listen");
        exit(EXIT_FAILURE);
    }

    while(1) {
        int client_sock;
        if((client_sock = accept(sock, NULL, NULL)) == -1) {
            perror("accept");
            exit(EXIT_FAILURE);
        }

        struct ucred cr;
        int cr_len = sizeof cr;
        if(getsockopt(client_sock, SOL_SOCKET, SO_PEERCRED, &cr, &cr_len) != 0) {
            perror("getsockopt: SO_PEERCRED");
            exit(EXIT_FAILURE);
        }

        fprintf(stderr, "PID: %d, UID: %d, GID: %d\n", cr.pid, cr.uid, cr.gid);

        char buffer[4096] = {0};
        ssize_t data_len = recv(client_sock, buffer, 4096 - 1, 0);
        if(!data_len) {
            perror("recv");
            exit(EXIT_FAILURE);
        }

        buffer[data_len] = '\0';
        fprintf(stderr, "Data read: %s\n", buffer);
        close(client_sock);

    }
    exit(EXIT_SUCCESS);
}