Is there a way to get the uid of the other end of a unix socket connection

Question

Is there a way for a UNIX domain socket listener to only accept connection from certain user (chmod/chown does not work for abstract socket afaik), or

Answer 1

Yes -- this operation, along with FD passing, is supported via an ancilliary message with the SCM_CREDENTIALS type. The calls involved are documented in man 7 unix.

Answer 2

The easiest way to check peer credentials is with SO_PEERCRED. To do this for socket sock:

int len;
struct ucred ucred;

len = sizeof(struct ucred);
if (getsockopt(sock, SOL_SOCKET, SO_PEERCRED, &ucred, &len) == -1)
    // check errno

printf("Credentials from SO_PEERCRED: pid=%ld, euid=%ld, egid=%ld\n",
        (long) ucred.pid, (long) ucred.uid, (long) ucred.gid);

SO_PEERCRED
          Return the credentials of the foreign process connected to
          this socket.  This is possible only for connected AF_UNIX
          stream sockets and AF_UNIX stream and datagram socket pairs
          created using socketpair(2); see unix(7).  The returned
          credentials are those that were in effect at the time of the
          call to connect(2) or socketpair(2).  The argument is a ucred
          structure; define the _GNU_SOURCE feature test macro to obtain
          the definition of that structure from <sys/socket.h>.  This
          socket option is read-only.

From a tlpi example. PostgreSQL has a few variants for other unices.