See - IO::FDPass

https://keithp.com/cgit/fdpassing.git/tree/README.md

## FD passing for DRI.Next

Using the DMA-BUF interfaces to pass DRI objects between the client
and server, as discussed in my previous blog posting on [[DRI-Next]],
requires that we successfully pass file descriptors over the X
protocol socket.

Rumor has it that this has been tried and found to be difficult, and
so I decided to do a bit of experimentation to see how this could be
made to work within the existing X implementation.

(All of the examples shown here are licensed under the GPL, version 2
and are available from git://keithp.com/git/fdpassing)

### Basics of FD passing

The kernel internals that support FD passing are actually quite simple
-- POSIX already require that two processes be able to share the same
underlying reference to a file because of the semantics of the fork(2)
call. Adding some ability to share arbitrary file descriptors between
two processes then is far more about how you ask the kernel than the
actual file descriptor sharing operation.

In Linux, file descriptors can be passed through local network
sockets. The sender constructs a mystic-looking sendmsg(2) call,
placing the file descriptor in the control field of that
operation. The kernel pulls the file descriptor out of the control
field, allocates a file descriptor in the target process which
references the same file object and then sticks the file descriptor in
a queue for the receiving process to fetch.

The receiver then constructs a matching call to recvmsg that provides
a place for the kernel to stick the new file descriptor.

### A helper API for testing

I first write a stand-alone program that created a socketpair, forked
and then passed an fd from the parent to the child. Once that was
working, I decided that some short helper functions would make further
testing a whole lot easier.

Here's a function that writes some data and an optional file
descriptor:

	ssize_t
	sock_fd_write(int sock, void *buf, ssize_t buflen, int fd)
	{
		ssize_t		size;
		struct msghdr	msg;
		struct iovec	iov;
		union {
			struct cmsghdr	cmsghdr;
			char		control[CMSG_SPACE(sizeof (int))];
		} cmsgu;
		struct cmsghdr	*cmsg;

		iov.iov_base = buf;
		iov.iov_len = buflen;

		msg.msg_name = NULL;
		msg.msg_namelen = 0;
		msg.msg_iov = &iov;
		msg.msg_iovlen = 1;

		if (fd != -1) {
			msg.msg_control = cmsgu.control;
			msg.msg_controllen = sizeof(cmsgu.control);

			cmsg = CMSG_FIRSTHDR(&msg);
			cmsg->cmsg_len = CMSG_LEN(sizeof (int));
			cmsg->cmsg_level = SOL_SOCKET;
			cmsg->cmsg_type = SCM_RIGHTS;

			printf ("passing fd %d\n", fd);
			*((int *) CMSG_DATA(cmsg)) = fd;
		} else {
			msg.msg_control = NULL;
			msg.msg_controllen = 0;
			printf ("not passing fd\n");
		}

		size = sendmsg(sock, &msg, 0);

		if (size < 0)
			perror ("sendmsg");
		return size;
	}

And here's the matching receiver function:

	ssize_t
	sock_fd_read(int sock, void *buf, ssize_t bufsize, int *fd)
	{
		ssize_t		size;

		if (fd) {
			struct msghdr	msg;
			struct iovec	iov;
			union {
				struct cmsghdr	cmsghdr;
				char		control[CMSG_SPACE(sizeof (int))];
			} cmsgu;
			struct cmsghdr	*cmsg;

			iov.iov_base = buf;
			iov.iov_len = bufsize;

			msg.msg_name = NULL;
			msg.msg_namelen = 0;
			msg.msg_iov = &iov;
			msg.msg_iovlen = 1;
			msg.msg_control = cmsgu.control;
			msg.msg_controllen = sizeof(cmsgu.control);
			size = recvmsg (sock, &msg, 0);
			if (size < 0) {
				perror ("recvmsg");
				exit(1);
			}
			cmsg = CMSG_FIRSTHDR(&msg);
			if (cmsg && cmsg->cmsg_len == CMSG_LEN(sizeof(int))) {
				if (cmsg->cmsg_level != SOL_SOCKET) {
					fprintf (stderr, "invalid cmsg_level %d\n",
						 cmsg->cmsg_level);
					exit(1);
				}
				if (cmsg->cmsg_type != SCM_RIGHTS) {
					fprintf (stderr, "invalid cmsg_type %d\n",
						 cmsg->cmsg_type);
					exit(1);
				}

				*fd = *((int *) CMSG_DATA(cmsg));
				printf ("received fd %d\n", *fd);
			} else
				*fd = -1;
		} else {
			size = read (sock, buf, bufsize);
			if (size < 0) {
				perror("read");
				exit(1);
			}
		}
		return size;
	}

With these two functions, I rewrote the simple example as follows:

	void
	child(int sock)
	{
		int	fd;
		char	buf[16];
		ssize_t	size;

		sleep(1);
		for (;;) {
			size = sock_fd_read(sock, buf, sizeof(buf), &fd);
			if (size <= 0)
				break;
			printf ("read %d\n", size);
			if (fd != -1) {
				write(fd, "hello, world\n", 13);
				close(fd);
			}
		}
	}

	void
	parent(int sock)
	{
		ssize_t	size;
		int	i;
		int	fd;

		fd = 1;
		size = sock_fd_write(sock, "1", 1, 1);
		printf ("wrote %d\n", size);
	}

	int
	main(int argc, char **argv)
	{
		int	sv[2];
		int	pid;

		if (socketpair(AF_LOCAL, SOCK_STREAM, 0, sv) < 0) {
			perror("socketpair");
			exit(1);
		}
		switch ((pid = fork())) {
		case 0:
			close(sv[0]);
			child(sv[1]);
			break;
		case -1:
			perror("fork");
			exit(1);
		default:
			close(sv[1]);
			parent(sv[0]);
			break;
		}
		return 0;
	}

### Experimenting with multiple writes

I wanted to know what would happen if multiple writes were made, some
with file descriptors and some without. So I changed the simple example
parent function to look like:

	void
	parent(int sock)
	{
		ssize_t	size;
		int	i;
		int	fd;

		fd = 1;
		size = sock_fd_write(sock, "1", 1, -1);
		printf ("wrote %d without fd\n", size);
		size = sock_fd_write(sock, "1", 1, 1);
		printf ("wrote %d with fd\n", size);
		size = sock_fd_write(sock, "1", 1, -1);
		printf ("wrote %d without fd\n", size);
	}

When run, this demonstrates that the reader gets two bytes in the
first read along with a file descriptor followed by one byte in a
second read, without a file descriptor. This demonstrates that
a file descriptor message forms a barrier within the socket; multiple
messages will be merged together, but not past a message containing a
file descriptor.

### Reading without accepting a file descriptor

What happens when the reader isn't expecting a file descriptor? Does
it just get lost? Does the reader not get the message until it asks
for the file descriptor? What about the boundary issue described above?

Here's my test case:

	void
	child(int sock)
	{
		int	fd;
		char	buf[16];
		ssize_t	size;

		sleep(1);
		size = sock_fd_read(sock, buf, sizeof(buf), NULL);
		if (size <= 0)
			return;
		printf ("read %d\n", size);
		size = sock_fd_read(sock, buf, sizeof(buf), &fd);
		if (size <= 0)
			return;
		printf ("read %d\n", size);
		if (fd != -1) {
			write(fd, "hello, world\n", 13);
			close(fd);
		}
	}

	void
	parent(int sock)
	{
		ssize_t	size;
		int	i;
		int	fd;

		fd = 1;
		size = sock_fd_write(sock, "1", 1, 1);
		printf ("wrote %d without fd\n", size);
		size = sock_fd_write(sock, "1", 1, 2);
		printf ("wrote %d with fd\n", size);
	}

This shows that the first passed file descriptor is picked up by the
first sock_fd_read call, but the file descriptor is closed. The second
file descriptor passed is picked up by the second sock_fd_read call.

### Zero-length writes

Can a file descriptor be passed without sending any data?

	void
	parent(int sock)
	{
		ssize_t	size;
		int	i;
		int	fd;

		fd = 1;
		size = sock_fd_write(sock, "1", 1, -1);
		printf ("wrote %d without fd\n", size);
		size = sock_fd_write(sock, NULL, 0, 1);
		printf ("wrote %d with fd\n", size);
		size = sock_fd_write(sock, "1", 1, -1);
		printf ("wrote %d without fd\n", size);
	}

And the answer is clearly "no" -- the file descriptor is not passed
when no data are included in the write.

(update, 2019-5-8 from Yicholas Rishel)

This is true for SOCK_STREAM sockets, but for SOCK_SEQPACKET sockets,
you *can* do zero-length writes and pass an fd.

### A summary of results


 1. read and recvmsg don't merge data across a file descriptor message boundary.

 2. failing to accept an fd in the receiver results in the fd being
    closed by the kernel.

 3. a file descriptor must be accompanied by some data.

