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During fd passing, receive_fd() tries to read the result value and
the file descriptor.  If the fd limit is exhausted, recvmsg(2)
fails.  The kernel discards the fd, but the result value stays in
the socket.  It has to be read on its own to keep the privsep parent
and syslogd child in sync.
OK benno@

/*	$OpenBSD: privsep_fdpass.c,v 1.11 2015/07/09 20:16:04 bluhm Exp $	*/

/*
 * Copyright 2001 Niels Provos <provos@citi.umich.edu>
 * All rights reserved.
 *
 * Copyright (c) 2002 Matthieu Herrb
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 *    - Redistributions of source code must retain the above copyright
 *      notice, this list of conditions and the following disclaimer.
 *    - Redistributions in binary form must reproduce the above
 *      copyright notice, this list of conditions and the following
 *      disclaimer in the documentation and/or other materials provided
 *      with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
 * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
 * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
 * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

#include <err.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#include "syslogd.h"

void
send_fd(int sock, int fd)
{
	struct msghdr msg;
	union {
		struct cmsghdr hdr;
		char buf[CMSG_SPACE(sizeof(int))];
	} cmsgbuf;
	struct cmsghdr *cmsg;
	struct iovec vec;
	int result = 0;
	ssize_t n;

	memset(&msg, 0, sizeof(msg));

	if (fd >= 0) {
		msg.msg_control = (caddr_t)&cmsgbuf.buf;
		msg.msg_controllen = sizeof(cmsgbuf.buf);
		cmsg = CMSG_FIRSTHDR(&msg);
		cmsg->cmsg_len = CMSG_LEN(sizeof(int));
		cmsg->cmsg_level = SOL_SOCKET;
		cmsg->cmsg_type = SCM_RIGHTS;
		*(int *)CMSG_DATA(cmsg) = fd;
	} else {
		result = errno;
	}

	vec.iov_base = &result;
	vec.iov_len = sizeof(int);
	msg.msg_iov = &vec;
	msg.msg_iovlen = 1;

	if ((n = sendmsg(sock, &msg, 0)) == -1)
		warn("%s: sendmsg(%d)", "send_fd", sock);
	if (n != sizeof(int))
		warnx("%s: sendmsg: expected sent 1 got %ld",
		    "send_fd", (long)n);
}

int
receive_fd(int sock)
{
	struct msghdr msg;
	union {
		struct cmsghdr hdr;
		char buf[CMSG_SPACE(sizeof(int))];
	} cmsgbuf;
	struct cmsghdr *cmsg;
	struct iovec vec;
	ssize_t n;
	int result;
	int fd;

	memset(&msg, 0, sizeof(msg));
	vec.iov_base = &result;
	vec.iov_len = sizeof(int);
	msg.msg_iov = &vec;
	msg.msg_iovlen = 1;
	msg.msg_control = &cmsgbuf.buf;
	msg.msg_controllen = sizeof(cmsgbuf.buf);

	if ((n = recvmsg(sock, &msg, 0)) == -1) {
		warn("%s: recvmsg", "receive_fd");
		/* receive message failed, but the result is in the socket */
		if (errno == EMSGSIZE)
			recv(sock, &result, sizeof(int), MSG_DONTWAIT);
		return -1;
	}
	if (n != sizeof(int))
		warnx("%s: recvmsg: expected received 1 got %ld",
		    "receive_fd", (long)n);
	if (result == 0) {
		cmsg = CMSG_FIRSTHDR(&msg);
		if (cmsg == NULL) {
			warnx("%s: no message header", "receive_fd");
			return (-1);
		}
		if (cmsg->cmsg_type != SCM_RIGHTS)
			warnx("%s: expected type %d got %d", "receive_fd",
			    SCM_RIGHTS, cmsg->cmsg_type);
		fd = (*(int *)CMSG_DATA(cmsg));
		return fd;
	} else {
		errno = result;
		return -1;
	}
}