Introduce the daemonfd function.

The daemonfd function is equivalent to the daemon(3) function expect that
arguments are descriptors. For example dhclient(8) which is sandboxed is
unable to open /dev/null to close stdio instead it's allows to fail
daemon(3) function to close the descriptors and then do it explicit in code.
Instead of such hacks we can use now daemonfd.

This API can be also helpful to migrate system to platforms like CheriBSD.

Reviewed by:	brooks@, bcr@, jilles@ (earlier version)
Differential Revision:	https://reviews.freebsd.org/D13433

include/stdlib.h

@@ -274,6 +274,7 @@ int	 cgetstr(char *, const char *, char **);
 int	 cgetustr(char *, const char *, char **);
 
 int	 daemon(int, int);
+int	 daemonfd(int, int);
 char	*devname(__dev_t, __mode_t);
 char	*devname_r(__dev_t, __mode_t, char *, int);
 char	*fdevname(int);

lib/libc/gen/Symbol.map

@@ -394,6 +394,7 @@ FBSD_1.4 {
 FBSD_1.5 {
 	alphasort;
 	basename;
+	daemonfd;
 	devname;
 	devname_r;
 	dirname;

lib/libc/gen/daemon.3

@@ -28,7 +28,7 @@
 .\"	@(#)daemon.3	8.1 (Berkeley) 6/9/93
 .\" $FreeBSD$
 .\"
-.Dd June 9, 1993
+.Dd December 23, 2017
 .Dt DAEMON 3
 .Os
 .Sh NAME
@@ -40,6 +40,8 @@
 .In stdlib.h
 .Ft int
 .Fn daemon "int nochdir" "int noclose"
+.Ft int
+.Fn daemonfd "int chdirfd" "int nullfd"
 .Sh DESCRIPTION
 The
 .Fn daemon
@@ -59,15 +61,39 @@ is non-zero,
 .Fn daemon
 will redirect standard input, standard output, and standard error to
 .Pa /dev/null .
+.Pp
+The
+.Fn daemonfd
+function is equivalent to the
+.Fn daemon
+function except that arguments are the descriptors for the current working
+directory and to the descriptor to
+.Pa /dev/null .
+.Pp
+If
+.Fa chdirfd
+is equal to
+.Pq -1
+the current working directory is not changed.
+.Pp
+If
+.Fa nullfd
+is equals to
+.Pq -1
+the redirection of standard input, standard output, and standard error is not
+closed.
 .Sh RETURN VALUES
-.Rv -std daemon
+.Rv -std daemon daemonfd
 .Sh ERRORS
 The
 .Fn daemon
+and
+.Fn daemonfd
 function may fail and set
 .Va errno
 for any of the errors specified for the library functions
 .Xr fork 2
+.Xr open 2,
 and
 .Xr setsid 2 .
 .Sh SEE ALSO
@@ -79,6 +105,10 @@ The
 .Fn daemon
 function first appeared in
 .Bx 4.4 .
+The
+.Fn daemonfd
+function first appeared in
+.Fx 12.0 .
 .Sh CAVEATS
 Unless the
 .Fa noclose

lib/libc/gen/daemon.c

@@ -1,8 +1,9 @@
 /*-
  * SPDX-License-Identifier: BSD-3-Clause
  *
- * Copyright (c) 1990, 1993
- *	The Regents of the University of California.  All rights reserved.
+ * Copyright (c) 1990, 1993 The Regents of the University of California.
+ * Copyright (c) 2017 Mariusz Zaborski <oshogbo@FreeBSD.org>
+ * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -46,10 +47,9 @@ __FBSDID("$FreeBSD$");
 #include "libc_private.h"
 
 int
-daemon(int nochdir, int noclose)
+daemonfd(int chdirfd, int nullfd)
 {
 	struct sigaction osa, sa;
-	int fd;
 	pid_t newgrp;
 	int oerrno;
 	int osa_ok;
@@ -83,15 +83,39 @@ daemon(int nochdir, int noclose)
 		return (-1);
 	}
 
-	if (!nochdir)
-		(void)chdir("/");
+	if (chdirfd != -1)
+		(void)fchdir(chdirfd);
 
-	if (!noclose && (fd = _open(_PATH_DEVNULL, O_RDWR, 0)) != -1) {
-		(void)_dup2(fd, STDIN_FILENO);
-		(void)_dup2(fd, STDOUT_FILENO);
-		(void)_dup2(fd, STDERR_FILENO);
-		if (fd > 2)
-			(void)_close(fd);
+	if (nullfd != -1) {
+		(void)_dup2(nullfd, STDIN_FILENO);
+		(void)_dup2(nullfd, STDOUT_FILENO);
+		(void)_dup2(nullfd, STDERR_FILENO);
 	}
 	return (0);
 }
+
+int
+daemon(int nochdir, int noclose)
+{
+	int chdirfd, nullfd, ret;
+
+	if (!noclose)
+		nullfd = _open(_PATH_DEVNULL, O_RDWR, 0);
+	else
+		nullfd = -1;
+
+	if (!nochdir)
+		chdirfd = _open("/", O_EXEC);
+	else
+		chdirfd = -1;
+
+	ret = daemonfd(chdirfd, nullfd);
+
+	if (chdirfd != -1)
+		_close(chdirfd);
+
+	if (nullfd > 2)
+		_close(nullfd);
+
+	return (ret);
+}