Provide cap_sandboxed(3) function, which is a wrapper around cap_getmode(2)

system call, which has a nice property - it never fails, so it is a bit
easier to use. If there is no support for capability mode in the kernel
the function will return false (not in a sandbox). If the kernel is compiled
with the support for capability mode, the function will return true or false
depending if the calling process is in the capability mode sandbox or not
respectively.

Sponsored by:	The FreeBSD Foundation

lib/libc/gen/Makefile.inc

@@ -16,6 +16,7 @@ SRCS+=	__getosreldate.c \
 	assert.c \
 	auxv.c \
 	basename.c \
+	cap_sandboxed.c \
 	check_utility_compat.c \
 	clock.c \
 	clock_getcpuclockid.c \
@@ -168,6 +169,7 @@ SYM_MAPS+=${.CURDIR}/gen/Symbol.map
 MAN+=	alarm.3 \
 	arc4random.3 \
 	basename.3 \
+	cap_sandboxed.3 \
 	check_utility_compat.3 \
 	clock.3 \
 	clock_getcpuclockid.3 \

lib/libc/gen/cap_sandboxed.3

@@ -0,0 +1,70 @@
+.\" Copyright (c) 2012 The FreeBSD Foundation
+.\" All rights reserved.
+.\"
+.\" This documentation was written by Pawel Jakub Dawidek under sponsorship
+.\" from the FreeBSD Foundation.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. 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 AUTHOR 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 AUTHOR 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.
+.\"
+.\" $FreeBSD$
+.\"
+.Dd September 18, 2012
+.Dt CAP_SANDBOXED 3
+.Os
+.Sh NAME
+.Nm cap_sandboxed
+.Nd Check if in a capability mode sandbox
+.Sh LIBRARY
+.Lb libc
+.Sh SYNOPSIS
+.In sys/capability.h
+.In stdbool.h
+.Ft bool
+.Fn cap_sandboxed "void"
+.Sh DESCRIPTION
+.Fn cap_sandboxed
+returns
+.Va true
+if the process is in a capability mode sandbox or
+.Va false
+if it is not.
+This function is a more handy alternative to the
+.Xr cap_getmode 2
+system call as it always succeeds, so there is no need for error checking.
+If the support for capability mode is not compiled into the kernel,
+.Fn cap_sandboxed
+will always return
+.Va false .
+.Sh RETURN VALUES
+Function
+.Fn cap_sandboxed
+is always successful and will return either
+.Va true
+or
+.Va false .
+.Sh SEE ALSO
+.Xr cap_enter 2 ,
+.Xr capsicum 4
+.Sh AUTHORS
+This function was implemented and manual page was written by
+.An Pawel Jakub Dawidek Aq pawel@dawidek.net
+under sponsorship of the FreeBSD Foundation.

lib/libc/gen/cap_sandboxed.c

@@ -0,0 +1,50 @@
+/*-
+ * Copyright (c) 2012 The FreeBSD Foundation
+ * All rights reserved.
+ *
+ * This software was developed by Pawel Jakub Dawidek under sponsorship from
+ * the FreeBSD Foundation.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. 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 AUTHORS 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 AUTHORS 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 <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
+#include <sys/capability.h>
+
+#include <assert.h>
+#include <errno.h>
+#include <stdbool.h>
+
+bool
+cap_sandboxed(void)
+{
+	u_int mode;
+
+	if (cap_getmode(&mode) != 0) {
+		assert(errno == ENOSYS);
+		return (false);
+	}
+	assert(mode == 0 || mode == 1);
+	return (mode == 1);
+}

lib/libc/sys/Symbol.map

@@ -379,6 +379,7 @@ FBSD_1.2 {
 };
 
 FBSD_1.3 {
+	cap_sandboxed;
 	clock_getcpuclockid2;
 	ffclock_getcounter;
 	ffclock_getestimate;

lib/libc/sys/cap_enter.2

@@ -90,6 +90,7 @@ acquired rights as possible.
 .Sh SEE ALSO
 .Xr cap_new 2 ,
 .Xr fexecve 2 ,
+.Xr cap_sandboxed 3 ,
 .Xr capsicum 4
 .Sh HISTORY
 Support for capabilities and capabilities mode was developed as part of the