libc: add issetugid()

issetugid() returns 1 if the process environment or memory address space
is considered tainted, and returns 0 otherwise.  This happens, for example,
when a process's privileges are elevated by the setuid or setgid flags on
an executable belonging to root.  This function first appeard in OpenBSD 2.0
and is needed for the LibreSSL.

This patch follows the same logic as the equivalent musl commit.  For more
information see the commit message at

http://git.musl-libc.org/cgit/musl/commit/?id=ddddec106fd17c3aca3287005d21e92f742aa9d4

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>

include/unistd.h

@@ -1168,6 +1168,20 @@ extern long int syscall (long int __sysno, ...) __THROW;
 
 #endif	/* Use misc.  */
 
+/* Are we in a secure process environment or are we dealing with setuid
+ * stuff?  This value is returned by issetugid().
+ */
+extern int _pe_secure;
+libc_hidden_proto(_pe_secure)
+
+#ifdef __USE_BSD
+/* issetugid() returns 1 if the process environment or memory address space
+   is considered tainted, and returns 0 otherwise.  This happens, for example,
+   when a process's privileges are elevated by the setuid or setgid flags on
+   an executable belonging to root.
+*/
+extern int issetugid(void);
+#endif
 
 #if (defined __USE_MISC || defined __USE_XOPEN_EXTENDED) && !defined F_LOCK
 /* NOTE: These declarations also appear in <fcntl.h>; be sure to keep both

libc/misc/file/issetugid.c

@@ -0,0 +1,10 @@
+/* Copyright (C) 2013 Gentoo Foundation
+ * Licensed under LGPL v2.1 or later, see the file COPYING.LIB in this tarball.
+ */
+
+#include <unistd.h>
+
+int issetugid(void)
+{
+	return _pe_secure;
+}

libc/misc/internals/__uClibc_main.c

@@ -40,6 +40,14 @@
 #include <locale.h>
 #endif
 
+/* Are we in a secure process environment or are we dealing
+ * with setuid stuff?  If we are dynamically linked, then we
+ * already have _dl_secure, otherwise we need to re-examine
+ * auxvt[] below.
+ */
+int _pe_secure = 0;
+libc_hidden_data_def(_pe_secure)
+
 #ifndef SHARED
 void *__libc_stack_end = NULL;
 
@@ -391,7 +399,10 @@ void __uClibc_main(int (*main)(int, char **, char **), int argc,
 	__check_one_fd (STDIN_FILENO, O_RDONLY | O_NOFOLLOW);
 	__check_one_fd (STDOUT_FILENO, O_RDWR | O_NOFOLLOW);
 	__check_one_fd (STDERR_FILENO, O_RDWR | O_NOFOLLOW);
+	_pe_secure = 1 ;
     }
+    else
+	_pe_secure = 0 ;
 #endif
 
     __uclibc_progname = *argv;