g_check_setuid: implement using getauxval(AT_SECURE) with glibc

See commit 4c2928a544 for why checking AT_SECURE is preferable compared
to UID checks as currently done in the fallback case.

getauxval() was added with glibc 2.16

While glibc <2.19 didn't provide a way to differentiate a 0 return value from an error,
passing AT_SECURE should always succeed according to
https://sourceware.org/ml/libc-alpha/2014-07/msg00407.html
I've added an errno check anyway, to be on the safe side.

config.h.meson

@@ -525,6 +525,9 @@
    */
 #mesondefine HAVE_SYS_DIR_H
 
+/* Define to 1 if you have the <sys/auxv.h> header file. */
+#mesondefine HAVE_SYS_AUXV_H
+
 /* Define to 1 if you have the <sys/event.h> header file. */
 #mesondefine HAVE_SYS_EVENT_H
 

configure.ac

@@ -674,7 +674,7 @@ fi
 # check for header files
 AC_CHECK_HEADERS([sys/param.h sys/resource.h mach/mach_time.h])
 AC_CHECK_HEADERS([sys/select.h stdint.h inttypes.h sched.h malloc.h])
-AC_CHECK_HEADERS([sys/vfs.h sys/vmount.h sys/statfs.h sys/statvfs.h sys/filio.h])
+AC_CHECK_HEADERS([sys/vfs.h sys/vmount.h sys/statfs.h sys/statvfs.h sys/filio.h sys/auxv.h])
 AC_CHECK_HEADERS([mntent.h sys/mnttab.h sys/vfstab.h sys/mntctl.h fstab.h])
 AC_CHECK_HEADERS([linux/magic.h])
 AC_CHECK_HEADERS([termios.h])

glib/gutils.c

@@ -50,6 +50,9 @@
 #ifdef HAVE_CRT_EXTERNS_H 
 #include <crt_externs.h> /* for _NSGetEnviron */
 #endif
+#ifdef HAVE_SYS_AUXV_H
+#include <sys/auxv.h>
+#endif
 
 #include "glib-init.h"
 #include "glib-private.h"
@@ -2500,9 +2503,17 @@ const gchar *g_get_tmp_dir_utf8 (void) { return g_get_tmp_dir (); }
 gboolean
 g_check_setuid (void)
 {
-/* TODO: use getauxval(AT_SECURE) if available */
+#if defined(HAVE_SYS_AUXV_H)
+  unsigned long value;
+  int errsv;
 
-#if defined(HAVE_ISSETUGID) && !defined(__BIONIC__)
+  errno = 0;
+  value = getauxval (AT_SECURE);
+  errsv = errno;
+  if (errsv)
+    g_error ("getauxval () failed: %s", g_strerror (errsv));
+  return value;
+#elif defined(HAVE_ISSETUGID) && !defined(__BIONIC__)
   /* BSD: http://www.freebsd.org/cgi/man.cgi?query=issetugid&sektion=2 */
 
   /* Android had it in older versions but the new 64 bit ABI does not

meson.build

@@ -229,6 +229,7 @@ headers = [
   'stdlib.h',
   'string.h',
   'strings.h',
+  'sys/auxv.h',
   'sys/event.h',
   'sys/filio.h',
   'sys/inotify.h',