Improve file descriptor checks for posix_spawn actions [BZ #19505]

posix/spawn_valid_fd.c

@@ -0,0 +1,31 @@
+/* File descriptor validity check for posix_spawn file actions.
+   Copyright (C) 2000-2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include "spawn_int.h"
+
+#include <unistd.h>
+
+bool
+internal_function
+__spawn_valid_fd (int fd)
+{
+  long maxfd = __sysconf (_SC_OPEN_MAX);
+  return __glibc_likely (fd >= 0)
+    && (__glibc_unlikely (maxfd < 0) /* No limit set.  */
+	|| __glibc_likely (fd < maxfd));
+}