Update.

* sysdeps/unix/sysv/linux/check_fds.c: New file.
	* sysdeps/generic/check_fds.c: Check that file opened is really
	/dev/null.

ChangeLog

@@ -1,5 +1,9 @@
 2000-09-25  Ulrich Drepper  <drepper@redhat.com>
 
+	* sysdeps/unix/sysv/linux/check_fds.c: New file.
+	* sysdeps/generic/check_fds.c: Check that file opened is really
+	/dev/null.
+
 	* elf/rtld.c (process_envvars): Open debug output file with O_NOFOLLOW.
 
 	* locale/Makefile (routines): Add nl_langinfo_l.

sysdeps/generic/check_fds.c

@@ -20,6 +20,8 @@
 #include <fcntl.h>
 #include <paths.h>
 #include <unistd.h>
+#include <sys/stat.h>
+#include <sys/sysmacros.h>
 
 /* Try to get a machine dependent instruction which will make the
    program crash.  This is used in case everything else fails.  */
@@ -38,11 +40,22 @@ check_one_fd (int fd, int mode)
   if (__builtin_expect (__libc_fcntl (fd, F_GETFD), 0) == -1
       && errno == EBADF)
     {
+      struct stat64 st;
+
       /* Something is wrong with this descriptor, it's probably not
 	 opened.  Open /dev/null so that the SUID program we are
 	 about to start does not accidently use this descriptor.  */
       int nullfd = __libc_open (_PATH_DEVNULL, mode);
-      if (__builtin_expect (nullfd, 0) == -1)
+      /* We are very paranoid here.  With all means we try to ensure
+	 that we are actually opening the /dev/null device and nothing
+	 else.  */
+      if (__builtin_expect (nullfd, 0) == -1
+	  || __builtin_expect (__fxstat64 (_STAT_VER, nullfd, &st), 0) != 0
+	  || __builtin_expect (S_ISCHR (st.st_mode), 1) == 0
+#if defined DEV_NULL_MAJOR && defined DEV_NULL_MINOR
+	  || st.st_rdev != makedev (DEV_NULL_MAJOR, DEV_NULL_MINOR)
+#endif
+	  )
 	/* We cannot even give an error message here since it would
 	   run into the same problems.  */
 	while (1)
@@ -55,8 +68,15 @@ check_one_fd (int fd, int mode)
 void
 __libc_check_standard_fds (void)
 {
-/* Check all three standard file descriptors.  */
+  /* This is really paranoid but some people actually are.  If /dev/null
-  check_one_fd (STDIN_FILENO, O_RDONLY);
+     should happen to be a symlink to somewhere else and not the device
-  check_one_fd (STDOUT_FILENO, O_RDWR);
+     commonly known as "/dev/null" be bail out.  We can detect this with
-  check_one_fd (STDERR_FILENO, O_RDWR);
+     the O_NOFOLLOW flag for open() but only on some system.  */
+#ifndef O_NOFOLLOW
+# define O_NOFOLLOW	0
+#endif
+  /* Check all three standard file descriptors.  */
+  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);
 }

sysdeps/unix/sysv/linux/check_fds.c

@@ -0,0 +1,22 @@
+/* Copyright (C) 2000 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 Library General Public License as
+   published by the Free Software Foundation; either version 2 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
+   Library General Public License for more details.
+
+   You should have received a copy of the GNU Library General Public
+   License along with the GNU C Library; see the file COPYING.LIB.  If not,
+   write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+   Boston, MA 02111-1307, USA.  */
+
+#define DEV_NULL_MAJOR	1
+#define DEV_NULL_MINOR	3
+
+#include <sysdeps/generic/check_fds.c>