openat: allow return of fd 0

Partially reverts patch fc33350 from 2009-09-02.

* modules/chdir-long (Depends-on): Relax openat-safer to openat.
* modules/save-cwd (Depends-on): Replace fcntl-safer with
unistd-safer.
* lib/chdir-long.c (includes): Replace "fcntl--.h" with
<fcntl.h>; this module does not leak fds.
* lib/openat.c (includes): Do not use "fcntl_safer"; plain openat
must be allowed to return 0, leaving openat_safer to add the
safety.
(openat_permissive): Avoid writing to just-opened fd 2 if
restoring the current directory fails.
* lib/openat-die.c (openat_restore_fail): Add comment.
* lib/save-cwd.c (includes): Make "fcntl--.h" conditional.
(save_cwd): Guarantee safe fd, but without use of open_safer.
* tests/test-openat.c: New test.
* modules/openat-tests (Files, Makefile.am): Distribute and build
new file.

Signed-off-by: Eric Blake <ebb9@byu.net>

9 files changed, 116 insertions, 20 deletions

diff --git a/ChangeLog b/ChangeLog
index a514b34144..3e8e24b6e3 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,23 @@
 2009-09-19  Eric Blake  <ebb9@byu.net>
 
+	openat: allow return of fd 0
+	* modules/chdir-long (Depends-on): Relax openat-safer to openat.
+	* modules/save-cwd (Depends-on): Replace fcntl-safer with
+	unistd-safer.
+	* lib/chdir-long.c (includes): Replace "fcntl--.h" with
+	<fcntl.h>; this module does not leak fds.
+	* lib/openat.c (includes): Do not use "fcntl_safer"; plain openat
+	must be allowed to return 0, leaving openat_safer to add the
+	safety.
+	(openat_permissive): Avoid writing to just-opened fd 2 if
+	restoring the current directory fails.
+	* lib/openat-die.c (openat_restore_fail): Add comment.
+	* lib/save-cwd.c (includes): Make "fcntl--.h" conditional.
+	(save_cwd): Guarantee safe fd, but without use of open_safer.
+	* tests/test-openat.c: New test.
+	* modules/openat-tests (Files, Makefile.am): Distribute and build
+	new file.
+
 	relocatable-prog-wrapper: fix build
 	* modules/relocatable-prog-wrapper (Files): Update name of
 	canonicalize m4 file, broken on 2009-09-17.
diff --git a/lib/chdir-long.c b/lib/chdir-long.c
index ba47d59973..afe018d168 100644
--- a/lib/chdir-long.c
+++ b/lib/chdir-long.c
@@ -20,19 +20,23 @@
 
 #include "chdir-long.h"
 
+#include <assert.h>
+#include <errno.h>
+#include <fcntl.h>
 #include <stdlib.h>
 #include <stdbool.h>
 #include <string.h>
-#include <errno.h>
 #include <stdio.h>
-#include <assert.h>
-
-#include "fcntl--.h"
 
 #ifndef PATH_MAX
 # error "compile this file only if your system defines PATH_MAX"
 #endif
 
+/* The results of openat() in this file are not leaked to any
+   single-threaded code that could use stdio.
+   FIXME - if the kernel ever adds support for multi-thread safety for
+   avoiding standard fds, then we should use openat_safer.  */
+
 struct cd_buf
 {
   int fd;
diff --git a/lib/openat-die.c b/lib/openat-die.c
index 677f3e02cb..a507eac10e 100644
--- a/lib/openat-die.c
+++ b/lib/openat-die.c
@@ -40,6 +40,11 @@ openat_save_fail (int errnum)
   abort ();
 }
 
+
+/* Exit with an error about failure to restore the working directory
+   during an openat emulation.  The caller must ensure that fd 2 is
+   not a just-opened fd, even when openat_safer is not in use.  */
+
 void
 openat_restore_fail (int errnum)
 {
diff --git a/lib/openat.c b/lib/openat.c
index e1471b8c0f..d22d830b10 100644
--- a/lib/openat.c
+++ b/lib/openat.c
@@ -28,13 +28,6 @@
 #include "openat-priv.h"
 #include "save-cwd.h"
 
-/* We can't use "fcntl--.h", so that openat_safer does not interfere.  */
-#if GNULIB_FCNTL_SAFER
-# include "fcntl-safer.h"
-# undef open
-# define open open_safer
-#endif
-
 /* Replacement for Solaris' openat function.
    <http://www.google.com/search?q=openat+site:docs.sun.com>
    First, try to simulate it via open ("/proc/self/fd/FD/FILE").
@@ -124,7 +117,13 @@ openat_permissive (int fd, char const *file, int flags, mode_t mode,
       if (save_ok && restore_cwd (&saved_cwd) != 0)
 	{
 	  if (! cwd_errno)
-	    openat_restore_fail (errno);
+	    {
+	      /* Don't write a message to just-created fd 2.  */
+	      saved_errno = errno;
+	      if (err == STDERR_FILENO)
+		close (err);
+	      openat_restore_fail (saved_errno);
+	    }
 	  *cwd_errno = errno;
 	}
     }
diff --git a/lib/save-cwd.c b/lib/save-cwd.c
index e158e8b6b2..7c2eb6d94f 100644
--- a/lib/save-cwd.c
+++ b/lib/save-cwd.c
@@ -1,6 +1,6 @@
 /* save-cwd.c -- Save and restore current working directory.
 
-   Copyright (C) 1995, 1997, 1998, 2003, 2004, 2005, 2006 Free
+   Copyright (C) 1995, 1997, 1998, 2003, 2004, 2005, 2006, 2009 Free
    Software Foundation, Inc.
 
    This program is free software: you can redistribute it and/or modify
@@ -23,15 +23,21 @@
 #include "save-cwd.h"
 
 #include <errno.h>
+#include <fcntl.h>
 #include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
-#include <unistd.h>
 
 #include "chdir-long.h"
-#include "fcntl--.h"
+#include "unistd--.h"
 #include "xgetcwd.h"
 
+#if GNULIB_FCNTL_SAFER
+# include "fcntl--.h"
+#else
+# define GNULIB_FCNTL_SAFER 0
+#endif
+
 /* On systems without the fchdir function (WOE), pretend that open
    always returns -1 so that save_cwd resorts to using xgetcwd.
    Since chdir_long requires fchdir, use chdir instead.  */
@@ -70,6 +76,8 @@ save_cwd (struct saved_cwd *cwd)
   cwd->name = NULL;
 
   cwd->desc = open (".", O_RDONLY);
+  if (!GNULIB_FCNTL_SAFER)
+    cwd->desc = fd_safer (cwd->desc);
   if (cwd->desc < 0)
     {
       cwd->name = xgetcwd ();
diff --git a/modules/chdir-long b/modules/chdir-long
index cdcb9eb70e..4025b45afc 100644
--- a/modules/chdir-long
+++ b/modules/chdir-long
@@ -10,7 +10,7 @@ Depends-on:
 atexit
 fchdir
 fcntl-h
-openat-safer
+openat
 memchr
 mempcpy
 memrchr
diff --git a/modules/openat-tests b/modules/openat-tests
index a82a4f33e8..54d0c61d7b 100644
--- a/modules/openat-tests
+++ b/modules/openat-tests
@@ -1,5 +1,6 @@
 Files:
 tests/test-rmdir.h
+tests/test-openat.c
 tests/test-unlinkat.c
 
 Depends-on:
@@ -8,6 +9,7 @@ configure.ac:
 AC_CHECK_FUNCS_ONCE([symlink])
 
 Makefile.am:
-TESTS += test-unlinkat
-check_PROGRAMS += test-unlinkat
+TESTS += test-openat test-unlinkat
+check_PROGRAMS += test-openat test-unlinkat
+test_openat_LDADD = $(LDADD) @LIBINTL@
 test_unlinkat_LDADD = $(LDADD) @LIBINTL@
diff --git a/modules/save-cwd b/modules/save-cwd
index 18c43b9ef7..46a1276abd 100644
--- a/modules/save-cwd
+++ b/modules/save-cwd
@@ -8,9 +8,9 @@ m4/save-cwd.m4
 
 Depends-on:
 chdir-long
-fcntl-safer
-xgetcwd
 stdbool
+unistd-safer
+xgetcwd
 
 configure.ac:
 gl_SAVE_CWD
diff --git a/tests/test-openat.c b/tests/test-openat.c
new file mode 100644
index 0000000000..8fa8f83c42
--- /dev/null
+++ b/tests/test-openat.c
@@ -0,0 +1,60 @@
+/* Test that openat works.
+   Copyright (C) 2009 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program 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 General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* Written by Eric Blake <ebb9@byu.net>, 2009.  */
+
+#include <config.h>
+
+#include <fcntl.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+#define ASSERT(expr) \
+  do                                                                         \
+    {                                                                        \
+      if (!(expr))                                                           \
+	{                                                                    \
+	  fprintf (stderr, "%s:%d: assertion failed\n", __FILE__, __LINE__); \
+	  fflush (stderr);                                                   \
+	  abort ();                                                          \
+	}                                                                    \
+    }                                                                        \
+  while (0)
+
+int
+main ()
+{
+  /* FIXME - add more tests.  For example, share /dev/null and
+     trailing slash tests with test-open, and do more checks for
+     proper fd handling.  */
+
+  /* Check that even when *-safer modules are in use, plain openat can
+     land in fd 0.  Do this test last, since it is destructive to
+     stdin.  */
+  ASSERT (close (STDIN_FILENO) == 0);
+  ASSERT (openat (AT_FDCWD, ".", O_RDONLY) == STDIN_FILENO);
+  {
+    int dfd = open (".", O_RDONLY);
+    ASSERT (STDIN_FILENO < dfd);
+    ASSERT (chdir ("..") == 0);
+    ASSERT (close (STDIN_FILENO) == 0);
+    ASSERT (openat (dfd, ".", O_RDONLY) == STDIN_FILENO);
+    ASSERT (close (dfd) == 0);
+  }
+  return 0;
+}