[v3] libio: Flush stream at freopen (BZ#21037)

Message ID 20181001194322.4104-1-adhemerval.zanella@linaro.org
State New
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  • [v3] libio: Flush stream at freopen (BZ#21037)
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Commit Message

Adhemerval Zanella Oct. 1, 2018, 7:43 p.m.
This is on my backlog for some time and based on previous review by 
Paul Eggert [1] I will commit it shortly if no one opposes it. Changes from
previous version are just to use better libsupport check macros and to use
an unsigned parameter on fd_to_filename to avoid require a larger value
for FD_TO_FILENAME_SIZE (since _fitoa_word VALUE argument is usually an
unsigned long int).

--

As POSIX states [2] a freopen call should first flush the stream as if by a
call fflush.  C99 (n1256) and C11 (n1570) only states the function should
first close any file associated with the specific stream.  Although current
implementation only follow C specification, current BSD and other libc
implementation (musl) are in sync with POSIX and fflush the stream.

This patch change freopen{64} to fflush the stream before actually reopening
it (or returning if the stream does not support reopen).  It also changes the
Linux implementation to avoid a dynamic allocation on 'fd_to_filename'.

Checked on x86_64-linux-gnu.

	[BZ #21037]
	* libio/Makefile (tests): Add tst-memstream4 and tst-wmemstream4.
	* libio/freopen.c (freopen): Sync stream before reopen and adjust to
	new fd_to_filename interface.
	* libio/freopen64.c (freopen64): Likewise.
	* libio/tst-memstream.h: New file.
	* libio/tst-memstream4.c: Likewise.
	* libio/tst-wmemstream4.c: Likewise.
	* sysdeps/generic/fd_to_filename.h (fd_to_filename): Change signature.
	* sysdeps/unix/sysv/linux/fd_to_filename.h (fd_to_filename): Likewise
	and remove internal dynamic allocation.

[1] https://sourceware.org/ml/libc-alpha/2018-06/msg00424.html
[2] http://pubs.opengroup.org/onlinepubs/9699919799/
---
 ChangeLog                                | 14 +++++
 libio/Makefile                           |  4 +-
 libio/freopen.c                          | 29 +++++-----
 libio/freopen64.c                        | 26 +++++----
 libio/tst-memstream.h                    | 68 ++++++++++++++++++++++++
 libio/tst-memstream4.c                   | 63 ++++++++++++++++++++++
 libio/tst-wmemstream4.c                  | 20 +++++++
 sysdeps/generic/fd_to_filename.h         |  4 +-
 sysdeps/unix/sysv/linux/fd_to_filename.h | 34 +++++-------
 9 files changed, 213 insertions(+), 49 deletions(-)
 create mode 100644 libio/tst-memstream.h
 create mode 100644 libio/tst-memstream4.c
 create mode 100644 libio/tst-wmemstream4.c

-- 
2.17.1

Comments

Florian Weimer Oct. 2, 2018, 10:39 a.m. | #1
* Adhemerval Zanella:

> diff --git a/libio/tst-memstream4.c b/libio/tst-memstream4.c

> new file mode 100644

> index 0000000000..bb2635e837

> --- /dev/null

> +++ b/libio/tst-memstream4.c


> +    //TEST_VERIFY (STRCMP (buf, W("new")) == 0);


This looks like a leftover.

> +    TEST_COMPARE_BLOB (buf, STRLEN (buf), W("new"), STRLEN (W ("new")));


I think you need STRLNE (buf) * sizeof (CHAR_T) here.
Adhemerval Zanella Oct. 2, 2018, 1:03 p.m. | #2
On 02/10/2018 07:39, Florian Weimer wrote:
> * Adhemerval Zanella:

> 

>> diff --git a/libio/tst-memstream4.c b/libio/tst-memstream4.c

>> new file mode 100644

>> index 0000000000..bb2635e837

>> --- /dev/null

>> +++ b/libio/tst-memstream4.c

> 

>> +    //TEST_VERIFY (STRCMP (buf, W("new")) == 0);


Ugh, I will fix it.

> 

> This looks like a leftover.

> 

>> +    TEST_COMPARE_BLOB (buf, STRLEN (buf), W("new"), STRLEN (W ("new")));

> 

> I think you need STRLNE (buf) * sizeof (CHAR_T) here.

> 


Indeed, thanks for check on this.
Andreas Schwab Oct. 2, 2018, 2:46 p.m. | #3
On Okt 01 2018, Adhemerval Zanella <adhemerval.zanella@linaro.org> wrote:

> diff --git a/sysdeps/unix/sysv/linux/fd_to_filename.h b/sysdeps/unix/sysv/linux/fd_to_filename.h

> index 297716b0ae..62bc9916b0 100644

> --- a/sysdeps/unix/sysv/linux/fd_to_filename.h

> +++ b/sysdeps/unix/sysv/linux/fd_to_filename.h

> @@ -16,30 +16,20 @@

>     License along with the GNU C Library; if not, see

>     <http://www.gnu.org/licenses/>.  */

>  

> -#include <stdlib.h>

> -#include <string.h>

>  #include <sys/stat.h>

> -#include <_itoa.h>

> +

> +#define FD_TO_FILENAME_SIZE ((sizeof ("/proc/self/fd/") - 1) \

> +			     + (sizeof ("4294967295") - 1) + 1)

>  

>  static inline const char *

> -fd_to_filename (int fd)

> +fd_to_filename (unsigned int fd, char *buf)

>  {

> -  char *ret = malloc (30);

> -

> -  if (ret != NULL)

> -    {

> -      struct stat64 st;

> -

> -      *_fitoa_word (fd, __stpcpy (ret, "/proc/self/fd/"), 10, 0) = '\0';

> -

> -      /* We must make sure the file exists.  */

> -      if (__lxstat64 (_STAT_VER, ret, &st) < 0)

> -	{

> -	  /* /proc is not mounted or something else happened.  Don't

> -	     return the file name.  */

> -	  free (ret);

> -	  ret = NULL;

> -	}

> -    }

> -  return ret;

> +  *_fitoa_word (fd, __stpcpy (buf, "/proc/self/fd/"), 10, 0) = '\0';

> +

> +  /* We must make sure the file exists.  */

> +  struct stat64 st;

> +  if (__lxstat64 (_STAT_VER, buf, &st) < 0)

> +    /* /proc is not mounted or something else happened.  */

> +    return NULL;

> +  return buf;

>  }


In file included from freopen.c:33:
../sysdeps/unix/sysv/linux/fd_to_filename.h: In function ‘fd_to_filename’:
../sysdeps/unix/sysv/linux/fd_to_filename.h:27:4: error: implicit declaration of function ‘_fitoa_word’ [-Werror=implicit-function-declaration]
   *_fitoa_word (fd, __stpcpy (buf, "/proc/self/fd/"), 10, 0) = '\0';
    ^~~~~~~~~~~
../sysdeps/unix/sysv/linux/fd_to_filename.h:27:21: error: implicit declaration of function ‘__stpcpy’; did you mean ‘__setpgid’? [-Werror=implicit-function-declaration]
   *_fitoa_word (fd, __stpcpy (buf, "/proc/self/fd/"), 10, 0) = '\0';
                     ^~~~~~~~
                     __setpgid
../sysdeps/unix/sysv/linux/fd_to_filename.h:27:3: error: invalid type argument of unary ‘*’ (have ‘int’)
   *_fitoa_word (fd, __stpcpy (buf, "/proc/self/fd/"), 10, 0) = '\0';
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
make[2]: *** [../o-iterator.mk:9: /suse/schwab/src/libc/n-riscv64/libio/freopen.o] Error 1

Andreas.

-- 
Andreas Schwab, SUSE Labs, schwab@suse.de
GPG Key fingerprint = 0196 BAD8 1CE9 1970 F4BE  1748 E4D4 88E3 0EEA B9D7
"And now for something completely different."
Adhemerval Zanella Oct. 2, 2018, 3:13 p.m. | #4
On 02/10/2018 11:46, Andreas Schwab wrote:
> On Okt 01 2018, Adhemerval Zanella <adhemerval.zanella@linaro.org> wrote:

> 

>> diff --git a/sysdeps/unix/sysv/linux/fd_to_filename.h b/sysdeps/unix/sysv/linux/fd_to_filename.h

>> index 297716b0ae..62bc9916b0 100644

>> --- a/sysdeps/unix/sysv/linux/fd_to_filename.h

>> +++ b/sysdeps/unix/sysv/linux/fd_to_filename.h

>> @@ -16,30 +16,20 @@

>>     License along with the GNU C Library; if not, see

>>     <http://www.gnu.org/licenses/>.  */

>>  

>> -#include <stdlib.h>

>> -#include <string.h>

>>  #include <sys/stat.h>

>> -#include <_itoa.h>

>> +

>> +#define FD_TO_FILENAME_SIZE ((sizeof ("/proc/self/fd/") - 1) \

>> +			     + (sizeof ("4294967295") - 1) + 1)

>>  

>>  static inline const char *

>> -fd_to_filename (int fd)

>> +fd_to_filename (unsigned int fd, char *buf)

>>  {

>> -  char *ret = malloc (30);

>> -

>> -  if (ret != NULL)

>> -    {

>> -      struct stat64 st;

>> -

>> -      *_fitoa_word (fd, __stpcpy (ret, "/proc/self/fd/"), 10, 0) = '\0';

>> -

>> -      /* We must make sure the file exists.  */

>> -      if (__lxstat64 (_STAT_VER, ret, &st) < 0)

>> -	{

>> -	  /* /proc is not mounted or something else happened.  Don't

>> -	     return the file name.  */

>> -	  free (ret);

>> -	  ret = NULL;

>> -	}

>> -    }

>> -  return ret;

>> +  *_fitoa_word (fd, __stpcpy (buf, "/proc/self/fd/"), 10, 0) = '\0';

>> +

>> +  /* We must make sure the file exists.  */

>> +  struct stat64 st;

>> +  if (__lxstat64 (_STAT_VER, buf, &st) < 0)

>> +    /* /proc is not mounted or something else happened.  */

>> +    return NULL;

>> +  return buf;

>>  }

> 

> In file included from freopen.c:33:

> ../sysdeps/unix/sysv/linux/fd_to_filename.h: In function ‘fd_to_filename’:

> ../sysdeps/unix/sysv/linux/fd_to_filename.h:27:4: error: implicit declaration of function ‘_fitoa_word’ [-Werror=implicit-function-declaration]

>    *_fitoa_word (fd, __stpcpy (buf, "/proc/self/fd/"), 10, 0) = '\0';

>     ^~~~~~~~~~~

> ../sysdeps/unix/sysv/linux/fd_to_filename.h:27:21: error: implicit declaration of function ‘__stpcpy’; did you mean ‘__setpgid’? [-Werror=implicit-function-declaration]

>    *_fitoa_word (fd, __stpcpy (buf, "/proc/self/fd/"), 10, 0) = '\0';

>                      ^~~~~~~~

>                      __setpgid

> ../sysdeps/unix/sysv/linux/fd_to_filename.h:27:3: error: invalid type argument of unary ‘*’ (have ‘int’)

>    *_fitoa_word (fd, __stpcpy (buf, "/proc/self/fd/"), 10, 0) = '\0';

>    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

> cc1: all warnings being treated as errors

> make[2]: *** [../o-iterator.mk:9: /suse/schwab/src/libc/n-riscv64/libio/freopen.o] Error 1

> 

> Andreas.

> 


I just rerunning a build on multiple abis (as I should have done)
to check it requires mode include fixing.

Patch

diff --git a/libio/Makefile b/libio/Makefile
index cab0eae946..8239fba828 100644
--- a/libio/Makefile
+++ b/libio/Makefile
@@ -59,8 +59,8 @@  tests = tst_swprintf tst_wprintf tst_swscanf tst_wscanf tst_getwc tst_putwc   \
 	tst-mmap-eofsync tst-mmap-fflushsync bug-mmap-fflush \
 	tst-mmap2-eofsync tst-mmap-offend bug-fopena+ bug-wfflush \
 	bug-ungetc2 bug-ftell bug-ungetc3 bug-ungetc4 tst-fopenloc2 \
-	tst-memstream1 tst-memstream2 tst-memstream3 \
-	tst-wmemstream1 tst-wmemstream2 tst-wmemstream3 \
+	tst-memstream1 tst-memstream2 tst-memstream3 tst-memstream4 \
+	tst-wmemstream1 tst-wmemstream2 tst-wmemstream3 tst-wmemstream4 \
 	bug-memstream1 bug-wmemstream1 \
 	tst-setvbuf1 tst-popen1 tst-fgetwc bug-wsetpos tst-fseek \
 	tst-fwrite-error tst-ftell-partial-wide tst-ftell-active-handler \
diff --git a/libio/freopen.c b/libio/freopen.c
index abf19e96f1..6ce74ae449 100644
--- a/libio/freopen.c
+++ b/libio/freopen.c
@@ -24,28 +24,34 @@ 
    This exception applies to code released by its copyright holders
    in files containing the exception.  */
 
-#include "libioP.h"
-#include "stdio.h"
+#include <stdio.h>
 #include <fcntl.h>
 #include <stdlib.h>
 #include <unistd.h>
 
-#include <shlib-compat.h>
+#include <libioP.h>
 #include <fd_to_filename.h>
-
-#include <kernel-features.h>
+#include <shlib-compat.h>
 
 FILE *
 freopen (const char *filename, const char *mode, FILE *fp)
 {
-  FILE *result;
+  FILE *result = NULL;
+  char fdfilename[FD_TO_FILENAME_SIZE];
+
   CHECK_FILE (fp, NULL);
-  if (!(fp->_flags & _IO_IS_FILEBUF))
-    return NULL;
+
   _IO_acquire_lock (fp);
+  /* First flush the stream (failure should be ignored).  */
+  _IO_SYNC (fp);
+
+  if (!(fp->_flags & _IO_IS_FILEBUF))
+    goto end;
+
   int fd = _IO_fileno (fp);
-  const char *gfilename = (filename == NULL && fd >= 0
-			   ? fd_to_filename (fd) : filename);
+  const char *gfilename
+    = filename != NULL ? filename : fd_to_filename (fd, fdfilename);
+
   fp->_flags2 |= _IO_FLAGS2_NOCLOSE;
 #if SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_1)
   if (&_IO_stdin_used == NULL)
@@ -101,9 +107,6 @@  freopen (const char *filename, const char *mode, FILE *fp)
     __close (fd);
 
 end:
-  if (filename == NULL)
-    free ((char *) gfilename);
-
   _IO_release_lock (fp);
   return result;
 }
diff --git a/libio/freopen64.c b/libio/freopen64.c
index ea7ebd3da4..055b999e59 100644
--- a/libio/freopen64.c
+++ b/libio/freopen64.c
@@ -24,27 +24,33 @@ 
    This exception applies to code released by its copyright holders
    in files containing the exception.  */
 
-#include "libioP.h"
-#include "stdio.h"
+#include <stdio.h>
 #include <fcntl.h>
 #include <stdlib.h>
 #include <unistd.h>
 
+#include <libioP.h>
 #include <fd_to_filename.h>
 
-#include <kernel-features.h>
-
 FILE *
 freopen64 (const char *filename, const char *mode, FILE *fp)
 {
-  FILE *result;
+  FILE *result = NULL;
+  char fdfilename[FD_TO_FILENAME_SIZE];
+
   CHECK_FILE (fp, NULL);
-  if (!(fp->_flags & _IO_IS_FILEBUF))
-    return NULL;
+
   _IO_acquire_lock (fp);
+  /* First flush the stream (failure should be ignored).  */
+  _IO_SYNC (fp);
+
+  if (!(fp->_flags & _IO_IS_FILEBUF))
+    goto end;
+
   int fd = _IO_fileno (fp);
-  const char *gfilename = (filename == NULL && fd >= 0
-			   ? fd_to_filename (fd) : filename);
+  const char *gfilename
+    = filename != NULL ? filename : fd_to_filename (fd, fdfilename);
+
   fp->_flags2 |= _IO_FLAGS2_NOCLOSE;
   _IO_file_close_it (fp);
   _IO_JUMPS_FILE_plus (fp) = &_IO_file_jumps;
@@ -84,8 +90,6 @@  freopen64 (const char *filename, const char *mode, FILE *fp)
     __close (fd);
 
 end:
-  if (filename == NULL)
-    free ((char *) gfilename);
   _IO_release_lock (fp);
   return result;
 }
diff --git a/libio/tst-memstream.h b/libio/tst-memstream.h
new file mode 100644
index 0000000000..b9b02bdb17
--- /dev/null
+++ b/libio/tst-memstream.h
@@ -0,0 +1,68 @@ 
+/* Common definitions for open_memstream tests.
+   Copyright (C) 2018 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 <mcheck.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdarg.h>
+#include <errno.h>
+
+#include <support/check.h>
+
+#ifdef TEST_WCHAR
+# include <wchar.h>
+
+/* Straighforward implementation so tst-memstream3 could use check
+   fwrite on open_memstream.  */
+static size_t __attribute__ ((used))
+fwwrite (const void *ptr, size_t size, size_t nmemb, FILE *arq)
+{
+  const wchar_t *wcs = (const wchar_t*) (ptr);
+  for (size_t s = 0; s < size; s++)
+    {
+      for (size_t n = 0; n < nmemb; n++)
+        if (fputwc (wcs[n], arq) == WEOF)
+          return n;
+    }
+  return size * nmemb;
+}
+
+# define CHAR_T wchar_t
+# define W(o) L##o
+# define OPEN_MEMSTREAM open_wmemstream
+# define PRINTF wprintf
+# define FWRITE fwwrite
+# define FPUTC  fputwc
+# define FPUTS  fputws
+# define STRCMP wcscmp
+# define STRLEN wcslen
+#else
+# define CHAR_T char
+# define W(o) o
+# define OPEN_MEMSTREAM open_memstream
+# define PRINTF printf
+# define FWRITE fwrite
+# define FPUTC fputc
+# define FPUTS  fputs
+# define STRCMP strcmp
+# define STRLEN strlen
+#endif
+
+#define S(s) S1 (s)
+#define S1(s) #s
diff --git a/libio/tst-memstream4.c b/libio/tst-memstream4.c
new file mode 100644
index 0000000000..bb2635e837
--- /dev/null
+++ b/libio/tst-memstream4.c
@@ -0,0 +1,63 @@ 
+/* Test for open_memstream BZ #21037.
+   Copyright (C) 2018 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 "tst-memstream.h"
+
+static void
+mcheck_abort (enum mcheck_status ev)
+{
+  printf ("mecheck failed with status %d\n", (int) ev);
+  exit (1);
+}
+
+static int
+do_test (void)
+{
+  mcheck_pedantic (mcheck_abort);
+
+  /* Check if freopen proper fflush the stream.  */
+  {
+    CHAR_T old[] = W("old");
+    CHAR_T *buf = old;
+    size_t size;
+
+    FILE *fp = OPEN_MEMSTREAM (&buf, &size);
+    TEST_VERIFY_EXIT (fp != NULL);
+
+    FPUTS (W("new"), fp);
+    /* The stream buffer pointer should be updated with only a fflush or
+       fclose.  */
+    TEST_COMPARE (STRCMP (buf, old), 0);
+
+    /* The old stream should be fflush the stream, even for an invalid
+       streams.  */
+    FILE *nfp = freopen ("invalid-file", "r", fp);
+    TEST_VERIFY_EXIT (nfp == NULL);
+
+    //TEST_VERIFY (STRCMP (buf, W("new")) == 0);
+    TEST_COMPARE_BLOB (buf, STRLEN (buf), W("new"), STRLEN (W ("new")));
+
+    TEST_COMPARE (fclose (fp), 0);
+
+    free (buf);
+  }
+
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/libio/tst-wmemstream4.c b/libio/tst-wmemstream4.c
new file mode 100644
index 0000000000..8ff146e2d9
--- /dev/null
+++ b/libio/tst-wmemstream4.c
@@ -0,0 +1,20 @@ 
+/* Test for open_wmemstream BZ #21037.
+   Copyright (C) 2018 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/>.  */
+
+#define TEST_WCHAR
+#include <libio/tst-memstream4.c>
diff --git a/sysdeps/generic/fd_to_filename.h b/sysdeps/generic/fd_to_filename.h
index bacfe5bf52..d41b345a9d 100644
--- a/sysdeps/generic/fd_to_filename.h
+++ b/sysdeps/generic/fd_to_filename.h
@@ -16,10 +16,12 @@ 
    License along with the GNU C Library; if not, see
    <http://www.gnu.org/licenses/>.  */
 
+#define FD_TO_FILENAME_SIZE 0
+
 /* In general there is no generic way to query filename for an open
    file descriptor.  */
 static inline const char *
-fd_to_filename (int fd)
+fd_to_filename (int fd, char *buf)
 {
   return NULL;
 }
diff --git a/sysdeps/unix/sysv/linux/fd_to_filename.h b/sysdeps/unix/sysv/linux/fd_to_filename.h
index 297716b0ae..62bc9916b0 100644
--- a/sysdeps/unix/sysv/linux/fd_to_filename.h
+++ b/sysdeps/unix/sysv/linux/fd_to_filename.h
@@ -16,30 +16,20 @@ 
    License along with the GNU C Library; if not, see
    <http://www.gnu.org/licenses/>.  */
 
-#include <stdlib.h>
-#include <string.h>
 #include <sys/stat.h>
-#include <_itoa.h>
+
+#define FD_TO_FILENAME_SIZE ((sizeof ("/proc/self/fd/") - 1) \
+			     + (sizeof ("4294967295") - 1) + 1)
 
 static inline const char *
-fd_to_filename (int fd)
+fd_to_filename (unsigned int fd, char *buf)
 {
-  char *ret = malloc (30);
-
-  if (ret != NULL)
-    {
-      struct stat64 st;
-
-      *_fitoa_word (fd, __stpcpy (ret, "/proc/self/fd/"), 10, 0) = '\0';
-
-      /* We must make sure the file exists.  */
-      if (__lxstat64 (_STAT_VER, ret, &st) < 0)
-	{
-	  /* /proc is not mounted or something else happened.  Don't
-	     return the file name.  */
-	  free (ret);
-	  ret = NULL;
-	}
-    }
-  return ret;
+  *_fitoa_word (fd, __stpcpy (buf, "/proc/self/fd/"), 10, 0) = '\0';
+
+  /* We must make sure the file exists.  */
+  struct stat64 st;
+  if (__lxstat64 (_STAT_VER, buf, &st) < 0)
+    /* /proc is not mounted or something else happened.  */
+    return NULL;
+  return buf;
 }