diff --git a/libio/freopen.c b/libio/freopen.c
index bab3ba204a..884cdb2961 100644
--- a/libio/freopen.c
+++ b/libio/freopen.c
@@ -37,7 +37,7 @@ FILE *
 freopen (const char *filename, const char *mode, FILE *fp)
 {
   FILE *result = NULL;
-  char fdfilename[FD_TO_FILENAME_SIZE];
+  struct fd_to_filename fdfilename;
 
   CHECK_FILE (fp, NULL);
 
@@ -50,7 +50,7 @@ freopen (const char *filename, const char *mode, FILE *fp)
 
   int fd = _IO_fileno (fp);
   const char *gfilename
-    = filename != NULL ? filename : fd_to_filename (fd, fdfilename);
+    = filename != NULL ? filename : __fd_to_filename (fd, &fdfilename);
 
   fp->_flags2 |= _IO_FLAGS2_NOCLOSE;
 #if SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_1)
diff --git a/libio/freopen64.c b/libio/freopen64.c
index c0ce604e6e..0d2c5264c7 100644
--- a/libio/freopen64.c
+++ b/libio/freopen64.c
@@ -36,7 +36,7 @@ FILE *
 freopen64 (const char *filename, const char *mode, FILE *fp)
 {
   FILE *result = NULL;
-  char fdfilename[FD_TO_FILENAME_SIZE];
+  struct fd_to_filename fdfilename;
 
   CHECK_FILE (fp, NULL);
 
@@ -49,7 +49,7 @@ freopen64 (const char *filename, const char *mode, FILE *fp)
 
   int fd = _IO_fileno (fp);
   const char *gfilename
-    = filename != NULL ? filename : fd_to_filename (fd, fdfilename);
+    = filename != NULL ? filename : __fd_to_filename (fd, &fdfilename);
 
   fp->_flags2 |= _IO_FLAGS2_NOCLOSE;
   _IO_file_close_it (fp);
diff --git a/misc/Makefile b/misc/Makefile
index e0465980c7..b8fed5783d 100644
--- a/misc/Makefile
+++ b/misc/Makefile
@@ -72,7 +72,7 @@ routines := brk sbrk sstk ioctl \
 	    fgetxattr flistxattr fremovexattr fsetxattr getxattr \
 	    listxattr lgetxattr llistxattr lremovexattr lsetxattr \
 	    removexattr setxattr getauxval ifunc-impl-list makedev \
-	    allocate_once
+	    allocate_once fd_to_filename
 
 generated += tst-error1.mtrace tst-error1-mem.out \
   tst-allocate_once.mtrace tst-allocate_once-mem.out
@@ -97,6 +97,10 @@ endif
 tests-internal := tst-atomic tst-atomic-long tst-allocate_once
 tests-static := tst-empty
 
+# Test for the internal, non-exported __fd_to_filename function.
+tests-internal += tst-fd_to_filename
+tests-static += tst-fd_to_filename
+
 ifeq ($(run-built-tests),yes)
 tests-special += $(objpfx)tst-error1-mem.out \
   $(objpfx)tst-allocate_once-mem.out
diff --git a/misc/fd_to_filename.c b/misc/fd_to_filename.c
new file mode 100644
index 0000000000..911c22cf2b
--- /dev/null
+++ b/misc/fd_to_filename.c
@@ -0,0 +1,104 @@
+/* Construct a pathname under /proc/self/fd (or /dev/fd for Hurd).
+   Copyright (C) 2020 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
+   <https://www.gnu.org/licenses/>.  */
+
+#include <fd_to_filename.h>
+#include <string.h>
+
+/* The split into groups of three digits is arbitrary, but it covers
+   the common case of descriptors less than 1000 fairly
+   efficiently.  */
+
+/* Writes two digits to P.  VALUE must be between 0 and 99
+   (inclusive).  Returns the address after the written digits.  */
+static char *
+digit2 (char *p, int value)
+{
+  p[0] = '0' + (value / 10);
+  p[1] = '0' + (value % 10);
+  return p + 2;
+}
+
+/* Writes three digits to P.  VALUE must be between 0 and 999
+   (inclusive).  Returns the address after the written digits.  */
+static char *
+digit3 (char *p, int value)
+{
+  p[0] = '0' + (value / 100);
+  p[1] = '0' + ((value / 10) % 10);
+  p[2] = '0' + (value % 10);
+  return p + 3;
+}
+
+/* Writes one to three digits to P, depending on VALUE, which must be
+   less than 1000.  Returns the address after the written digits.  */
+static char *
+digit123 (char *p, int value)
+{
+  if (value < 10)
+    {
+      p[0] = '0' + value;
+      return p + 1;
+    }
+  else if (value < 100)
+    return digit2 (p, value);
+  else
+    return digit3 (p, value);
+}
+
+char *
+__fd_to_filename (int descriptor, struct fd_to_filename *storage)
+{
+  char *p = mempcpy (storage->buffer, FD_TO_FILENAME_PREFIX,
+                     strlen (FD_TO_FILENAME_PREFIX));
+  if (__glibc_likely (descriptor >= 0))
+    {
+      if (descriptor < 1000)
+        p = digit123 (p, descriptor);
+      else if (descriptor < 1000 * 1000)
+        {
+          p = digit123 (p, descriptor / 1000);
+          p = digit3 (p, descriptor % 1000);
+        }
+      else if (descriptor < 1000 * 1000 * 1000)
+        {
+          p = digit123 (p, descriptor / (1000 * 1000));
+          p = digit3 (p, (descriptor / 1000) % 1000);
+          p = digit3 (p, descriptor % 1000);
+        }
+      else
+        {
+          if (descriptor < 2000 * 1000 * 1000)
+            {
+              *p = '1';
+              descriptor -= 1000 * 1000 * 1000;
+            }
+          else
+            {
+              *p = '2';
+              descriptor -= 2000 * 1000 * 1000;
+            }
+          ++p;
+          p = digit3 (p, descriptor / (1000 * 1000));
+          p = digit3 (p, (descriptor / 1000) % 1000);
+          p = digit3 (p, descriptor % 1000);
+        }
+    }
+
+  *p = '\0';
+  return storage->buffer;
+}
diff --git a/misc/tst-fd_to_filename.c b/misc/tst-fd_to_filename.c
new file mode 100644
index 0000000000..480f613dcf
--- /dev/null
+++ b/misc/tst-fd_to_filename.c
@@ -0,0 +1,105 @@
+/* Test for /proc/self/fd (or /dev/fd) pathname construction.
+   Copyright (C) 2020 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
+   <https://www.gnu.org/licenses/>.  */
+
+#include <fd_to_filename.h>
+#include <stdio.h>
+#include <support/check.h>
+#include <support/xunistd.h>
+
+/* Run a check on one value.  */
+static void
+check (int value)
+{
+  struct fd_to_filename storage;
+  char *actual = __fd_to_filename (value, &storage);
+  char expected[100];
+  snprintf (expected, sizeof (expected), FD_TO_FILENAME_PREFIX "%d", value);
+
+  if (value < 0)
+    /* Negative inputs are invalid, but the output should still be
+       deterministic.  */
+    TEST_COMPARE_STRING (actual, FD_TO_FILENAME_PREFIX);
+  else
+    TEST_COMPARE_STRING (actual, expected);
+}
+
+/* Check various ranges constructed around powers.  */
+static void
+check_ranges (int base)
+{
+  unsigned int power = 1;
+  do
+    {
+      for (int factor = 1; factor < base; ++factor)
+        for (int shift = -1000; shift <= 1000; ++shift)
+          {
+            check (factor * power + shift);
+            check (-(factor * power + shift));
+          }
+    }
+  while (!__builtin_mul_overflow (power, base, &power));
+}
+
+/* Check that it is actually possible to use a the constructed
+   name.  */
+static void
+check_open (void)
+{
+  int pipes[2];
+  xpipe (pipes);
+
+  struct fd_to_filename storage;
+  int read_alias = xopen (__fd_to_filename (pipes[0], &storage), O_RDONLY, 0);
+  int write_alias = xopen (__fd_to_filename (pipes[1], &storage), O_WRONLY, 0);
+
+  /* Ensure that all the descriptor numbers are different.  */
+  TEST_VERIFY (pipes[0] < pipes[1]);
+  TEST_VERIFY (pipes[1] < read_alias);
+  TEST_VERIFY (read_alias < write_alias);
+
+  xwrite (write_alias, "1", 1);
+  char buf[16];
+  TEST_COMPARE_BLOB ("1", 1, buf, read (pipes[0], buf, sizeof (buf)));
+
+  xwrite (pipes[1], "2", 1);
+  TEST_COMPARE_BLOB ("2", 1, buf, read (read_alias, buf, sizeof (buf)));
+
+  xwrite (write_alias, "3", 1);
+  TEST_COMPARE_BLOB ("3", 1, buf, read (read_alias, buf, sizeof (buf)));
+
+  xwrite (pipes[1], "4", 1);
+  TEST_COMPARE_BLOB ("4", 1, buf, read (pipes[0], buf, sizeof (buf)));
+
+  xclose (write_alias);
+  xclose (read_alias);
+  xclose (pipes[1]);
+  xclose (pipes[0]);
+}
+
+static int
+do_test (void)
+{
+  check_ranges (2);
+  check_ranges (10);
+
+  check_open ();
+
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/sysdeps/generic/arch-fd_to_filename.h b/sysdeps/generic/arch-fd_to_filename.h
new file mode 100644
index 0000000000..ecaaa14dba
--- /dev/null
+++ b/sysdeps/generic/arch-fd_to_filename.h
@@ -0,0 +1,19 @@
+/* Query filename corresponding to an open FD.  Generic stub.
+   Copyright (C) 2020 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
+   <https://www.gnu.org/licenses/>.  */
+
+#error "<arch-fd_to_filename.h> must be ported to this architecture"
diff --git a/sysdeps/generic/fd_to_filename.h b/sysdeps/generic/fd_to_filename.h
index eff6ca211b..7889db879c 100644
--- a/sysdeps/generic/fd_to_filename.h
+++ b/sysdeps/generic/fd_to_filename.h
@@ -1,4 +1,4 @@
-/* Query filename corresponding to an open FD.  Generic version.
+/* Query filename corresponding to an open FD.
    Copyright (C) 2001-2020 Free Software Foundation, Inc.
    This file is part of the GNU C Library.
 
@@ -16,12 +16,21 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
-#define FD_TO_FILENAME_SIZE 0
+#ifndef _FD_TO_FILENAME_H
+#define _FD_TO_FILENAME_H
 
-/* In general there is no generic way to query filename for an open
-   file descriptor.  */
-static inline const char *
-fd_to_filename (int fd, char *buf)
+#include <arch-fd_to_filename.h>
+
+struct fd_to_filename
 {
-  return NULL;
-}
+  /* A positive int value has at most 10 decimal digits.  */
+  char buffer[sizeof (FD_TO_FILENAME_PREFIX) + 10];
+};
+
+/* Writes a /proc/self/fd-style path for DESCRIPTOR to *STORAGE and
+   returns a pointer to the start of the string.  DESCRIPTOR must be
+   non-negative.  */
+char *__fd_to_filename (int descriptor, struct fd_to_filename *storage)
+  attribute_hidden;
+
+#endif /* _FD_TO_FILENAME_H */
diff --git a/sysdeps/mach/hurd/arch-fd_to_filename.h b/sysdeps/mach/hurd/arch-fd_to_filename.h
new file mode 100644
index 0000000000..b45cd8d836
--- /dev/null
+++ b/sysdeps/mach/hurd/arch-fd_to_filename.h
@@ -0,0 +1,19 @@
+/* Query filename corresponding to an open FD.  Hurd version.
+   Copyright (C) 2020 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
+   <https://www.gnu.org/licenses/>.  */
+
+#define FD_TO_FILENAME_PREFIX "/dev/fd/"
diff --git a/sysdeps/unix/sysv/linux/fd_to_filename.h b/sysdeps/unix/sysv/linux/arch-fd_to_filename.h
similarity index 58%
rename from sysdeps/unix/sysv/linux/fd_to_filename.h
rename to sysdeps/unix/sysv/linux/arch-fd_to_filename.h
index 92a5e02976..b6017214c7 100644
--- a/sysdeps/unix/sysv/linux/fd_to_filename.h
+++ b/sysdeps/unix/sysv/linux/arch-fd_to_filename.h
@@ -1,5 +1,5 @@
 /* Query filename corresponding to an open FD.  Linux version.
-   Copyright (C) 2001-2020 Free Software Foundation, Inc.
+   Copyright (C) 2020 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
@@ -16,22 +16,4 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
-#include <sys/stat.h>
-#include <string.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 (unsigned int fd, char *buf)
-{
-  *_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;
-}
+#define FD_TO_FILENAME_PREFIX "/proc/self/fd/"
