diff mbox

[1/3] <fd_to_filename.h>: Add type safety and port to Hurd

Message ID c6b3a5e519cd3b0d7d77c7f2dfe35384b36cb39a.1581703185.git.fweimer@redhat.com
State Superseded
Headers show

Commit Message

Florian Weimer Feb. 14, 2020, 6:11 p.m. UTC
The new type struct fd_to_filename makes the allocation of the
backing storage explicit.

Hurd uses /dev/fd, not /proc/self/fd.
---
 libio/freopen.c                               |   4 +-
 libio/freopen64.c                             |   4 +-
 misc/Makefile                                 |   6 +-
 misc/fd_to_filename.c                         | 104 +++++++++++++++++
 misc/tst-fd_to_filename.c                     | 105 ++++++++++++++++++
 sysdeps/generic/arch-fd_to_filename.h         |  19 ++++
 sysdeps/generic/fd_to_filename.h              |  25 +++--
 sysdeps/mach/hurd/arch-fd_to_filename.h       |  19 ++++
 ...fd_to_filename.h => arch-fd_to_filename.h} |  22 +---
 9 files changed, 275 insertions(+), 33 deletions(-)
 create mode 100644 misc/fd_to_filename.c
 create mode 100644 misc/tst-fd_to_filename.c
 create mode 100644 sysdeps/generic/arch-fd_to_filename.h
 create mode 100644 sysdeps/mach/hurd/arch-fd_to_filename.h
 rename sysdeps/unix/sysv/linux/{fd_to_filename.h => arch-fd_to_filename.h} (58%)

Comments

Samuel Thibault Feb. 14, 2020, 6:43 p.m. UTC | #1
Florian Weimer, le ven. 14 févr. 2020 19:11:09 +0100, a ecrit:
> The new type struct fd_to_filename makes the allocation of the
> backing storage explicit.
> 
> Hurd uses /dev/fd, not /proc/self/fd.

Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>

Thanks!

> ---
>  libio/freopen.c                               |   4 +-
>  libio/freopen64.c                             |   4 +-
>  misc/Makefile                                 |   6 +-
>  misc/fd_to_filename.c                         | 104 +++++++++++++++++
>  misc/tst-fd_to_filename.c                     | 105 ++++++++++++++++++
>  sysdeps/generic/arch-fd_to_filename.h         |  19 ++++
>  sysdeps/generic/fd_to_filename.h              |  25 +++--
>  sysdeps/mach/hurd/arch-fd_to_filename.h       |  19 ++++
>  ...fd_to_filename.h => arch-fd_to_filename.h} |  22 +---
>  9 files changed, 275 insertions(+), 33 deletions(-)
>  create mode 100644 misc/fd_to_filename.c
>  create mode 100644 misc/tst-fd_to_filename.c
>  create mode 100644 sysdeps/generic/arch-fd_to_filename.h
>  create mode 100644 sysdeps/mach/hurd/arch-fd_to_filename.h
>  rename sysdeps/unix/sysv/linux/{fd_to_filename.h => arch-fd_to_filename.h} (58%)
> 
> 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/"
> -- 
> 2.24.1
> 
>
Adhemerval Zanella Feb. 14, 2020, 7:08 p.m. UTC | #2
On 14/02/2020 15:11, Florian Weimer wrote:
> The new type struct fd_to_filename makes the allocation of the
> backing storage explicit.
> 
> Hurd uses /dev/fd, not /proc/self/fd.
> ---
>  libio/freopen.c                               |   4 +-
>  libio/freopen64.c                             |   4 +-
>  misc/Makefile                                 |   6 +-
>  misc/fd_to_filename.c                         | 104 +++++++++++++++++
>  misc/tst-fd_to_filename.c                     | 105 ++++++++++++++++++
>  sysdeps/generic/arch-fd_to_filename.h         |  19 ++++
>  sysdeps/generic/fd_to_filename.h              |  25 +++--
>  sysdeps/mach/hurd/arch-fd_to_filename.h       |  19 ++++
>  ...fd_to_filename.h => arch-fd_to_filename.h} |  22 +---
>  9 files changed, 275 insertions(+), 33 deletions(-)
>  create mode 100644 misc/fd_to_filename.c
>  create mode 100644 misc/tst-fd_to_filename.c
>  create mode 100644 sysdeps/generic/arch-fd_to_filename.h
>  create mode 100644 sysdeps/mach/hurd/arch-fd_to_filename.h
>  rename sysdeps/unix/sysv/linux/{fd_to_filename.h => arch-fd_to_filename.h} (58%)
> 
> 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;
> +}

Do we really such optimization which results in somewhat complicate
code? Couldn't we use something simple as:

char *
__fd_to_filename (int descriptor, struct fd_to_filename *storage)
{
  char *buf = storage->buffer;
  int i, j;

  for (i = 0; (buf[i] = FD_TO_FILENAME_PREFIX[i]); i++);
  if (descriptor == 0)
    {
      buf[i] = '0';
      buf[i+1] = '\0';
      return storage->buffer;
    }

  for (j = descriptor; j != 0; j /= 10, i++);
  buf[i] = '\0';
  for (; descriptor != 0; descriptor /= 10) 
    buf[--i] = '0' + descriptor % 10;

  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/"
>
Florian Weimer Feb. 14, 2020, 8:21 p.m. UTC | #3
* Adhemerval Zanella:

> Do we really such optimization which results in somewhat complicate
> code? Couldn't we use something simple as:
>
> char *
> __fd_to_filename (int descriptor, struct fd_to_filename *storage)
> {
>   char *buf = storage->buffer;
>   int i, j;
>
>   for (i = 0; (buf[i] = FD_TO_FILENAME_PREFIX[i]); i++);
>   if (descriptor == 0)
>     {
>       buf[i] = '0';
>       buf[i+1] = '\0';
>       return storage->buffer;
>     }
>
>   for (j = descriptor; j != 0; j /= 10, i++);
>   buf[i] = '\0';
>   for (; descriptor != 0; descriptor /= 10) 
>     buf[--i] = '0' + descriptor % 10;
>
>   return storage->buffer;
> }

I don't have a strong opinion about this (but I think we should probably
keep the mempcpy).  Most descriptors will be fairly small, so the
smaller code size is probably more important.

Thanks,
Florian
Paul Eggert Feb. 14, 2020, 8:28 p.m. UTC | #4
On 2/14/20 10:11 AM, Florian Weimer wrote:
> +  /* A positive int value has at most 10 decimal digits.  */
> +  char buffer[sizeof (FD_TO_FILENAME_PREFIX) + 10];

This should use 'INT_STRLEN_BOUND (int)' rather than '10', where 
INT_STRLEN_BOUND is taken from intprops.h. Then you don't need the 
comment (and the code won't break on future ILP64 platforms :-).

> +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) ...

It's not clear what that "descriptor >= 0" test is doing. I assume that 
a precondition is that DESCRIPTOR is nonnegative; if so, this should be 
mentioned and the test removed. If it's not a precondition, the code 
should do the right thing if DESCRIPTOR is negative (I suppose, return a 
filename that cannot be opened, though it doesn't do that currently) -- 
which should also be documented but I think this'd be overkill.

As Adhemerval writes, there's no need for special cases for powers of 
1000 etc. Something like the following should do (this is a bit shorter 
than Adhemerval's version):

   /* Convert nonnegative DESCRIPTOR to a file name.  */

   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) - 1);
     for (int d = descriptor; p++, (d /= 10) != 0; )
       continue;
     *p = '\0';
     for (int d = descriptor; *--p = '0' + d % 10, (d /= 10) != 0; )
       continue;
     return storage->buffer;
   }

GCC turns those divisions and remainders into the usual 
multiply+shift+add and this should be good enough.
Florian Weimer Feb. 15, 2020, 1:16 p.m. UTC | #5
* Paul Eggert:

> On 2/14/20 10:11 AM, Florian Weimer wrote:
>> +  /* A positive int value has at most 10 decimal digits.  */
>> +  char buffer[sizeof (FD_TO_FILENAME_PREFIX) + 10];
>
> This should use 'INT_STRLEN_BOUND (int)' rather than '10', where
> INT_STRLEN_BOUND is taken from intprops.h. Then you don't need the
> comment (and the code won't break on future ILP64 platforms :-).

But INT_STRLEN_BOUND is 11, right?

>> +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) ...
>
> It's not clear what that "descriptor >= 0" test is doing. I assume
> that a precondition is that DESCRIPTOR is nonnegative; if so, this
> should be mentioned and the test removed. If it's not a precondition,
> the code should do the right thing if DESCRIPTOR is negative (I
> suppose, return a filename that cannot be opened, though it doesn't do
> that currently) -- which should also be documented but I think this'd
> be overkill.

The problem is when an application passes an invalid descriptor to some
libc function and that ends up with __fd_to_filename.  We should not
make matters worse in that case.

Maybe we should use an assert?

Thanks,
Florian
Paul Eggert Feb. 16, 2020, 12:02 a.m. UTC | #6
On 2/15/20 5:16 AM, Florian Weimer wrote:

> INT_STRLEN_BOUND is 11, right?

Yes, it's a bound on the string length of a printed int, and that's 11 in the 
typical case of 32-bit int because the int might be negative.  I didn't lose 
sleep over the wasted byte, but if we want a tighter bound then we could use 
INT_STRLEN_BOUND (int) - 1 instead. However, it might be better to leave it 
alone so that we can use the code below.

> The problem is when an application passes an invalid descriptor to some
> libc function and that ends up with __fd_to_filename.  We should not
> make matters worse in that case.

If it's not a precondition that the descriptor is nonnegative, we can't simply 
return a copy of FD_TO_FILENAME_PREFIX as that's an existing filename. Instead, 
how about the following? It uses a randomish garbage filename beginning with "-" 
which should be good enough, and it doesn't cost a conditional branch to handle 
negative descriptors.

   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) - 1);

     /* If DESCRIPTOR is negative, arrange for the filename to not exist
        by prepending any byte other than '/', '.', '\0' or an ASCII digit.
        The rest of the filename will be gibberish that fits.  */
     *p = '-';
     p += descriptor < 0;

     for (int d = descriptor; p++, (d /= 10) != 0; )
       continue;
     *p = '\0';
     for (int d = descriptor; *--p = '0' + d % 10, (d /= 10) != 0; )
       continue;
     return storage->buffer;
   }
diff mbox

Patch

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/"