Subject: bug#39236: [musl] coreutils cp mishandles error return from lchmod

From: Paul Eggert <[email-masked]>
Date: Fri, 7 Feb 2020 16:37:56 -0800

On 1/22/20 2:05 PM, Rich Felker wrote:
> I think we're approaching a consensus that glibc should fix this too,
> so then it would just be gnulib matching the fix.

I installed the attached patch to Gnulib in preparation for the upcoming 
glibc fix. The patch causes fchmodat with AT_SYMLINK_NOFOLLOW to work on 
non-symlinks, and similarly for lchmod on non-symlinks. The idea is to 
avoid this sort of problem in the future, and to let Coreutils etc. work 
on older platforms as if glibc 2.32 (or whatever) is already in place.
>From b16a04394121e7396569a13161dba02c6752b19f Mon Sep 17 00:00:00 2001
From: Paul Eggert <[email-masked]>
Date: Fri, 7 Feb 2020 16:34:12 -0800
Subject: [PATCH] fchmodat: AT_SYMLINK_NOFOLLOW fix for non-symlinks

Fix lchmod, and fchmodat with AT_SYMLINK_NOFOLLOW, so that
they act like chmod on non-symlinks.
* NEWS:
* doc/glibc-functions/lchmod.texi (lchmod):
* doc/posix-functions/fchmodat.texi (fchmodat):
Mention this.
* lib/fchmodat.c: Define __need_system_sys_stat_h before including
config.h, and undef it after including sys/stat.h the first time.
Include fcntl.h, stdio.h, unistd.h, intprops.h, and include
sys/stat.h a second time after defining orig_fchmodat.
(orig_fchmodat) [HAVE_FCHMODAT]: New function.
(fchmodat) [HAVE_FCHMODAT]: Work around the AT_SYMLINK_NOFOLLOW bug.
* lib/lchmod.c: New file.
* lib/sys_stat.in.h (fchmodat, lchmod):
Support replacing these functions.
* m4/fchmodat.m4 (gl_FUNC_FCHMODAT): If fchmodat exists,
test that AT_SYMLINK_NOFOLLOW works on non-symlinks.
* m4/lchmod.m4 (gl_FUNC_LCHMOD): Check for lstat.
Test that lchmod works on non-symlinks.
* m4/sys_stat_h.m4 (gl_SYS_STAT_H_DEFAULTS):
Default REPLACE_FCHMODAT and REPLACE_LCHMOD to 0.
* modules/fchmodat (Depends-on): Add fstatat, intprops, lchmod, unistd.
(Depends-on, configure.ac): Check REPLACE_FCHMODAT too.
* modules/lchmod (Files): Add lib/lchmod.c.
(Depends-on): Add errno, fcntl-h, fchmodat, intprops, lstat, unistd.
(configure.ac): Compile lchmod.c if needed.
(lib_SOURCES): Add lchmod.c.
* modules/sys_stat (sys/stat.h): Substitute REPLACE_FCHMODAT
and REPLACE_LCHMOD.
* tests/test-fchmodat.c: Include fcntl.h, sys/stat.h.
(main): Test fchmodat with AT_SYMLINK_NOFOLLOW on non-symlinks.
---
 ChangeLog                         | 35 ++++++++++++
 NEWS                              |  7 +++
 doc/glibc-functions/lchmod.texi   |  4 ++
 doc/posix-functions/fchmodat.texi | 11 ++--
 lib/fchmodat.c                    | 89 +++++++++++++++++++++++++------
 lib/lchmod.c                      | 72 +++++++++++++++++++++++++
 lib/sys_stat.in.h                 | 41 +++++++-------
 m4/fchmodat.m4                    | 48 ++++++++++++++++-
 m4/lchmod.m4                      | 52 ++++++++++++++++--
 m4/sys_stat_h.m4                  |  4 +-
 modules/fchmodat                  | 10 ++--
 modules/lchmod                    | 13 ++++-
 modules/sys_stat                  |  2 +
 tests/test-fchmodat.c             | 10 ++++
 14 files changed, 348 insertions(+), 50 deletions(-)
 create mode 100644 lib/lchmod.c

diff --git a/ChangeLog b/ChangeLog
index 99e0c2e9e..71dcaba6c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,38 @@
+2020-02-07  Paul Eggert  <[email-masked]>
+
+	fchmodat: AT_SYMLINK_NOFOLLOW fix for non-symlinks
+	Fix lchmod, and fchmodat with AT_SYMLINK_NOFOLLOW, so that
+	they act like chmod on non-symlinks.
+	* NEWS:
+	* doc/glibc-functions/lchmod.texi (lchmod):
+	* doc/posix-functions/fchmodat.texi (fchmodat):
+	Mention this.
+	* lib/fchmodat.c: Define __need_system_sys_stat_h before including
+	config.h, and undef it after including sys/stat.h the first time.
+	Include fcntl.h, stdio.h, unistd.h, intprops.h, and include
+	sys/stat.h a second time after defining orig_fchmodat.
+	(orig_fchmodat) [HAVE_FCHMODAT]: New function.
+	(fchmodat) [HAVE_FCHMODAT]: Work around the AT_SYMLINK_NOFOLLOW bug.
+	* lib/lchmod.c: New file.
+	* lib/sys_stat.in.h (fchmodat, lchmod):
+	Support replacing these functions.
+	* m4/fchmodat.m4 (gl_FUNC_FCHMODAT): If fchmodat exists,
+	test that AT_SYMLINK_NOFOLLOW works on non-symlinks.
+	* m4/lchmod.m4 (gl_FUNC_LCHMOD): Check for lstat.
+	Test that lchmod works on non-symlinks.
+	* m4/sys_stat_h.m4 (gl_SYS_STAT_H_DEFAULTS):
+	Default REPLACE_FCHMODAT and REPLACE_LCHMOD to 0.
+	* modules/fchmodat (Depends-on): Add fstatat, intprops, lchmod, unistd.
+	(Depends-on, configure.ac): Check REPLACE_FCHMODAT too.
+	* modules/lchmod (Files): Add lib/lchmod.c.
+	(Depends-on): Add errno, fcntl-h, fchmodat, intprops, lstat, unistd.
+	(configure.ac): Compile lchmod.c if needed.
+	(lib_SOURCES): Add lchmod.c.
+	* modules/sys_stat (sys/stat.h): Substitute REPLACE_FCHMODAT
+	and REPLACE_LCHMOD.
+	* tests/test-fchmodat.c: Include fcntl.h, sys/stat.h.
+	(main): Test fchmodat with AT_SYMLINK_NOFOLLOW on non-symlinks.
+
 2020-02-05  Marc Dionne  <[email-masked]>  (tiny change)
 
 	mountlist: Consider AFS filesystems as remote
diff --git a/NEWS b/NEWS
index dc5cc71f9..bc81dfc28 100644
--- a/NEWS
+++ b/NEWS
@@ -58,6 +58,13 @@ User visible incompatible changes
 
 Date        Modules         Changes
 
+2020-02-07  fchmodat        When applied to non-symlinks, these now act like
+            lchmod          chmod (the BSD behavior, which POSIX requires for
+                            fchmodat + AT_SYMLINK_NOFOLLOW), instead of failing
+                            (the GNU/Linux behavior through glibc 2.31).
+                            Future versions of GNU/Linux are planned to act as
+                            per POSIX and BSD.
+
 2020-01-15  gc-pbkdf2-sha1  This module is deprecated.  Use gc-pbkdf2 instead.
 
 2019-12-12  dfa             Its API now uses ptrdiff_t instead of size_t.
diff --git a/doc/glibc-functions/lchmod.texi b/doc/glibc-functions/lchmod.texi
index aea782dc2..6cc48b453 100644
--- a/doc/glibc-functions/lchmod.texi
+++ b/doc/glibc-functions/lchmod.texi
@@ -9,6 +9,10 @@ Portability problems fixed by Gnulib:
 @item
 This function is missing on some platforms:
 OpenBSD 3.8, Minix 3.1.8, AIX 5.1, IRIX 6.5, Solaris 11.4, Cygwin, mingw, MSVC 14, Android 9.0.
+@item
+This function always fails with @code{errno} set to @code{ENOSYS},
+even when the file is not a symbolic link:
+GNU/Linux with glibc 2.31.
 @end itemize
 
 Portability problems not fixed by Gnulib:
diff --git a/doc/posix-functions/fchmodat.texi b/doc/posix-functions/fchmodat.texi
index 6add5ba89..4d190310e 100644
--- a/doc/posix-functions/fchmodat.texi
+++ b/doc/posix-functions/fchmodat.texi
@@ -9,6 +9,11 @@ Gnulib module: fchmodat
 Portability problems fixed by Gnulib:
 @itemize
 @item
+When given the @code{AT_SYMLINK_NOFOLLOW} flag,
+this function fails with @code{errno} set to @code{ENOTSUP},
+even when the file is not a symbolic link:
+GNU/Linux and Cygwin with glibc 2.31.
+@item
 This function is missing on some platforms:
 glibc 2.3.6, Mac OS X 10.5, FreeBSD 6.0, NetBSD 5.0, OpenBSD 3.8, Minix 3.1.8,
 AIX 5.1, HP-UX 11, IRIX 6.5, Solaris 10, Cygwin 1.5.x, mingw, MSVC 14.
@@ -19,9 +24,5 @@ Portability problems not fixed by Gnulib:
 @itemize
 @item
 Some platforms do not allow changing the access bits on symbolic
-links.  POSIX states that @code{fchmodat(@dots{},AT_SYMLINK_NOFOLLOW)}
-may fail with @code{EOPNOTSUPP} when called on a symlink, but some
-platforms, as well as the gnulib replacement, fail for any use of
-AT_SYMLINK_NOFOLLOW even if the target was not a symlink:
-glibc, Cygwin.
+links.
 @end itemize
diff --git a/lib/fchmodat.c b/lib/fchmodat.c
index 0d4891b3e..c6b8ef768 100644
--- a/lib/fchmodat.c
+++ b/lib/fchmodat.c
@@ -16,29 +16,42 @@
 
 /* written by Jim Meyering */
 
+/* If the user's config.h happens to include <sys/stat.h>, let it include only
+   the system's <sys/stat.h> here, so that orig_fchmodat doesn't recurse to
+   rpl_fchmodat.  */
+#define __need_system_sys_stat_h
 #include <config.h>
 
 /* Specification.  */
 #include <sys/stat.h>
+#undef __need_system_sys_stat_h
 
 #include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
 #include <stdlib.h>
+#include <unistd.h>
 
-#ifndef HAVE_LCHMOD
-/* Use a different name, to avoid conflicting with any
-   system-supplied declaration.  */
-# undef lchmod
-# define lchmod lchmod_rpl
+#if HAVE_FCHMODAT
 static int
-lchmod (char const *f _GL_UNUSED, mode_t m _GL_UNUSED)
+orig_fchmodat (int dir, char const *file, mode_t mode, int flags)
 {
-  errno = ENOSYS;
-  return -1;
+  return fchmodat (dir, file, mode, flags);
 }
 #endif
 
-/* Solaris 10 has no function like this.
-   Invoke chmod or lchmod on file, FILE, using mode MODE, in the directory
+#ifdef __osf__
+/* Write "sys/stat.h" here, not <sys/stat.h>, otherwise OSF/1 5.1 DTK cc
+   eliminates this include because of the preliminary #include <sys/stat.h>
+   above.  */
+# include "sys/stat.h"
+#else
+# include <sys/stat.h>
+#endif
+
+#include <intprops.h>
+
+/* Invoke chmod or lchmod on FILE, using mode MODE, in the directory
    open on descriptor FD.  If possible, do it without changing the
    working directory.  Otherwise, resort to using save_cwd/fchdir,
    then (chmod|lchmod)/restore_cwd.  If either the save_cwd or the
@@ -46,10 +59,52 @@ lchmod (char const *f _GL_UNUSED, mode_t m _GL_UNUSED)
    Note that an attempt to use a FLAG value of AT_SYMLINK_NOFOLLOW
    on a system without lchmod support causes this function to fail.  */
 
-#define AT_FUNC_NAME fchmodat
-#define AT_FUNC_F1 lchmod
-#define AT_FUNC_F2 chmod
-#define AT_FUNC_USE_F1_COND AT_SYMLINK_NOFOLLOW
-#define AT_FUNC_POST_FILE_PARAM_DECLS , mode_t mode, int flag
-#define AT_FUNC_POST_FILE_ARGS        , mode
-#include "at-func.c"
+#if HAVE_FCHMODAT
+int
+fchmodat (int dir, char const *file, mode_t mode, int flags)
+{
+  if (flags & AT_SYMLINK_NOFOLLOW)
+    {
+# ifdef O_PATH
+      int fd = openat (dir, file, O_PATH | O_NOFOLLOW | O_CLOEXEC);
+      if (fd < 0)
+        return fd;
+      static char const fmt[] = "/proc/self/fd/%d";
+      char buf[sizeof fmt - sizeof "%d" + INT_BUFSIZE_BOUND (int)];
+      sprintf (buf, fmt, fd);
+      int chmod_result = chmod (buf, mode);
+      int chmod_errno = errno;
+      close (fd);
+      if (chmod_result == 0)
+        return chmod_result;
+      if (chmod_errno != ENOENT)
+        {
+          errno = chmod_errno;
+          return chmod_result;
+        }
+      /* /proc is not mounted; fall back on racy implementation.  */
+# endif
+
+      struct stat st;
+      int fstatat_result = fstatat (dir, file, &st, AT_SYMLINK_NOFOLLOW);
+      if (fstatat_result != 0)
+        return fstatat_result;
+      if (S_ISLNK (st.st_mode))
+        {
+          errno = EOPNOTSUPP;
+          return -1;
+        }
+      flags &= ~AT_SYMLINK_NOFOLLOW;
+    }
+
+  return orig_fchmodat (dir, file, mode, flags);
+}
+#else
+# define AT_FUNC_NAME fchmodat
+# define AT_FUNC_F1 lchmod
+# define AT_FUNC_F2 chmod
+# define AT_FUNC_USE_F1_COND AT_SYMLINK_NOFOLLOW
+# define AT_FUNC_POST_FILE_PARAM_DECLS , mode_t mode, int flag
+# define AT_FUNC_POST_FILE_ARGS        , mode
+# include "at-func.c"
+#endif
diff --git a/lib/lchmod.c b/lib/lchmod.c
new file mode 100644
index 000000000..cc260ce4d
--- /dev/null
+++ b/lib/lchmod.c
@@ -0,0 +1,72 @@
+/* Implement lchmod on platforms where it does not work correctly.
+
+   Copyright 2020 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 <https://www.gnu.org/licenses/>.  */
+
+/* written by Paul Eggert */
+
+#include <config.h>
+
+#include <sys/stat.h>
+
+#include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <unistd.h>
+
+#include <intprops.h>
+
+/* Work like lchmod, except when FILE is a symbolic link.
+   In that case, set errno to EOPNOTSUPP and return -1.  */
+
+int
+lchmod (char const *file, mode_t mode)
+{
+#if HAVE_FCHMODAT
+  return fchmodat (AT_FDCWD, file, mode, AT_SYMLINK_NOFOLLOW);
+#elif defined O_PATH && defined AT_FDCWD
+  int fd = openat (AT_FDCWD, file, O_PATH | O_NOFOLLOW | O_CLOEXEC);
+  if (fd < 0)
+    return fd;
+  static char const fmt[] = "/proc/self/fd/%d";
+  char buf[sizeof fmt - sizeof "%d" + INT_BUFSIZE_BOUND (int)];
+  sprintf (buf, fmt, fd);
+  int chmod_result = chmod (buf, mode);
+  int chmod_errno = errno;
+  close (fd);
+  if (chmod_result == 0)
+    return chmod_result;
+  if (chmod_errno != ENOENT)
+    {
+      errno = chmod_errno;
+      return chmod_result;
+    }
+  /* /proc is not mounted; fall back on racy implementation.  */
+#endif
+
+#if HAVE_LSTAT
+  struct stat st;
+  int lstat_result = lstat (file, &st);
+  if (lstat_result != 0)
+    return lstat_result;
+  if (S_ISLNK (st.st_mode))
+    {
+      errno = EOPNOTSUPP;
+      return -1;
+    }
+#endif
+
+  return chmod (file, mode);
+}
diff --git a/lib/sys_stat.in.h b/lib/sys_stat.in.h
index 5c891e7b8..4f9eb5976 100644
--- a/lib/sys_stat.in.h
+++ b/lib/sys_stat.in.h
@@ -392,13 +392,25 @@ struct stat

 #if @GNULIB_FCHMODAT@
-# if !@HAVE_FCHMODAT@
+# if @REPLACE_FCHMODAT@
+#  if !(defined __cplusplus && defined GNULIB_NAMESPACE)
+#   undef fchmodat
+#   define fchmodat rpl_fchmodat
+#  endif
+_GL_FUNCDECL_RPL (fchmodat, int,
+                  (int fd, char const *file, mode_t mode, int flag)
+                  _GL_ARG_NONNULL ((2)));
+_GL_CXXALIAS_RPL (fchmodat, int,
+                  (int fd, char const *file, mode_t mode, int flag));
+# else
+#  if !@HAVE_FCHMODAT@
 _GL_FUNCDECL_SYS (fchmodat, int,
                   (int fd, char const *file, mode_t mode, int flag)
                   _GL_ARG_NONNULL ((2)));
-# endif
+#  endif
 _GL_CXXALIAS_SYS (fchmodat, int,
                   (int fd, char const *file, mode_t mode, int flag));
+# endif
 _GL_CXXALIASWARN (fchmodat);
 #elif defined GNULIB_POSIXCHECK
 # undef fchmodat
@@ -502,31 +514,24 @@ _GL_WARN_ON_USE (futimens, "futimens is not portable - "
 #if @GNULIB_LCHMOD@
 /* Change the mode of FILENAME to MODE, without dereferencing it if FILENAME
    denotes a symbolic link.  */
-# if !@HAVE_LCHMOD@
-/* The lchmod replacement follows symbolic links.  Callers should take
-   this into account; lchmod should be applied only to arguments that
-   are known to not be symbolic links.  On hosts that lack lchmod,
-   this can lead to race conditions between the check and the
-   invocation of lchmod, but we know of no workarounds that are
-   reliable in general.  You might try requesting support for lchmod
-   from your operating system supplier.  */
+# if @REPLACE_LCHMOD@
 #  if !(defined __cplusplus && defined GNULIB_NAMESPACE)
-#   define lchmod chmod
+#   undef lchmod
+#   define lchmod rpl_lchmod
 #  endif
-/* Need to cast, because on mingw, the second parameter of chmod is
-                                                int mode.  */
-_GL_CXXALIAS_RPL_CAST_1 (lchmod, chmod, int,
-                         (const char *filename, mode_t mode));
+_GL_FUNCDECL_RPL (lchmod, int,
+                  (char const *filename, mode_t mode)
+                  _GL_ARG_NONNULL ((1)));
+_GL_CXXALIAS_RPL (lchmod, int,
+                  (char const *filename, mode_t mode));
 # else
-#  if 0 /* assume already declared */
+#  if !@HAVE_LCHMOD@
 _GL_FUNCDECL_SYS (lchmod, int, (const char *filename, mode_t mode)
                                _GL_ARG_NONNULL ((1)));
 #  endif
 _GL_CXXALIAS_SYS (lchmod, int, (const char *filename, mode_t mode));
 # endif
-# if @HAVE_LCHMOD@
 _GL_CXXALIASWARN (lchmod);
-# endif
 #elif defined GNULIB_POSIXCHECK
 # undef lchmod
 # if HAVE_RAW_DECL_LCHMOD
diff --git a/m4/fchmodat.m4 b/m4/fchmodat.m4
index 460a4dc17..8195ef668 100644
--- a/m4/fchmodat.m4
+++ b/m4/fchmodat.m4
@@ -1,4 +1,4 @@
-# fchmodat.m4 serial 1
+# fchmodat.m4 serial 2
 dnl Copyright (C) 2004-2020 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -13,5 +13,51 @@ AC_DEFUN([gl_FUNC_FCHMODAT],
   AC_CHECK_FUNCS_ONCE([fchmodat lchmod])
   if test $ac_cv_func_fchmodat != yes; then
     HAVE_FCHMODAT=0
+  else
+    AC_CACHE_CHECK(
+      [whether fchmodat+AT_SYMLINK_NOFOLLOW works on non-symlinks],
+      [gl_cv_func_fchmodat_works],
+      [AC_RUN_IFELSE(
+         [AC_LANG_PROGRAM(
+            [
+              AC_INCLUDES_DEFAULT[
+              #include <fcntl.h>
+              #ifndef S_IRUSR
+               #define S_IRUSR 0400
+              #endif
+              #ifndef S_IWUSR
+               #define S_IWUSR 0200
+              #endif
+              #ifndef S_IRWXU
+               #define S_IRWXU 0700
+              #endif
+              #ifndef S_IRWXG
+               #define S_IRWXG 0070
+              #endif
+              #ifndef S_IRWXO
+               #define S_IRWXO 0007
+              #endif
+            ]],
+            [[
+              int permissive = S_IRWXU | S_IRWXG | S_IRWXO;
+              int desired = S_IRUSR | S_IWUSR;
+              static char const f[] = "conftest.fchmodat";
+              struct stat st;
+              if (creat (f, permissive) < 0)
+                return 1;
+              if (fchmodat (AT_FDCWD, f, desired, AT_SYMLINK_NOFOLLOW) != 0)
+                return 1;
+              if (stat (f, &st) != 0)
+                return 1;
+              return ! ((st.st_mode & permissive) == desired);
+            ]])],
+         [gl_cv_func_fchmodat_works=yes],
+         [gl_cv_func_fchmodat_works=no],
+         [gl_cv_func_fchmodat_works=$gl_cross_guess_normal])
+       rm -f conftest.fchmodat])
+    case $gl_cv_func_fchmodat_works in
+      *yes) ;;
+      *) REPLACE_FCHMODAT=1;;
+    esac
   fi
 ])
diff --git a/m4/lchmod.m4 b/m4/lchmod.m4
index eeca7ac20..68dab7a4a 100644
--- a/m4/lchmod.m4
+++ b/m4/lchmod.m4
@@ -1,4 +1,4 @@
-#serial 3
+#serial 4
 
 dnl Copyright (C) 2005-2006, 2008-2020 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
@@ -6,7 +6,7 @@ dnl gives unlimited permission to copy and/or distribute it,
 dnl with or without modifications, as long as this notice is preserved.
 
 dnl From Paul Eggert.
-dnl Provide a replacement for lchmod on hosts that lack it.
+dnl Provide a replacement for lchmod on hosts that lack a working version.
 
 AC_DEFUN([gl_FUNC_LCHMOD],
 [
@@ -15,8 +15,52 @@ AC_DEFUN([gl_FUNC_LCHMOD],
   dnl Persuade glibc <sys/stat.h> to declare lchmod().
   AC_REQUIRE([AC_USE_SYSTEM_EXTENSIONS])
 
-  AC_CHECK_FUNCS_ONCE([lchmod])
-  if test $ac_cv_func_lchmod = no; then
+  AC_CHECK_FUNCS_ONCE([fchmodat lchmod lstat])
+  if test "$ac_cv_func_lchmod" = no; then
     HAVE_LCHMOD=0
+  else
+    AC_CACHE_CHECK([whether lchmod works on non-symlinks],
+      [gl_cv_func_lchmod_works],
+      [AC_RUN_IFELSE(
+         [AC_LANG_PROGRAM(
+            [
+              AC_INCLUDES_DEFAULT[
+              #ifndef S_IRUSR
+               #define S_IRUSR 0400
+              #endif
+              #ifndef S_IWUSR
+               #define S_IWUSR 0200
+              #endif
+              #ifndef S_IRWXU
+               #define S_IRWXU 0700
+              #endif
+              #ifndef S_IRWXG
+               #define S_IRWXG 0070
+              #endif
+              #ifndef S_IRWXO
+               #define S_IRWXO 0007
+              #endif
+            ]],
+            [[
+              int permissive = S_IRWXU | S_IRWXG | S_IRWXO;
+              int desired = S_IRUSR | S_IWUSR;
+              static char const f[] = "conftest.lchmod";
+              struct stat st;
+              if (creat (f, permissive) < 0)
+                return 1;
+              if (lchmod (f, desired) != 0)
+                return 1;
+              if (stat (f, &st) != 0)
+                return 1;
+              return ! ((st.st_mode & permissive) == desired);
+            ]])],
+         [gl_cv_func_lchmod_works=yes],
+         [gl_cv_func_lchmod_works=no],
+         [gl_cv_func_lchmod_works=$gl_cross_guess_normal])
+       rm -f conftest.lchmod])
+    case $gl_cv_func_lchmod_works in
+      *yes) ;;
+      *) REPLACE_LCHMOD=1;;
+    esac
   fi
 ])
diff --git a/m4/sys_stat_h.m4 b/m4/sys_stat_h.m4
index d63df9ebf..a5f35d46d 100644
--- a/m4/sys_stat_h.m4
+++ b/m4/sys_stat_h.m4
@@ -1,4 +1,4 @@
-# sys_stat_h.m4 serial 31   -*- Autoconf -*-
+# sys_stat_h.m4 serial 32   -*- Autoconf -*-
 dnl Copyright (C) 2006-2020 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -88,9 +88,11 @@ AC_DEFUN([gl_SYS_STAT_H_DEFAULTS],
   HAVE_MKNOD=1;         AC_SUBST([HAVE_MKNOD])
   HAVE_MKNODAT=1;       AC_SUBST([HAVE_MKNODAT])
   HAVE_UTIMENSAT=1;     AC_SUBST([HAVE_UTIMENSAT])
+  REPLACE_FCHMODAT=0;   AC_SUBST([REPLACE_FCHMODAT])
   REPLACE_FSTAT=0;      AC_SUBST([REPLACE_FSTAT])
   REPLACE_FSTATAT=0;    AC_SUBST([REPLACE_FSTATAT])
   REPLACE_FUTIMENS=0;   AC_SUBST([REPLACE_FUTIMENS])
+  REPLACE_LCHMOD=0;     AC_SUBST([REPLACE_LCHMOD])
   REPLACE_LSTAT=0;      AC_SUBST([REPLACE_LSTAT])
   REPLACE_MKDIR=0;      AC_SUBST([REPLACE_MKDIR])
   REPLACE_MKFIFO=0;     AC_SUBST([REPLACE_MKFIFO])
diff --git a/modules/fchmodat b/modules/fchmodat
index 7a5e1a688..b0f06e862 100644
--- a/modules/fchmodat
+++ b/modules/fchmodat
@@ -12,17 +12,21 @@ sys_stat
 extensions
 at-internal     [test $HAVE_FCHMODAT = 0]
 dosname         [test $HAVE_FCHMODAT = 0]
-errno           [test $HAVE_FCHMODAT = 0]
+errno           [test $HAVE_FCHMODAT = 0 || test $REPLACE_FCHMODAT = 1]
 extern-inline   [test $HAVE_FCHMODAT = 0]
 fchdir          [test $HAVE_FCHMODAT = 0]
-fcntl-h         [test $HAVE_FCHMODAT = 0]
+fcntl-h         [test $HAVE_FCHMODAT = 0 || test $REPLACE_FCHMODAT = 1]
+fstatat         [test $REPLACE_FCHMODAT = 1]
+intprops        [test $HAVE_FCHMODAT = 0 || test $REPLACE_FCHMODAT = 1]
+lchmod          [test $HAVE_FCHMODAT = 0]
 openat-die      [test $HAVE_FCHMODAT = 0]
 openat-h        [test $HAVE_FCHMODAT = 0]
 save-cwd        [test $HAVE_FCHMODAT = 0]
+unistd          [test $HAVE_FCHMODAT = 0 || test $REPLACE_FCHMODAT = 1]
 
 configure.ac:
 gl_FUNC_FCHMODAT
-if test $HAVE_FCHMODAT = 0; then
+if test $HAVE_FCHMODAT = 0 || test $REPLACE_FCHMODAT = 1; then
   AC_LIBOBJ([fchmodat])
 fi
 gl_MODULE_INDICATOR([fchmodat]) dnl for lib/openat.h
diff --git a/modules/lchmod b/modules/lchmod
index a0ac7a534..e1054f6a4 100644
--- a/modules/lchmod
+++ b/modules/lchmod
@@ -2,17 +2,28 @@ Description:
 lchmod that is actually chmod (!) on hosts lacking lchmod
 
 Files:
+lib/lchmod.c
 m4/lchmod.m4
 
 Depends-on:
-sys_stat
+errno         [test $HAVE_LCHMOD = 0 || test $REPLACE_LCHMOD = 1]
 extensions
+fcntl-h       [test $HAVE_LCHMOD = 0 || test $REPLACE_LCHMOD = 1]
+fchmodat      [test $HAVE_LCHMOD = 0 || test $REPLACE_LCHMOD = 1]
+intprops      [test $HAVE_LCHMOD = 0 || test $REPLACE_LCHMOD = 1]
+lstat         [test $HAVE_LCHMOD = 0 || test $REPLACE_LCHMOD = 1]
+sys_stat
+unistd        [test $HAVE_LCHMOD = 0 || test $REPLACE_LCHMOD = 1]
 
 configure.ac:
 gl_FUNC_LCHMOD
+if test $HAVE_LCHMOD = 0 || test $REPLACE_LCHMOD = 1; then
+  AC_LIBOBJ([lchmod])
+fi
 gl_SYS_STAT_MODULE_INDICATOR([lchmod])
 
 Makefile.am:
+lib_SOURCES += lchmod.c
 
 Include:
 <sys/stat.h>
diff --git a/modules/sys_stat b/modules/sys_stat
index 867cc8549..93e0cf07e 100644
--- a/modules/sys_stat
+++ b/modules/sys_stat
@@ -59,9 +59,11 @@ sys/stat.h: sys_stat.in.h $(top_builddir)/config.status $(CXXDEFS_H) $(ARG_NONNU
 	      -e 's|@''HAVE_MKNOD''@|$(HAVE_MKNOD)|g' \
 	      -e 's|@''HAVE_MKNODAT''@|$(HAVE_MKNODAT)|g' \
 	      -e 's|@''HAVE_UTIMENSAT''@|$(HAVE_UTIMENSAT)|g' \
+	      -e 's|@''REPLACE_FCHMODAT''@|$(REPLACE_FCHMODAT)|g' \
 	      -e 's|@''REPLACE_FSTAT''@|$(REPLACE_FSTAT)|g' \
 	      -e 's|@''REPLACE_FSTATAT''@|$(REPLACE_FSTATAT)|g' \
 	      -e 's|@''REPLACE_FUTIMENS''@|$(REPLACE_FUTIMENS)|g' \
+	      -e 's|@''REPLACE_LCHMOD''@|$(REPLACE_LCHMOD)|g' \
 	      -e 's|@''REPLACE_LSTAT''@|$(REPLACE_LSTAT)|g' \
 	      -e 's|@''REPLACE_MKDIR''@|$(REPLACE_MKDIR)|g' \
 	      -e 's|@''REPLACE_MKFIFO''@|$(REPLACE_MKFIFO)|g' \
diff --git a/tests/test-fchmodat.c b/tests/test-fchmodat.c
index 395b2b78e..df0f604d8 100644
--- a/tests/test-fchmodat.c
+++ b/tests/test-fchmodat.c
@@ -22,6 +22,8 @@
 SIGNATURE_CHECK (fchmodat, int, (int, const char *, mode_t, int));
 
 #include <errno.h>
+#include <fcntl.h>
+#include <sys/stat.h>
 #include <unistd.h>
 
 #include "macros.h"
@@ -42,5 +44,13 @@ main (void)
     ASSERT (errno == EBADF);
   }
 
+  /* Test that fchmodat works on non-symlinks, when given
+     the AT_SYMLINK_NOFOLLOW flag.  */
+  {
+    ASSERT (close (creat ("file", 0600)) == 0);
+    ASSERT (fchmodat (AT_FDCWD, "file", 0700, AT_SYMLINK_NOFOLLOW) == 0);
+    ASSERT (unlink ("file") == 0);
+  }
+
   return 0;
 }
-- 
2.24.1