fchmodat: port better to MS-Windows etc.

MS-Windows problem reported by Bruno Haible in:
https://lists.gnu.org/r/bug-gnulib/2022-06/msg00041.html
Although I don’t use MS-Windows I see some related fstatat etc.
problems and am trying to fix them with this further patch.
* lib/fchmodat.c (fchmodat):
* lib/lchmod.c (lchmod):
* lib/lchown.c (lchown)
[!HAVE_LCHOWN && HAVE_CHOWN && !CHOWN_MODIFIES_SYMLINK]:
* lib/renameatu.c (renameatu)
[HAVE_RENAME && RENAME_TRAILING_SLASH_SOURCE_BUG]:
Use readlinkat/readlink instead of fstatat/lstat to test merely
whether a string names a symlink, as this avoids problems
with EOVERFLOW.  Also, I hope it works around the MS-Windows
issues that Bruno noted.
* m4/fchmodat.m4 (gl_PREREQ_FCHMODAT):
Check for readlinkat, not lchmod.
* m4/lchmod.m4 (gl_FUNC_LCHMOD): Do not require AC_CANONICAL_HOST
or check for lstat.
(gl_PREREQ_LCHMOD): Check for readlink.
* modules/lchown (Depends-on): Add readlink.  Do not depend on
lstat merely because !HAVE_LCHOWN.
* modules/renameatu (Depends-on): Add fstatat, readlinkat.

9 files changed, 56 insertions, 30 deletions

diff --git a/ChangeLog b/ChangeLog
index 2d0340b933..2daa6d8c81 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,29 @@
+2022-06-12  Paul Eggert  <eggert@cs.ucla.edu>
+
+	fchmodat: port better to MS-Windows etc.
+	MS-Windows problem reported by Bruno Haible in:
+	https://lists.gnu.org/r/bug-gnulib/2022-06/msg00041.html
+	Although I don’t use MS-Windows I see some related fstatat etc.
+	problems and am trying to fix them with this further patch.
+	* lib/fchmodat.c (fchmodat):
+	* lib/lchmod.c (lchmod):
+	* lib/lchown.c (lchown)
+	[!HAVE_LCHOWN && HAVE_CHOWN && !CHOWN_MODIFIES_SYMLINK]:
+	* lib/renameatu.c (renameatu)
+	[HAVE_RENAME && RENAME_TRAILING_SLASH_SOURCE_BUG]:
+	Use readlinkat/readlink instead of fstatat/lstat to test merely
+	whether a string names a symlink, as this avoids problems
+	with EOVERFLOW.  Also, I hope it works around the MS-Windows
+	issues that Bruno noted.
+	* m4/fchmodat.m4 (gl_PREREQ_FCHMODAT):
+	Check for readlinkat, not lchmod.
+	* m4/lchmod.m4 (gl_FUNC_LCHMOD): Do not require AC_CANONICAL_HOST
+	or check for lstat.
+	(gl_PREREQ_LCHMOD): Check for readlink.
+	* modules/lchown (Depends-on): Add readlink.  Do not depend on
+	lstat merely because !HAVE_LCHOWN.
+	* modules/renameatu (Depends-on): Add fstatat, readlinkat.
+
 2022-06-12  Bruno Haible  <bruno@clisp.org>
 
 	doc: Update O_PATH platforms list.
diff --git a/lib/fchmodat.c b/lib/fchmodat.c
index b233c366de..8ed4cb7398 100644
--- a/lib/fchmodat.c
+++ b/lib/fchmodat.c
@@ -83,9 +83,10 @@ fchmodat (int dir, char const *file, mode_t mode, int flags)
 # if NEED_FCHMODAT_NONSYMLINK_FIX
   if (flags == AT_SYMLINK_NOFOLLOW)
     {
-      struct stat st;
+#  if HAVE_READLINKAT
+      char readlink_buf[1];
 
-#  ifdef O_PATH
+#   ifdef O_PATH
       /* Open a file descriptor with O_NOFOLLOW, to make sure we don't
          follow symbolic links, if /proc is mounted.  O_PATH is used to
          avoid a failure if the file is not readable.
@@ -96,7 +97,7 @@ fchmodat (int dir, char const *file, mode_t mode, int flags)
 
       int err;
       char buf[1];
-      if (0 <= readlinkat (fd, "", buf, sizeof buf))
+      if (0 <= readlinkat (fd, "", readlink_buf, sizeof readlink_buf))
         err = EOPNOTSUPP;
       else if (errno == EINVAL)
         {
@@ -113,17 +114,16 @@ fchmodat (int dir, char const *file, mode_t mode, int flags)
       errno = err;
       if (0 <= err)
         return err == 0 ? 0 : -1;
-#  endif
+#   endif
 
       /* O_PATH + /proc is not supported.  */
-      int fstatat_result = fstatat (dir, file, &st, AT_SYMLINK_NOFOLLOW);
-      if (fstatat_result != 0)
-        return fstatat_result;
-      if (S_ISLNK (st.st_mode))
+
+      if (0 <= readlinkat (dir, file, readlink_buf, sizeof readlink_buf))
         {
           errno = EOPNOTSUPP;
           return -1;
         }
+#  endif
 
       /* Fall back on orig_fchmodat with no flags, despite a possible race.  */
       flags = 0;
diff --git a/lib/lchmod.c b/lib/lchmod.c
index b4cc0a8176..f21bf7f652 100644
--- a/lib/lchmod.c
+++ b/lib/lchmod.c
@@ -45,7 +45,10 @@
 int
 lchmod (char const *file, mode_t mode)
 {
-#ifdef O_PATH
+#if HAVE_READLINK
+  char readlink_buf[1];
+
+# ifdef O_PATH
   /* Open a file descriptor with O_NOFOLLOW, to make sure we don't
      follow symbolic links, if /proc is mounted.  O_PATH is used to
      avoid a failure if the file is not readable.
@@ -55,8 +58,7 @@ lchmod (char const *file, mode_t mode)
     return fd;
 
   int err;
-  char buf[1];
-  if (0 <= readlinkat (fd, "", buf, sizeof buf))
+  if (0 <= readlinkat (fd, "", readlink_buf, sizeof readlink_buf))
     err = EOPNOTSUPP;
   else if (errno == EINVAL)
     {
@@ -73,16 +75,11 @@ lchmod (char const *file, mode_t mode)
   errno = err;
   if (0 <= err)
     return err == 0 ? 0 : -1;
-#endif
+# endif
 
   /* O_PATH + /proc is not supported.  */
 
-#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))
+  if (0 <= readlink (file, readlink_buf, sizeof readlink_buf))
     {
       errno = EOPNOTSUPP;
       return -1;
diff --git a/lib/lchown.c b/lib/lchown.c
index 105c2d9990..8b0d871a27 100644
--- a/lib/lchown.c
+++ b/lib/lchown.c
@@ -45,9 +45,9 @@ lchown (const char *file, uid_t uid, gid_t gid)
 {
 # if HAVE_CHOWN
 #  if ! CHOWN_MODIFIES_SYMLINK
-  struct stat stats;
+  char readlink_buf[1];
 
-  if (lstat (file, &stats) == 0 && S_ISLNK (stats.st_mode))
+  if (0 <= readlink (file, readlink_buf, sizeof readlink_buf))
     {
       errno = EOPNOTSUPP;
       return -1;
diff --git a/lib/renameatu.c b/lib/renameatu.c
index b4e317d6e5..7ba186cae7 100644
--- a/lib/renameatu.c
+++ b/lib/renameatu.c
@@ -214,15 +214,16 @@ renameatu (int fd1, char const *src, int fd2, char const *dst,
           goto out;
         }
       strip_trailing_slashes (dst_temp);
-      if (fstatat (fd2, dst_temp, &dst_st, AT_SYMLINK_NOFOLLOW))
+      char readlink_buf[1];
+      if (readlinkat (fd2, dst_temp, readlink_buf, sizeof readlink_buf) < 0)
         {
-          if (errno != ENOENT)
+          if (errno != ENOENT && errno != EINVAL)
             {
               rename_errno = errno;
               goto out;
             }
         }
-      else if (S_ISLNK (dst_st.st_mode))
+      else
         goto out;
     }
 # endif /* RENAME_TRAILING_SLASH_SOURCE_BUG */
diff --git a/m4/fchmodat.m4 b/m4/fchmodat.m4
index a5cf95a88b..f743ce1b02 100644
--- a/m4/fchmodat.m4
+++ b/m4/fchmodat.m4
@@ -1,4 +1,4 @@
-# fchmodat.m4 serial 6
+# fchmodat.m4 serial 7
 dnl Copyright (C) 2004-2022 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -97,6 +97,6 @@ AC_DEFUN([gl_FUNC_FCHMODAT],
 # Prerequisites of lib/fchmodat.c.
 AC_DEFUN([gl_PREREQ_FCHMODAT],
 [
-  AC_CHECK_FUNCS_ONCE([lchmod])
+  AC_CHECK_FUNCS_ONCE([readlinkat])
   :
 ])
diff --git a/m4/lchmod.m4 b/m4/lchmod.m4
index 5baee738ef..bfc925fbe4 100644
--- a/m4/lchmod.m4
+++ b/m4/lchmod.m4
@@ -1,4 +1,4 @@
-#serial 8
+#serial 9
 
 dnl Copyright (C) 2005-2006, 2008-2022 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
@@ -15,9 +15,7 @@ AC_DEFUN([gl_FUNC_LCHMOD],
   dnl Persuade glibc <sys/stat.h> to declare lchmod().
   AC_REQUIRE([AC_USE_SYSTEM_EXTENSIONS])
 
-  AC_REQUIRE([AC_CANONICAL_HOST]) dnl for cross-compiles
-
-  AC_CHECK_FUNCS_ONCE([lchmod lstat])
+  AC_CHECK_FUNCS_ONCE([lchmod])
   if test "$ac_cv_func_lchmod" = no; then
     HAVE_LCHMOD=0
   fi
@@ -26,5 +24,6 @@ AC_DEFUN([gl_FUNC_LCHMOD],
 # Prerequisites of lib/lchmod.c.
 AC_DEFUN([gl_PREREQ_LCHMOD],
 [
+  AC_CHECK_FUNCS_ONCE([readlink])
   :
 ])
diff --git a/modules/lchown b/modules/lchown
index 335d191f9f..217dd75cce 100644
--- a/modules/lchown
+++ b/modules/lchown
@@ -7,11 +7,12 @@ m4/lchown.m4
 
 Depends-on:
 unistd
+readlink        [test $HAVE_LCHOWN = 0]
 chown           [test $HAVE_LCHOWN = 0 || test $REPLACE_LCHOWN = 1]
 errno           [test $HAVE_LCHOWN = 0 || test $REPLACE_LCHOWN = 1]
-lstat           [test $HAVE_LCHOWN = 0 || test $REPLACE_LCHOWN = 1]
 stdbool         [test $HAVE_LCHOWN = 0 || test $REPLACE_LCHOWN = 1]
 sys_stat        [test $HAVE_LCHOWN = 0 || test $REPLACE_LCHOWN = 1]
+lstat           [test $REPLACE_LCHOWN = 1]
 
 configure.ac:
 gl_FUNC_LCHOWN
diff --git a/modules/renameatu b/modules/renameatu
index e0c1ce3176..3fc68a91dc 100644
--- a/modules/renameatu
+++ b/modules/renameatu
@@ -13,6 +13,8 @@ extensions
 fcntl-h
 filenamecat-lgpl [test $HAVE_RENAMEAT = 0 || test $REPLACE_RENAMEAT = 1]
 openat-h         [test $HAVE_RENAMEAT = 0 || test $REPLACE_RENAMEAT = 1]
+fstatat          [test $REPLACE_RENAMEAT = 1]
+readlinkat       [test $REPLACE_RENAMEAT = 1]
 stdbool          [test $REPLACE_RENAMEAT = 1]
 at-internal      [test $HAVE_RENAMEAT = 0]
 filename         [test $HAVE_RENAMEAT = 0]