chmod: fix erroneous warnings with -R --changes

For files with "special" bits set, we would stat the relative
file name in the wrong directory, giving an erroneous ENOENT diagnostic.
This issue was introduced with commit v5.92-653-gc1994c1
which changed fts to not change directory on traversal.

* src/chmod.c (mode_changed): Use fts->fts_cwd_fd with fstatat rather
than stat.  All callers changed.
* tests/chmod/c-option.sh: Add a test case.
* NEWS: Mention the fix.
Fixes http://bugs.gnu.org/17035

NEWS

@@ -4,6 +4,9 @@ GNU coreutils NEWS                                    -*- outline -*-
 
 ** Bug fixes
 
+  chmod -Rc no longer issues erroneous warnings for files with special bits set.
+  [bug introduced in coreutils-6.0]
+
   cp -a, mv, and install --preserve-context, once again set the correct SELinux
   context for existing directories in the destination.  Previously they set
   the context of an existing directory to that of its last copied descendent.

src/chmod.c

@@ -111,7 +111,8 @@ static struct option const long_options[] =
    The old mode was OLD_MODE, but it was changed to NEW_MODE.  */
 
 static bool
-mode_changed (char const *file, mode_t old_mode, mode_t new_mode)
+mode_changed (int dir_fd, char const *file, char const *file_full_name,
+              mode_t old_mode, mode_t new_mode)
 {
   if (new_mode & (S_ISUID | S_ISGID | S_ISVTX))
     {
@@ -120,10 +121,11 @@ mode_changed (char const *file, mode_t old_mode, mode_t new_mode)
 
       struct stat new_stats;
 
-      if (stat (file, &new_stats) != 0)
+      if (fstatat (dir_fd, file, &new_stats, 0) != 0)
         {
           if (! force_silent)
-            error (0, errno, _("getting new attributes of %s"), quote (file));
+            error (0, errno, _("getting new attributes of %s"),
+                   quote (file_full_name));
           return false;
         }
 
@@ -283,7 +285,8 @@ process_file (FTS *fts, FTSENT *ent)
   if (verbosity != V_off)
     {
       bool changed = (chmod_succeeded
-                      && mode_changed (file, old_mode, new_mode));
+                      && mode_changed (fts->fts_cwd_fd, file, file_full_name,
+                                       old_mode, new_mode));
 
       if (changed || verbosity == V_high)
         {

tests/chmod/c-option.sh

@@ -37,4 +37,15 @@ case "$(cat out)" in
   *) cat out; fail=1 ;;
 esac
 
+# From V5.1.0 to 8.22 this would stat the wrong file and
+# give an erroneous ENOENT diagnostic
+mkdir -p a/b || framework_failure_
+# chmod g+s might fail as detailed in setgid.sh
+# but we don't care about those edge cases here
+chmod g+s a/b
+# This should never warn, but it did when special
+# bits are set on b (the common case under test)
+chmod -c -R g+w a 2>err
+compare /dev/null err || fail=1
+
 Exit $fail