[PATCH] fstatat: support NULL if AT_EMPTY_PATH

On platforms that define AT_EMPTY_PATH, allow a null pointer to be
passed as a file name.  This is GNU behavior as of glibc 2.41 +
Linux kernel 6.11, and can be supported on older kernels, older
glibcs, and on recent FreeBSD and z/OS.
* lib/fstatat.c (rpl_fstatat): Support AT_EMPTY_PATH,
if it exists, on null pointers.
* m4/fstatat.m4 (gl_FUNC_FSTATAT): Replace fstatat if
AT_EMPTY_PATH is defined but does not work on null pointers.
Also, define HAVE_WORKING_FSTATAT_ZERO_FLAG, if it is discovered,
regardless of whether replacing fstatat.  This refactoring makes
config.h a bit less confusing.
* tests/test-fstatat.c (main): Test AT_EMPTY_PATH if defined.
---
 ChangeLog                        | 14 ++++++++++
 doc/glibc-functions/statx.texi   |  2 +-
 doc/posix-functions/fstatat.texi | 14 ++++++----
 lib/fstatat.c                    | 17 ++++++++---
 m4/fstatat.m4                    | 48 ++++++++++++++++++++++++++------
 tests/test-fstatat.c             | 12 ++++++++
 6 files changed, 88 insertions(+), 19 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index cc4ca63526..8b4a239eaf 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,19 @@
 2026-06-18  Paul Eggert  <[email protected]>
 
+	fstatat: support NULL if AT_EMPTY_PATH
+	On platforms that define AT_EMPTY_PATH, allow a null pointer to be
+	passed as a file name.  This is GNU behavior as of glibc 2.41 +
+	Linux kernel 6.11, and can be supported on older kernels, older
+	glibcs, and on recent FreeBSD and z/OS.
+	* lib/fstatat.c (rpl_fstatat): Support AT_EMPTY_PATH,
+	if it exists, on null pointers.
+	* m4/fstatat.m4 (gl_FUNC_FSTATAT): Replace fstatat if
+	AT_EMPTY_PATH is defined but does not work on null pointers.
+	Also, define HAVE_WORKING_FSTATAT_ZERO_FLAG, if it is discovered,
+	regardless of whether replacing fstatat.  This refactoring makes
+	config.h a bit less confusing.
+	* tests/test-fstatat.c (main): Test AT_EMPTY_PATH if defined.
+
 	fchmodat: fix AT_FDCWD + AT_EMPTY_PATH
 	* lib/fchmodat.c (fchmodat): When following symlinks and with
 	AT_FDCWD and an empty filename, don’t use fchmod as it will fail.
diff --git a/doc/glibc-functions/statx.texi b/doc/glibc-functions/statx.texi
index d57b488dd2..30c2a5cecd 100644
--- a/doc/glibc-functions/statx.texi
+++ b/doc/glibc-functions/statx.texi
@@ -18,7 +18,7 @@ statx
 @item
 When @code{AT_EMPTY_PATH} is used,
 this function does not allow the file name to be a null pointer:
-Linux kernel 6.10.
+glibc 2.40, Linux kernel 6.10.
 @item
 There is an incompatible function of the same name on some platforms:
 AIX 5.2 or newer.
diff --git a/doc/posix-functions/fstatat.texi b/doc/posix-functions/fstatat.texi
index 1f7ab3124b..45331beb51 100644
--- a/doc/posix-functions/fstatat.texi
+++ b/doc/posix-functions/fstatat.texi
@@ -13,9 +13,15 @@ fstatat
 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, Cygwin 1.5.x, mingw, MSVC 14.
-But the replacement function is not safe to be used in libraries and is not
+But when the function is missing, its Gnulib substitute
+is not safe to be used in libraries and is not
 thread-safe.
 @item
+On platforms with @code{AT_EMPTY_PATH},
+this function does not work when @code{AT_EMPTY_PATH}
+is used with a null pointer file name:
+glibc 2.40, Linux kernel 6.10, Cygwin 3.6, FreeBSD 15.1.
+@item
 On platforms where @code{off_t} is a 32-bit type, @code{fstatat} may
 not correctly report the size of files or block devices larger than 2
 GB@.  @xref{Large File Support}.
@@ -40,11 +46,7 @@ fstatat
 @item
 This function does not fail when the second argument is an empty string
 on some platforms, even when @code{AT_EMPTY_PATH} is not used:
-glibc 2.7, Linux 2.6.38.
-@item
-When @code{AT_EMPTY_PATH} is used,
-this function does not allow the file name to be a null pointer:
-Linux kernel 6.10, Cygwin 3.6, FreeBSD 15.1.
+glibc 2.7, Linux kernel 2.6.38.
 @item
 This function sets @code{st_ino} only to the low-order 32 bits of
 the inode number of a socket or pipe, which thus can disagree
diff --git a/lib/fstatat.c b/lib/fstatat.c
index 8d26e44501..6b0c564a21 100644
--- a/lib/fstatat.c
+++ b/lib/fstatat.c
@@ -46,6 +46,10 @@ orig_fstatat (int fd, char const *filename, struct stat *buf, int flags)
 #include <stdlib.h>
 #include <string.h>
 
+#ifndef AT_EMPTY_PATH
+# define AT_EMPTY_PATH 0
+#endif
+
 #if HAVE_FSTATAT && HAVE_WORKING_FSTATAT_ZERO_FLAG
 
 # ifndef LSTAT_FOLLOWS_SLASHED_SYMLINK
@@ -68,14 +72,19 @@ normal_fstatat (int fd, char const *file, struct stat *st, int flag)
    Work around this bug if FSTATAT_AT_FDCWD_0_BROKEN is nonzero.  */
 
 int
-rpl_fstatat (int fd, char const *file, struct stat *st, int flag)
+rpl_fstatat (int fd, char const *file, struct stat *st, int flags)
 {
-  int result = normal_fstatat (fd, file, st, flag);
+  /* Implement Linux kernel 6.11+ behavior on platforms that have
+     AT_EMPTY_PATH but do not support it on null pointers.  */
+  if (flags & AT_EMPTY_PATH && !file)
+    file = "";
+
+  int result = normal_fstatat (fd, file, st, flags);
   if (LSTAT_FOLLOWS_SLASHED_SYMLINK || result != 0)
     return result;
 
   size_t len = strlen (file);
-  if (flag & AT_SYMLINK_NOFOLLOW)
+  if (flags & AT_SYMLINK_NOFOLLOW)
     {
       /* Fix lstat behavior.  */
       if (file[len - 1] != '/' || S_ISDIR (st->st_mode))
@@ -85,7 +94,7 @@ rpl_fstatat (int fd, char const *file, struct stat *st, int flag)
           errno = ENOTDIR;
           return -1;
         }
-      result = normal_fstatat (fd, file, st, flag & ~AT_SYMLINK_NOFOLLOW);
+      result = normal_fstatat (fd, file, st, flags & ~AT_SYMLINK_NOFOLLOW);
     }
   /* Fix stat behavior.  */
   if (result == 0 && !S_ISDIR (st->st_mode) && file[len - 1] == '/')
diff --git a/m4/fstatat.m4 b/m4/fstatat.m4
index d53e8d9196..ceeb211d34 100644
--- a/m4/fstatat.m4
+++ b/m4/fstatat.m4
@@ -1,5 +1,5 @@
 # fstatat.m4
-# serial 5
+# serial 6
 dnl Copyright (C) 2004-2026 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -45,6 +45,11 @@ AC_DEFUN([gl_FUNC_FSTATAT]
           esac
          ])
       ])
+    AS_CASE([$gl_cv_func_fstatat_zero_flag],
+      [*yes],
+        [AC_DEFINE([HAVE_WORKING_FSTATAT_ZERO_FLAG], [1],
+           [Define to 1 if fstatat (..., 0) works.
+            For example, it does not work in AIX 7.1.])])
 
     case $gl_cv_func_fstatat_zero_flag+$gl_cv_func_lstat_dereferences_slashed_symlink in
     *yes+*yes) ;;
@@ -56,12 +61,39 @@ AC_DEFUN([gl_FUNC_FSTATAT]
         REPLACE_FSTATAT=1 ;;
     esac
 
-    case $REPLACE_FSTATAT,$gl_cv_func_fstatat_zero_flag in
-      1,*yes)
-         AC_DEFINE([HAVE_WORKING_FSTATAT_ZERO_FLAG], [1],
-           [Define to 1 if fstatat (..., 0) works.
-            For example, it does not work in AIX 7.1.])
-         ;;
-    esac
+    dnl Check for the AT_EMPTY_PATH compatibility issue with null pointers
+    dnl only if not already replacing fstatat.
+    dnl There is no need to AC_DEFINE anything here, as the Gnulib
+    dnl replacement works around the compatibility bug even if the bug
+    dnl is not present, and it is not worth the trouble to tune this.
+    AS_CASE([$REPLACE_FSTATAT],
+      [0],
+        [AC_CACHE_CHECK([whether fstatat+AT_EMPTY_PATH allows null file],
+           [gl_cv_func_fstatat_null_file],
+           [gl_saved_CFLAGS=$CFLAGS
+            CFLAGS="$CFLAGS -Wno-nonnull"
+            AC_RUN_IFELSE(
+              [AC_LANG_PROGRAM(
+                 [[#include <stddef.h>
+                   #include <fcntl.h>
+                   #include <sys/stat.h>
+                   #ifndef AT_EMPTY_PATH
+                    #define AT_EMPTY_PATH 0
+                   #endif
+                   #if __GLIBC__ && ! (2 < __GLIBC__ + (41 <= __GLIBC_MINOR__))
+                    #error "glibc 2.40 and earlier can fail with null file"
+                   #endif
+                 ]],
+                 [[struct stat st;
+                   return
+                     (AT_EMPTY_PATH
+                      && fstatat (AT_FDCWD, NULL, &st, AT_EMPTY_PATH) < 0);
+                 ]])],
+              [gl_cv_func_fstatat_null_file=yes],
+              [gl_cv_func_fstatat_null_file=no],
+              [gl_cv_func_fstatat_null_file="guessing no"])])
+         AS_CASE([$gl_cv_func_fstatat_null_file],
+           [*no],
+             [REPLACE_FSTATAT=1])])
   fi
 ])
diff --git a/tests/test-fstatat.c b/tests/test-fstatat.c
index bd25905ca0..bbcbaae31e 100644
--- a/tests/test-fstatat.c
+++ b/tests/test-fstatat.c
@@ -101,6 +101,18 @@ main (_GL_UNUSED int argc, _GL_UNUSED char *argv[])
   ASSERT (0 <= dfd);
   ASSERT (test_stat_func (do_stat, false) == result);
   ASSERT (test_lstat_func (do_lstat, false) == result);
+#ifdef AT_EMPTY_PATH
+  struct stat dotst;
+  ASSERT (stat (".", &dotst) == 0);
+  for (int i = 0; i < 2; i++)
+    for (int j = 0; j < 2; j++)
+      {
+        struct stat st;
+        ASSERT (fstatat (i ? AT_FDCWD : dfd, j ? "" : NULL, &st, AT_EMPTY_PATH)
+                == 0);
+        ASSERT (st.st_dev == dotst.st_dev && st.st_ino == dotst.st_ino);
+      }
+#endif
   ASSERT (close (dfd) == 0);
 
   /* FIXME - add additional tests of dfd not at current directory.  */
-- 
2.54.0