[PATCH] linux: Fix stat layout divergence under pre-POSIX.1-2008 macros [BZ #32119]

The struct stat layout must not depend on the feature-test macros in
use. Currently there are three divergences:

  - bits/struct_stat_time64_helper.h tests __BYTE_ORDER without
    including <bits/endian.h>.  In strict pre-POSIX.1-2008 modes both
    operands are undefined, the big-endian member order is selected
    on little-endian targets, displacing the time64 nanosecond fields
    on targets whose bits/struct_stat.h does not already include
    <bits/endian.h> (x86, mips o32/n32, microblazeel).

  - The same helper adds two reserved words only when POSIX.1-2008
    interfaces are hidden, making the public time64 structure larger
    under older feature-test macros.

  - The generic bits/struct_stat.h declares the strict-mode timestamps
    as a bare seconds/nanoseconds pair with no endianness handling or
    explicit padding.  On 32-bit ports with 64-bit time_t whose ABI
    aligns 64-bit types to 4 bytes, the timestamps end up packed 12
    bytes apart instead of 16 (arc); on big-endian the nanoseconds also
    land on the wrong side of the padding (or1k).  This triggers with
    plain -D_POSIX_C_SOURCE=200112L, without any _TIME_BITS opt-in,
    since time_t is always 64-bit on such ports.

Fix the helper by including <bits/endian.h>, and give the generic
header the same endianness-aware nanosecond padding as
<bits/types/struct_timespec.h>.

For the reserved words, instead of removing them unify all
feature-test-macro modes on the largest layout.  The internal
__stat64_t64 does not write on them, so libc keeps writing the same
number of bytes and previously built objects using the smaller
default-mode structure remain correct (newly built objects will
over-allocate bytes).

The resulting sizeof (struct stat) changes, with _TIME_BITS=64 for
the first two rows (the time32 layouts are unchanged) and with any
feature-test macro combination on the time64-only arc and or1k:

  ABI                            default and       pre-POSIX.1-2008
                                 POSIX.1-2008      strict modes
  i686, csky, m68k, microblaze,
  sh                             108 -> 116        116 (unchanged)
  arm, hppa, mips o32/n32,
  powerpc32, sparc32             112 -> 120        120 (unchanged)
  arc, or1k                      128 (unchanged)   116 -> 128

Add a compile-only Python test based on glibcextract that compares
sizeof and the member offsets of struct stat across several feature
modes on all configurations.

Co-authored-by: Matthias Goergens <[email protected]>
---
 sysdeps/unix/sysv/linux/Makefile              | 11 +++
 sysdeps/unix/sysv/linux/bits/struct_stat.h    | 36 +++++---
 .../linux/bits/struct_stat_time64_helper.h    | 13 ++-
 sysdeps/unix/sysv/linux/struct_stat_time64.h  |  2 +
 .../unix/sysv/linux/tst-stat-layout-time64.py | 91 +++++++++++++++++++
 5 files changed, 138 insertions(+), 15 deletions(-)
 create mode 100644 sysdeps/unix/sysv/linux/tst-stat-layout-time64.py

diff --git a/sysdeps/unix/sysv/linux/Makefile b/sysdeps/unix/sysv/linux/Makefile
index 14a56d5cc3f..a5811977d4a 100644
--- a/sysdeps/unix/sysv/linux/Makefile
+++ b/sysdeps/unix/sysv/linux/Makefile
@@ -413,6 +413,17 @@ $(objpfx)tst-sched-consts.out: ../sysdeps/unix/sysv/linux/tst-sched-consts.py
 	  < /dev/null > $@ 2>&1; $(evaluate-test)
 $(objpfx)tst-sched-consts.out: $(sysdeps-linux-python-deps)
 
+tests-special += \
+  $(objpfx)tst-stat-layout-time64.out \
+  # tests-special
+$(objpfx)tst-stat-layout-time64.out: \
+  ../sysdeps/unix/sysv/linux/tst-stat-layout-time64.py
+	$(sysdeps-linux-python) \
+	  ../sysdeps/unix/sysv/linux/tst-stat-layout-time64.py \
+	    $(sysdeps-linux-python-cc) \
+	  < /dev/null > $@ 2>&1; $(evaluate-test)
+$(objpfx)tst-stat-layout-time64.out: $(sysdeps-linux-python-deps)
+
 tst-rseq-disable-TUNABLES += glibc.pthread.rseq=0
 tst-rseq-disable-static-TUNABLES += glibc.pthread.rseq=0
 
diff --git a/sysdeps/unix/sysv/linux/bits/struct_stat.h b/sysdeps/unix/sysv/linux/bits/struct_stat.h
index e912c3f6ba5..cdbe1f8f762 100644
--- a/sysdeps/unix/sysv/linux/bits/struct_stat.h
+++ b/sysdeps/unix/sysv/linux/bits/struct_stat.h
@@ -41,6 +41,22 @@
   int __##name##_pad __attribute__((__aligned__ (__alignof__ (type64)))); type name
 #endif
 
+/* The pre-POSIX.1-2008 timestamp fields must match the layout of the
+   'struct timespec' members used in the POSIX.1-2008 case, including the
+   padding required when the seconds field is wider than the word size.  */
+#if __WORDSIZE == 64 \
+  || (defined __SYSCALL_WORDSIZE && __SYSCALL_WORDSIZE == 64) \
+  || (__TIMESIZE == 32 && !defined __USE_TIME64_REDIRECTS)
+# define __fieldts(name) \
+  __time_t name; unsigned long int name ## nsec
+#elif __BYTE_ORDER == __BIG_ENDIAN
+# define __fieldts(name) \
+  __time_t name; int: 32; unsigned long int name ## nsec
+#else
+# define __fieldts(name) \
+  __time_t name; unsigned long int name ## nsec; int: 32
+#endif
+
 struct stat
   {
     __dev_t st_dev;		/* Device.  */
@@ -69,12 +85,9 @@ struct stat
 # define st_mtime st_mtim.tv_sec
 # define st_ctime st_ctim.tv_sec
 #else
-    __time_t st_atime;			/* Time of last access.  */
-    unsigned long int st_atimensec;	/* Nscecs of last access.  */
-    __time_t st_mtime;			/* Time of last modification.  */
-    unsigned long int st_mtimensec;	/* Nsecs of last modification.  */
-    __time_t st_ctime;			/* Time of last status change.  */
-    unsigned long int st_ctimensec;	/* Nsecs of last status change.  */
+    __fieldts (st_atime);		/* Time of last access.  */
+    __fieldts (st_mtime);		/* Time of last modification.  */
+    __fieldts (st_ctime);		/* Time of last status change.  */
 #endif
     int __glibc_reserved[2];
   };
@@ -107,17 +120,16 @@ struct stat64
     struct timespec st_mtim;		/* Time of last modification.  */
     struct timespec st_ctim;		/* Time of last status change.  */
 #else
-    __time_t st_atime;			/* Time of last access.  */
-    unsigned long int st_atimensec;	/* Nscecs of last access.  */
-    __time_t st_mtime;			/* Time of last modification.  */
-    unsigned long int st_mtimensec;	/* Nsecs of last modification.  */
-    __time_t st_ctime;			/* Time of last status change.  */
-    unsigned long int st_ctimensec;	/* Nsecs of last status change.  */
+    __fieldts (st_atime);		/* Time of last access.  */
+    __fieldts (st_mtime);		/* Time of last modification.  */
+    __fieldts (st_ctime);		/* Time of last status change.  */
 #endif
     int __glibc_reserved[2];
   };
 #endif
 
+#undef __fieldts
+
 /* Tell code we have these members.  */
 #define	_STATBUF_ST_BLKSIZE
 #define _STATBUF_ST_RDEV
diff --git a/sysdeps/unix/sysv/linux/bits/struct_stat_time64_helper.h b/sysdeps/unix/sysv/linux/bits/struct_stat_time64_helper.h
index e0b78b314ad..7cbb664d6df 100644
--- a/sysdeps/unix/sysv/linux/bits/struct_stat_time64_helper.h
+++ b/sysdeps/unix/sysv/linux/bits/struct_stat_time64_helper.h
@@ -16,6 +16,8 @@
    License along with the GNU C Library.  If not, see
    <https://www.gnu.org/licenses/>.  */
 
+#include <bits/endian.h>
+
   /* Content of internal __stat64_t64 struct.  */
   __dev_t st_dev;		/* Device.  */
   __ino64_t st_ino;		/* file serial number.	*/
@@ -59,8 +61,13 @@
   __fieldts64 (st_mtime);
   __fieldts64 (st_ctime);
 
-  unsigned long int __glibc_reserved4;
-  unsigned long int __glibc_reserved5;
-
 # undef __fieldts64
 #endif
+
+#ifndef __struct_stat_time64_internal
+  /* Trailing padding, never set by the stat functions.  It keeps
+     sizeof (struct stat) as the largest one for all feature-test-macro
+     modes.  */
+  unsigned long int __glibc_reserved4;
+  unsigned long int __glibc_reserved5;
+#endif
diff --git a/sysdeps/unix/sysv/linux/struct_stat_time64.h b/sysdeps/unix/sysv/linux/struct_stat_time64.h
index e3b878e1bed..38e603febdb 100644
--- a/sysdeps/unix/sysv/linux/struct_stat_time64.h
+++ b/sysdeps/unix/sysv/linux/struct_stat_time64.h
@@ -27,8 +27,10 @@
 struct __stat64_t64
   {
 # define __struct_timespec struct __timespec64
+# define __struct_stat_time64_internal 1
 # include <bits/struct_stat_time64_helper.h>
   };
+# undef __struct_stat_time64_internal
 #endif /* __TIMESIZE == 64  */
 
 #endif /* _BITS_STRUCT_STAT_TIME64_H  */
diff --git a/sysdeps/unix/sysv/linux/tst-stat-layout-time64.py b/sysdeps/unix/sysv/linux/tst-stat-layout-time64.py
new file mode 100644
index 00000000000..288a03bb661
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/tst-stat-layout-time64.py
@@ -0,0 +1,91 @@
+#!/usr/bin/python3
+# Check that feature-test macros do not change the time64 stat layout.
+# Copyright (C) 2026 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/>.
+
+import argparse
+import sys
+
+import glibcextract
+
+MODES = {
+    'POSIX.1-1996': '#define _POSIX_C_SOURCE 199506L',
+    'POSIX.1-2001': '#define _POSIX_C_SOURCE 200112L',
+    'POSIX.1-2008': '#define _POSIX_C_SOURCE 200809L',
+    'XPG6': '#define _XOPEN_SOURCE 600',
+}
+
+
+def compute_stat_layout(cc, mode_define):
+    sym_data = [
+        '#undef _GNU_SOURCE',
+        mode_define,
+        '#define _TIME_BITS 64',
+        '#define _FILE_OFFSET_BITS 64',
+        '#include <stddef.h>',
+        '#include <sys/stat.h>',
+        'START',
+        ('sizeof_stat', 'sizeof (struct stat)'),
+        ('st_dev', 'offsetof (struct stat, st_dev)'),
+        ('st_ino', 'offsetof (struct stat, st_ino)'),
+        ('st_mode', 'offsetof (struct stat, st_mode)'),
+        ('st_nlink', 'offsetof (struct stat, st_nlink)'),
+        ('st_uid', 'offsetof (struct stat, st_uid)'),
+        ('st_gid', 'offsetof (struct stat, st_gid)'),
+        ('st_rdev', 'offsetof (struct stat, st_rdev)'),
+        ('st_size', 'offsetof (struct stat, st_size)'),
+        ('st_blksize', 'offsetof (struct stat, st_blksize)'),
+        ('st_blocks', 'offsetof (struct stat, st_blocks)'),
+        ('st_atime', 'offsetof (struct stat, st_atime)'),
+        ('st_mtime', 'offsetof (struct stat, st_mtime)'),
+        ('st_ctime', 'offsetof (struct stat, st_ctime)'),
+        '#ifdef __USE_XOPEN2K8',
+        ('st_atimensec', 'offsetof (struct stat, st_atim.tv_nsec)'),
+        ('st_mtimensec', 'offsetof (struct stat, st_mtim.tv_nsec)'),
+        ('st_ctimensec', 'offsetof (struct stat, st_ctim.tv_nsec)'),
+        '#else',
+        ('st_atimensec', 'offsetof (struct stat, st_atimensec)'),
+        ('st_mtimensec', 'offsetof (struct stat, st_mtimensec)'),
+        ('st_ctimensec', 'offsetof (struct stat, st_ctimensec)'),
+        '#endif',
+    ]
+    return glibcextract.compute_c_consts(sym_data, cc)
+
+
+def main():
+    parser = argparse.ArgumentParser(
+        description='Check that feature-test macros do not change '
+        'the time64 stat layout.')
+    parser.add_argument('--cc', metavar='CC',
+                        help='C compiler (including options) to use')
+    args = parser.parse_args()
+    default_layout = compute_stat_layout(args.cc, '#define _GNU_SOURCE 1')
+    status = 0
+    for mode, mode_define in sorted(MODES.items()):
+        mode_layout = compute_stat_layout(args.cc, mode_define)
+        for name, value in default_layout.items():
+            if mode_layout[name] != value:
+                print('FAIL: %s: %s is %s, %s in default mode'
+                      % (mode, name, mode_layout[name], value))
+                status = 1
+    if status == 0:
+        print('PASS: struct stat layout is feature-test-macro invariant')
+    sys.exit(status)
+
+
+if __name__ == '__main__':
+    main()
-- 
2.53.0