Re: [PATCH 0/2] linux: Keep time64 stat layout independent of feature macros

On 05/08/26 05:03, Matthias Goergens wrote:
> Bug 32119 reports that feature-test macros change the public time64
> struct stat layout on 32-bit targets. The series separates two causes.
> 
> Patch 1 includes the endian definitions used by the shared helper. Without
> them, strict pre-POSIX.1-2008 modes select big-endian timestamp ordering on
> little-endian targets. This is an unambiguous layout bug.
> 
> Patch 2 removes two trailing reserved words which the helper exposes only in
> those strict modes. History shows that they were imported accidentally when
> the public and internal time64 declarations were consolidated. The stat
> implementation does not write them, and the default public and internal
> layouts omit them.
> 
> Patch 2 nevertheless changes sizeof(struct stat) for 32-bit objects already
> compiled with both 64-bit time and strict pre-POSIX.1-2008 feature macros.
> Please advise whether preserving that niche accidental layout is required by
> glibc ABI policy. Patch 1 is independent if the size correction is unsuitable.

I still strictly an ABI break, but I also think the blast radius is quite minimal.
The fields were unused by glibc, so it would be mostly between TU built with
different flags.

And I do not think it would worth adding compat symbols, but I also think it
would be good to backport this fix to all affected branches.  

> 
> An i386 build and the io tests pass. The regression compares timestamp
> offsets and sizes across translation units built in default and POSIX.1-2001
> modes. Compile-time layout models also pass for 32-bit Arm, PowerPC, and MIPS;
> these models are supplementary and are not full cross builds.

I think it would be better to move this test to script that uses glibcextracy.py,
so these tests can be checked in cross-compiling mode.  Something like:

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/tst-stat-layout-time64.py b/sysdeps/unix/sysv/linux/tst-stat-layout-time64.py
new file mode 100644
index 00000000000..2c6dd66a491
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/tst-stat-layout-time64.py
@@ -0,0 +1,94 @@
+#!/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):
+    # The nanosecond members are named differently in the two helper
+    # branches: st_Xtim.tv_nsec with __USE_XOPEN2K8, st_Xtimensec
+    #otherwise.
+    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()


And this approach uncovered a similar issue on arc and ork1, which are both
32-bit with 64-bit time_t by default.  Different than riscv32, another 32-bit
with 64-bit time_t, alignof(int64) == 4 and thus 'struct stat' size and
internal layout (sigh...) differ.

The fix would require to change de generic implementation
sysdeps/unix/sysv/linux/bits/struct_stat.h:

iff --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

I am not sure if this would require a different bug, but we will need to fix
it along with BZ#34466.


> 
> I do not have an FSF copyright assignment on file. This series is offered
> under the Developer Certificate of Origin 1.1, as certified by the
> Signed-off-by line in each commit.
> 
> Matthias Goergens (2):
>   linux: Fix time64 stat nanosecond layout in strict modes
>   linux: Keep time64 stat size independent of feature macros [BZ #32119]
> 
>  io/Makefile                                   | 15 +++++++
>  io/tst-stat-layout-time64-legacy.c            | 41 +++++++++++++++++++
>  io/tst-stat-layout-time64.c                   | 38 +++++++++++++++++
>  .../linux/bits/struct_stat_time64_helper.h    |  5 +--
>  4 files changed, 96 insertions(+), 3 deletions(-)
>  create mode 100644 io/tst-stat-layout-time64-legacy.c
>  create mode 100644 io/tst-stat-layout-time64.c
>