[PATCH] linux: Fix stat layout divergence under pre-POSIX.1-2008 macros [BZ #32119]
Adhemerval Zanella Netto
adhemerval.zanella@linaro.org
Sat Aug 15 14:01:27 GMT 2026
Sign, I forgot to properly tested on armhf and the CI triggered a
regression because the patch does not update the internal __stat64_t64.
I will send a new version.
On 14/08/26 12:53, Adhemerval Zanella wrote:
> 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 <matthias.goergens@gmail.com>
> ---
> 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()
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