[PATCH v2] io: Do not implement fstat with fstatat

[PATCH v2] io: Do not implement fstat with fstatat

[Adhemerval Zanella]

AT_EMPTY_PATH is a requirement to implement fstat over fstatat,
however it does not prevent the kernel to read the path argument.
It is not an issue, but on x86-64 with SMAP-capable CPUs the kernel is
forced to perform expensive user memory access.  After that regular
lookup is performed which adds even more overhead.

Instead, issue the fstat syscall directly on LFS fstat implementation
(32 bit architectures will still continue to use statx, which is
required to have 64 bit time_t support).  it should be even a
small performance gain on non x86_64, since there is no need
to handle the path argument.

Checked on x86_64-linux-gnu.
---
 sysdeps/unix/sysv/linux/fstat64.c       | 37 +++++++++++++++++++++++--
 sysdeps/unix/sysv/linux/fstatat64.c     | 12 ++------
 sysdeps/unix/sysv/linux/internal-stat.h | 31 +++++++++++++++++++++
 3 files changed, 68 insertions(+), 12 deletions(-)
 create mode 100644 sysdeps/unix/sysv/linux/internal-stat.h

diff --git a/sysdeps/unix/sysv/linux/fstat64.c b/sysdeps/unix/sysv/linux/fstat64.c
index 124384e57f..a291f0825b 100644
--- a/sysdeps/unix/sysv/linux/fstat64.c
+++ b/sysdeps/unix/sysv/linux/fstat64.c
@@ -19,20 +19,53 @@
 #define __fstat __redirect___fstat
 #define fstat   __redirect_fstat
 #include <sys/stat.h>
+#undef __fstat
+#undef fstat
 #include <fcntl.h>
-#include <kernel_stat.h>
-#include <stat_t64_cp.h>
+#include <internal-stat.h>
 #include <errno.h>
 
 int
 __fstat64_time64 (int fd, struct __stat64_t64 *buf)
 {
+#if !FSTATAT_USE_STATX
+# if XSTAT_IS_XSTAT64
+#  ifdef __NR_fstat
+  /* 64-bit kABI, e.g. aarch64, ia64, powerpc64*, s390x, riscv64, and
+     x86_64.  */
+  return INLINE_SYSCALL_CALL (fstat, fd, buf);
+#  elif defined __NR_fstat64
+#   if STAT64_IS_KERNEL_STAT64
+  /* 64-bit kABI outlier, e.g. alpha  */
+  return INLINE_SYSCALL_CALL (fstat64, fd, buf);
+#   else
+  /* 64-bit kABI outlier, e.g. sparc64.  */
+  struct kernel_stat64 kst64;
+  int r = INLINE_SYSCALL_CALL (fstat64, fd, &kst64);
+  if (r == 0)
+    __cp_stat64_kstat64 (buf, &kst64);
+  return r;
+#   endif /* STAT64_IS_KERNEL_STAT64 */
+#  endif
+# else /* XSTAT_IS_XSTAT64 */
+  /* 64-bit kabi outlier, e.g. mips64 and mips64-n32.  */
+  struct kernel_stat kst;
+  int r = INLINE_SYSCALL_CALL (fstat, fd, &kst);
+  if (r == 0)
+    __cp_kstat_stat64_t64 (&kst, buf);
+  return r;
+# endif
+#else /* !FSTATAT_USE_STATX  */
+  /* All kABIs with non-LFS support and with old 32-bit time_t support
+     e.g. arm, csky, i386, hppa, m68k, microblaze, nios2, sh, powerpc32,
+     and sparc32.  */
   if (fd < 0)
     {
       __set_errno (EBADF);
       return -1;
     }
   return __fstatat64_time64 (fd, "", buf, AT_EMPTY_PATH);
+#endif
 }
 #if __TIMESIZE != 64
 hidden_def (__fstat64_time64)
diff --git a/sysdeps/unix/sysv/linux/fstatat64.c b/sysdeps/unix/sysv/linux/fstatat64.c
index 3509d3ca6d..127c6ff601 100644
--- a/sysdeps/unix/sysv/linux/fstatat64.c
+++ b/sysdeps/unix/sysv/linux/fstatat64.c
@@ -21,12 +21,10 @@
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <string.h>
-#include <kernel_stat.h>
 #include <sysdep.h>
 #include <time.h>
-#include <kstat_cp.h>
-#include <stat_t64_cp.h>
 #include <sys/sysmacros.h>
+#include <internal-stat.h>
 
 #if __TIMESIZE == 64 \
      && (__WORDSIZE == 32 \
@@ -40,11 +38,7 @@ _Static_assert (sizeof (__blkcnt_t) == sizeof (__blkcnt64_t),
                 "__blkcnt_t and __blkcnt64_t must match");
 #endif
 
-#if (__WORDSIZE == 32 \
-     && (!defined __SYSCALL_WORDSIZE || __SYSCALL_WORDSIZE == 32)) \
-     || defined STAT_HAS_TIME32 \
-     || (!defined __NR_newfstatat && !defined __NR_fstatat64)
-# define FSTATAT_USE_STATX 1
+#if FSTATAT_USE_STATX
 
 static inline int
 fstatat64_time64_statx (int fd, const char *file, struct __stat64_t64 *buf,
@@ -79,8 +73,6 @@ fstatat64_time64_statx (int fd, const char *file, struct __stat64_t64 *buf,
 
   return r;
 }
-#else
-# define FSTATAT_USE_STATX 0
 #endif
 
 /* Only statx supports 64-bit timestamps for 32-bit architectures with
diff --git a/sysdeps/unix/sysv/linux/internal-stat.h b/sysdeps/unix/sysv/linux/internal-stat.h
new file mode 100644
index 0000000000..e3b0569853
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/internal-stat.h
@@ -0,0 +1,31 @@
+/* Internal stat definitions.
+   Copyright (C) 2023 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/>.  */
+
+#include <sysdep.h>
+#include <stat_t64_cp.h>
+#include <kernel_stat.h>
+#include <kstat_cp.h>
+
+#if (__WORDSIZE == 32 \
+     && (!defined __SYSCALL_WORDSIZE || __SYSCALL_WORDSIZE == 32)) \
+     || defined STAT_HAS_TIME32 \
+     || (!defined __NR_newfstatat && !defined __NR_fstatat64)
+# define FSTATAT_USE_STATX 1
+#else
+# define FSTATAT_USE_STATX 0
+#endif
-- 
2.34.1

Re: [PATCH v2] io: Do not implement fstat with fstatat

[Andreas Schwab]

On Sep 11 2023, Adhemerval Zanella wrote:

> AT_EMPTY_PATH is a requirement to implement fstat over fstatat,
> however it does not prevent the kernel to read the path argument.
> It is not an issue, but on x86-64 with SMAP-capable CPUs the kernel is
> forced to perform expensive user memory access.  After that regular
> lookup is performed which adds even more overhead.
>
> Instead, issue the fstat syscall directly on LFS fstat implementation
> (32 bit architectures will still continue to use statx, which is
> required to have 64 bit time_t support).  it should be even a
> small performance gain on non x86_64, since there is no need
> to handle the path argument.

Ok.

-- 
Andreas Schwab, SUSE Labs, schwab@suse.de
GPG Key fingerprint = 0196 BAD8 1CE9 1970 F4BE  1748 E4D4 88E3 0EEA B9D7
"And now for something completely different."