Re: [PATCH] Linux: Use 32-bit vDSO for clock_gettime, gettimeofday, time (bug 28071)

[Adhemerval Zanella via Libc-alpha <libc-alpha@sourceware.org>]

On 10/07/2021 17:00, Florian Weimer wrote:
> * Adhemerval Zanella:
> 
>> On 10/07/2021 15:54, Florian Weimer wrote:
>>> * Adhemerval Zanella:
>>>
>>>> This does not fix the issue for __ASSUME_TIME64_SYSCALLS where it still uses
>>>> INLINE_SYSCALL_CALL which might clobber the errno, besides adding another
>>>> ifdef code path (which I really want to avoid).  Instead I think we need to 
>>>> open-coded the INLINE_VSYSCALL macro and replace all INLINE_SYSCALL_CALL 
>>>> with INTERNAL_SYSCALL_CALLS:
>>>
>>> But for __ASSUME_TIME64_SYSCALLS, clock_gettime64 will not fail.
>>>
>>> What am I missing?  Is the issue that INLINE_VSYSCALL may set ENOSYS
>>> artificially?
>>
>> I meant for __clock_gettime64, where it may still clobber the errno
>> on older kernels (although it might be a fringe case).  In any case,
>> I still think making all time32 call to call time64 is a better
>> implementation than add some specific calls for time32.
> 
> So do you want to send an alternative patch?  I can add my tests on top
> of that.

Could you check the following?  I checked on a 5.11 kernel (64-bit vDSO),
4.4 kernel (32-bit vDSO without CLOCK_MONOTONIC support) and on a 
3.10 kernel (no vDSO support).

Using the test (a slight modified one from the bug report):

--
#include <time.h>
#include <stdio.h>
#include <errno.h>

int
main (void)
{
  struct timespec ts;
  errno = 0;
  clock_gettime (CLOCK_REALTIME, &ts);
  printf ("errno = %m (%d)\n", errno);
  errno = 0;
  clock_gettime (CLOCK_MONOTONIC, &ts);
  printf ("errno = %m (%d)\n", errno);
}
--

I see no syscall on 5.11 kernel, only a clock_gettime (CLOCK_MONOTONIC)
on the 4.4 and a clock_gettime_time64 plus a clock_gettime on the 3.10.

---

[PATCH] Linux: Use 32-bit vDSO for clock_gettime, gettimeofday, time
 (BZ# 28071)

The previous approach defeats the vDSO optimization on older kernels
because a failing clock_gettime64 system call is performed on every
function call.  It also results in a clobbered errno value, exposing
an OpenJDK bug (JDK-8270244).

This patch fixes by open-code INLINE_VSYSCALL macro and replace all
INLINE_SYSCALL_CALL with INTERNAL_SYSCALL_CALLS.  Now for
__clock_gettime64, the 64-bit vDSO is used and the 32-bit vDSO is
tried before falling back to 64-bit syscalls.

The previous code preferred 64-bit syscall for the case where the kernel
provides 64-bit time_t syscalls *and* also a 32-bit vDSO (in this case
the *64-bit* syscall should be preferable over the vDSO).  All
architectures that provides 32-bit vDSO (i386, mips, powerpc, s390)
modulo sparc; but I am not sure if some kernels versions do provide
only 32-bit vDSO while still providing 64-bit time_t syscall.
Regardless, for such cases the 64-bit time_t syscall is used if the
vDSO returns overflowed 32-bit time_t.

Tested on i686-linux-gnu (with a time64 and non-time64 kernel),
x86_64-linux-gnu.  Built with build-many-glibcs.py.

Co-authored-by: Florian Weimer <fweimer@redhat.com>
---
 sysdeps/unix/sysv/linux/Makefile              |  6 ++
 sysdeps/unix/sysv/linux/clock_gettime.c       | 50 ++++++++++++----
 .../sysv/linux/tst-clock_gettime-clobber.c    | 57 +++++++++++++++++++
 .../sysv/linux/tst-gettimeofday-clobber.c     | 37 ++++++++++++
 sysdeps/unix/sysv/linux/tst-time-clobber.c    | 36 ++++++++++++
 5 files changed, 174 insertions(+), 12 deletions(-)
 create mode 100644 sysdeps/unix/sysv/linux/tst-clock_gettime-clobber.c
 create mode 100644 sysdeps/unix/sysv/linux/tst-gettimeofday-clobber.c
 create mode 100644 sysdeps/unix/sysv/linux/tst-time-clobber.c

diff --git a/sysdeps/unix/sysv/linux/Makefile b/sysdeps/unix/sysv/linux/Makefile
index 7d43dc95f2..a4769aa9db 100644
--- a/sysdeps/unix/sysv/linux/Makefile
+++ b/sysdeps/unix/sysv/linux/Makefile
@@ -213,6 +213,12 @@ ifeq ($(subdir),time)
 sysdep_headers += sys/timex.h bits/timex.h
 
 sysdep_routines += ntp_gettime ntp_gettimex
+
+tests += \
+  tst-clock_gettime-clobber \
+  tst-gettimeofday-clobber \
+  tst-time-clobber \
+  # tests
 endif
 
 ifeq ($(subdir),signal)
diff --git a/sysdeps/unix/sysv/linux/clock_gettime.c b/sysdeps/unix/sysv/linux/clock_gettime.c
index cfe9370455..59b146702f 100644
--- a/sysdeps/unix/sysv/linux/clock_gettime.c
+++ b/sysdeps/unix/sysv/linux/clock_gettime.c
@@ -35,27 +35,53 @@ __clock_gettime64 (clockid_t clock_id, struct __timespec64 *tp)
 #endif
 
 #ifdef HAVE_CLOCK_GETTIME64_VSYSCALL
-  r = INLINE_VSYSCALL (clock_gettime64, 2, clock_id, tp);
-#else
-  r = INLINE_SYSCALL_CALL (clock_gettime64, clock_id, tp);
+  int (*vdso_time64)(clockid_t clock_id, struct __timespec64 *tp)
+    = GLRO(dl_vdso_clock_gettime64);
+  if (vdso_time64 != NULL)
+    {
+      r = INTERNAL_VSYSCALL_CALL (vdso_time64, 2, clock_id, tp);
+      if (r == 0)
+	return 0;
+      return INLINE_SYSCALL_ERROR_RETURN_VALUE (-r);
+    }
 #endif
 
-  if (r == 0 || errno != ENOSYS)
-    return r;
+#ifdef HAVE_CLOCK_GETTIME_VSYSCALL
+  int (*vdso_time)(clockid_t clock_id, struct timespec *tp)
+    = GLRO(dl_vdso_clock_gettime);
+  if (vdso_time != NULL)
+    {
+      struct timespec tp32;
+      r = INTERNAL_VSYSCALL_CALL (vdso_time, 2, clock_id, &tp32);
+      if (r == 0 && tp32.tv_sec > 0)
+	{
+	  *tp = valid_timespec_to_timespec64 (tp32);
+	  return 0;
+	}
+      else if (r != 0)
+	return INLINE_SYSCALL_ERROR_RETURN_VALUE (-r);
+      /* Fallback to syscall if time32 overflow.  */
+    }
+#endif
+
+  r = INTERNAL_SYSCALL_CALL (clock_gettime64, clock_id, tp);
+  if (r == 0)
+    return 0;
+  if (-r != ENOSYS)
+    return INLINE_SYSCALL_ERROR_RETURN_VALUE (-r);
 
 #ifndef __ASSUME_TIME64_SYSCALLS
   /* Fallback code that uses 32-bit support.  */
   struct timespec tp32;
-# ifdef HAVE_CLOCK_GETTIME_VSYSCALL
-  r = INLINE_VSYSCALL (clock_gettime, 2, clock_id, &tp32);
-# else
-  r = INLINE_SYSCALL_CALL (clock_gettime, clock_id, &tp32);
-# endif
+  r = INTERNAL_SYSCALL_CALL (clock_gettime, clock_id, &tp32);
   if (r == 0)
-    *tp = valid_timespec_to_timespec64 (tp32);
+    {
+      *tp = valid_timespec_to_timespec64 (tp32);
+      return 0;
+    }
 #endif
 
-  return r;
+  return INLINE_SYSCALL_ERROR_RETURN_VALUE (-r);
 }
 
 #if __TIMESIZE != 64
diff --git a/sysdeps/unix/sysv/linux/tst-clock_gettime-clobber.c b/sysdeps/unix/sysv/linux/tst-clock_gettime-clobber.c
new file mode 100644
index 0000000000..81a32bd15a
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/tst-clock_gettime-clobber.c
@@ -0,0 +1,57 @@
+/* Check that clock_gettime does not clobber errno on success.
+   Copyright (C) 2021 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 <errno.h>
+#include <time.h>
+#include <support/check.h>
+#include <stdio.h>
+
+static void
+test_clock (clockid_t clk)
+{
+  printf ("info: testing clock: %d\n", (int) clk);
+
+  for (int original_errno = 0; original_errno < 2; ++original_errno)
+    {
+      errno = original_errno;
+      struct timespec ts;
+      if (clock_gettime (clk, &ts) == 0)
+        TEST_COMPARE (errno, original_errno);
+    }
+}
+
+static int
+do_test (void)
+{
+  test_clock (CLOCK_BOOTTIME);
+  test_clock (CLOCK_BOOTTIME_ALARM);
+  test_clock (CLOCK_MONOTONIC);
+  test_clock (CLOCK_MONOTONIC_COARSE);
+  test_clock (CLOCK_MONOTONIC_RAW);
+  test_clock (CLOCK_PROCESS_CPUTIME_ID);
+  test_clock (CLOCK_REALTIME);
+  test_clock (CLOCK_REALTIME_ALARM);
+  test_clock (CLOCK_REALTIME_COARSE);
+#ifdef CLOCK_TAI
+  test_clock (CLOCK_TAI);
+#endif
+  test_clock (CLOCK_THREAD_CPUTIME_ID);
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/sysdeps/unix/sysv/linux/tst-gettimeofday-clobber.c b/sysdeps/unix/sysv/linux/tst-gettimeofday-clobber.c
new file mode 100644
index 0000000000..bb9bdb67b0
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/tst-gettimeofday-clobber.c
@@ -0,0 +1,37 @@
+/* Check that gettimeofday does not clobber errno.
+   Copyright (C) 2021 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 <errno.h>
+#include <stddef.h>
+#include <support/check.h>
+#include <sys/time.h>
+
+static int
+do_test (void)
+{
+  for (int original_errno = 0; original_errno < 2; ++original_errno)
+    {
+      errno = original_errno;
+      struct timeval tv;
+      gettimeofday (&tv, NULL);
+      TEST_COMPARE (errno, original_errno);
+    }
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/sysdeps/unix/sysv/linux/tst-time-clobber.c b/sysdeps/unix/sysv/linux/tst-time-clobber.c
new file mode 100644
index 0000000000..ad83be9be8
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/tst-time-clobber.c
@@ -0,0 +1,36 @@
+/* Check that time does not clobber errno.
+   Copyright (C) 2021 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 <errno.h>
+#include <stddef.h>
+#include <support/check.h>
+#include <time.h>
+
+static int
+do_test (void)
+{
+  for (int original_errno = 0; original_errno < 2; ++original_errno)
+    {
+      errno = original_errno;
+      time (NULL);
+      TEST_COMPARE (errno, original_errno);
+    }
+  return 0;
+}
+
+#include <support/test-driver.c>