[v3,4/5] y2038: linux: Provide __clock_settime64 implementation
diff mbox series

Message ID 20190507131848.30980-5-lukma@denx.de
State New
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Series
  • y2038: Linux: Provide __clock_settime function supporting 64 bit time
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Commit Message

Lukasz Majewski May 7, 2019, 1:18 p.m. UTC
This patch provides new __clock_settime64 explicit 64 bit function for
setting the time. Moreover, a 32 bit version - __clock_settime has been
refactored to internally use __clock_settime64.

The __clock_settime is now supposed to be used on systems still supporting
32 bit time - hence the necessary checks and conversion to 64 bit type.
After this change it is intrinsically Y2038 safe.

The new 64 bit syscall (clock_settime64) available from Linux
5.1+ has been used when applicable on 32 bit systems.

The __ASSUME_TIME64_SYSCALLS flag indicates if the Linux kernel provides
64 bit version of clock_settime (i.e. clock_settime64). If this syscall is
not provided by the kernel - the 32 bit version of it is executed instead.

When working on 32 bit systems without Y2038 time support the clock_settime
returns error when one wants to set time with wrong (overflowed) tv_sec
value. Moreover, the correctness of tv_nsec is checked.

In this patch the internal padding (tv_pad) of struct __timespec64 is
left untouched (on 32 bit systems) as Linux kernel ignores upper 32 bits
of tv_nsec.

The execution path on 64 bit systems has not been changed or affected in
any way.

Tests:
- The code has been tested with x86_64/x86 (native compilation):
make PARALLELMFLAGS="-j8" && make xcheck PARALLELMFLAGS="-j8"

- Run specific tests on ARM/x86 32bit systems (qemu):
https://github.com/lmajewski/meta-y2038
and run tests:
https://github.com/lmajewski/y2038-tests/commits/master
on kernels with and without 64 bit time support.

No regressions were observed.

* include/time.h (__clock_settime64):
  Add __clock_settime alias according to __TIMESIZE define
* sysdeps/unix/sysv/linux/clock_settime.c (__clock_settime):
  Refactor this function to be used only on 32 bit machines as a wrapper
  on __clock_settime64.
* sysdeps/unix/sysv/linux/clock_settime.c (__clock_settime64): Add
* sysdeps/unix/sysv/linux/clock_settime.c (__clock_settime64):
  Use clock_settime64 kernel syscall (available from 5.1-rc1+ Linux) by
  32 bit Y2038 safe systems

---
Changes for v3:
- Rename __ASSUME_64BIT_TIME to __ASSUME_TIME64_SYSCALLS
- Refactor in-code comment (add information regarding Linux kernel ignorance
  of padding
- Do not use __TIMESIZE to select main execution path (for Y2038 systems
  __TIMESIZE would be changed from 32 to 64 bits at some point to indicate
  full Y2038 support

Changes for v2:
- Add support for __ASSUME_64BIT_TIME flag when Linux kernel provides syscalls
  supporting 64 bit time on 32 bit systems
- Provide fallback to 32 bit version of clock_settime when clock_settime64
  is not available
- Do not copy *tp to timespec - this seems like an overkill as in clock_settime()
  the 32 bit struct timespec is copied to internal 64 bit struct __timespec64
---
 include/time.h                          |  8 +++++
 sysdeps/unix/sysv/linux/clock_settime.c | 53 +++++++++++++++++++++++++++++++--
 2 files changed, 58 insertions(+), 3 deletions(-)

Comments

Joseph Myers May 7, 2019, 4:40 p.m. UTC | #1
On Tue, 7 May 2019, Lukasz Majewski wrote:

> +#ifdef __ASSUME_TIME64_SYSCALLS
> +# ifdef __NR_clock_settime64
> +  /* For Y2038 safe systems with __WORDSIZE==32 and __TIMESIZE==64
> +     (x86, arm) the glibc exported struct timespec has 64 bit tv_sec,
> +     32 bit tv_nsec (to be still POSIX compliant -> long tv_nsec )
> +     and 32 bits of unnamed padding.
> +
> +     It may happen that due to dynamic allocation the tv_pad, which
> +     corresponds to upper 32 bits of kernel's 64 bit tv_nsec accepted
> +     by syscalls, may not be zero.
> +
> +     However, the Linux kernel is ignoring those 32 bits (to be more
> +     precise - as of 5.1 - it casts 64 bit tv_nsec to internal's 32 bit
> +     representation) and hence the padding clearing is not needed.  */
> +  int ret = INLINE_SYSCALL_CALL (clock_settime64, clock_id, tp);
> +  if (ret == 0 || errno != ENOSYS)
> +    return ret;
> +# endif

This logic is wrong.  If __ASSUME_TIME64_SYSCALLS is defined, then:

(a) There should be no special ENOSYS check, as the syscall is guaranteed 
to be present; just return its return value.

(b) There is no need for a check of whether __NR_clock_settime64 is 
defined, because the kernel headers are always at least as new as the 
minimum runtime kernel version; a build failure if __NR_clock_settime64 is 
somehow not defined, despite __ASSUME_TIME64_SYSCALLS being defined, is 
entirely appropriate.

Tests of __NR_clock_settime64 being defined are entirely appropriate 
if __ASSUME_TIME64_SYSCALLS is *not* defined; likewise runtime tests for 
an ENOSYS return to determine whether in fact the syscall existed or 
whether a fallback to the older syscalls is needed.

Patch
diff mbox series

diff --git a/include/time.h b/include/time.h
index 18587fdd8b..670226df0c 100644
--- a/include/time.h
+++ b/include/time.h
@@ -127,6 +127,14 @@  extern __time64_t __timegm64 (struct tm *__tp) __THROW;
 libc_hidden_proto (__timegm64)
 #endif
 
+#if __TIMESIZE == 64
+# define __clock_settime64 __clock_settime
+#else
+extern int __clock_settime64 (clockid_t clock_id,
+                              const struct __timespec64 *tp);
+libc_hidden_proto (__clock_settime64)
+#endif
+
 /* Compute the `struct tm' representation of T,
    offset OFFSET seconds east of UTC,
    and store year, yday, mon, mday, wday, hour, min, sec into *TP.
diff --git a/sysdeps/unix/sysv/linux/clock_settime.c b/sysdeps/unix/sysv/linux/clock_settime.c
index d837e3019c..084edeaa61 100644
--- a/sysdeps/unix/sysv/linux/clock_settime.c
+++ b/sysdeps/unix/sysv/linux/clock_settime.c
@@ -19,11 +19,9 @@ 
 #include <sysdep.h>
 #include <time.h>
 
-#include "kernel-posix-cpu-timers.h"
-
 /* Set CLOCK to value TP.  */
 int
-__clock_settime (clockid_t clock_id, const struct timespec *tp)
+__clock_settime64 (clockid_t clock_id, const struct __timespec64 *tp)
 {
   /* Make sure the time cvalue is OK.  */
   if (tp->tv_nsec < 0 || tp->tv_nsec >= 1000000000)
@@ -32,6 +30,55 @@  __clock_settime (clockid_t clock_id, const struct timespec *tp)
       return -1;
     }
 
+#ifdef __ASSUME_TIME64_SYSCALLS
+# ifdef __NR_clock_settime64
+  /* For Y2038 safe systems with __WORDSIZE==32 and __TIMESIZE==64
+     (x86, arm) the glibc exported struct timespec has 64 bit tv_sec,
+     32 bit tv_nsec (to be still POSIX compliant -> long tv_nsec )
+     and 32 bits of unnamed padding.
+
+     It may happen that due to dynamic allocation the tv_pad, which
+     corresponds to upper 32 bits of kernel's 64 bit tv_nsec accepted
+     by syscalls, may not be zero.
+
+     However, the Linux kernel is ignoring those 32 bits (to be more
+     precise - as of 5.1 - it casts 64 bit tv_nsec to internal's 32 bit
+     representation) and hence the padding clearing is not needed.  */
+  int ret = INLINE_SYSCALL_CALL (clock_settime64, clock_id, tp);
+  if (ret == 0 || errno != ENOSYS)
+    return ret;
+# endif
+#endif
+
+/* For systems supporting 32 bit time only __WORDSIZE==32 and
+   __TIMESIZE==32 (!=64) the passed struct __timespec64 must be
+   converted to 32 bit one before invoking Linux syscall.  */
+#if __WORDSIZE == 32 && __TIMESIZE != 64
+  /* Fall back to syscall supporting 32bit struct timespec.  */
+  struct timespec ts32;
+  valid_timespec64_to_timespec (tp, &ts32);
+  return INLINE_SYSCALL_CALL (clock_settime, clock_id, &ts32);
+#else
+/* Systems with __WORDSIZE==64 (i.e. x86_64, aarch64) or
+   __WORDSIZE==32 && __TIMESIZE == 64 (i.e. x32 - special case).  */
   return INLINE_SYSCALL_CALL (clock_settime, clock_id, tp);
+#endif
 }
 weak_alias (__clock_settime, clock_settime)
+
+#if __TIMESIZE != 64
+int
+__clock_settime (clockid_t clock_id, const struct timespec *tp)
+{
+  struct __timespec64 ts64;
+
+  if (! in_time_t_range (tp->tv_sec))
+    {
+      __set_errno (EOVERFLOW);
+      return -1;
+    }
+
+  valid_timespec_to_timespec64 (tp, &ts64);
+  return __clock_settime64 (clock_id, &ts64);
+}
+#endif