y2038: linux: Provide __time64 implementation

In the glibc the time function can use vDSO (on power and x86 the
USE_IFUNC_TIME is defined), time syscall or 'default' time() from
./time/time.c (as a fallback).

In this patch the last function (time) has been refactored and moved
to ./sysdeps/unix/sysv/linux/time.c to be Linux specific.

The new __time64 explicit 64 bit function for providing 64 bit value of
seconds after epoch (by internally calling __clock_gettime64) has been
introduced.

Moreover, a 32 bit version - __time has been refactored to internally
use __time64.

The __time is now supposed to be used on systems still supporting 32 bit
time (__TIMESIZE != 64) - hence the necessary check for time_t potential
overflow.

The iFUNC vDSO direct call optimization has been removed from both i686 and
powerpc32 (USE_IFUNC_TIME is not defined for those architectures
anymore). The Linux kernel does not provide a y2038 safe implementation of
time neither it plans to provide it in the future, __clock_gettime64
should be used instead. Keeping support for this optimization would require
to handle another build permutation (!__ASSUME_TIME64_SYSCALLS &&
USE_IFUNC_TIME which adds more complexity and has limited use (since the
idea is to eventually have a y2038 safe glibc build).

Build tests:
./src/scripts/build-many-glibcs.py glibcs

Run-time tests:
- 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

Above tests were performed with Y2038 redirection applied as well as
without to test proper usage of both __time64 and __time.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>

include/time.h

@@ -317,6 +317,13 @@ extern int __timespec_get64 (struct __timespec64 *ts, int base);
 libc_hidden_proto (__timespec_get64)
 #endif
 
+#if __TIMESIZE == 64
+# define __time64 __time
+#else
+extern __time64_t __time64 (__time64_t *timer);
+libc_hidden_proto (__time64)
+#endif
+
 /* Use in the clock_* functions.  Size of the field representing the
    actual clock ID.  */
 #define CLOCK_IDFIELD_SIZE	3

sysdeps/unix/sysv/linux/powerpc/time.c

@@ -16,5 +16,7 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
-#define USE_IFUNC_TIME
+#ifdef __powerpc64__
+# define USE_IFUNC_TIME
+#endif
 #include <sysdeps/unix/sysv/linux/time.c>

sysdeps/unix/sysv/linux/time.c

@@ -47,5 +47,41 @@ time (time_t *t)
 }
 # endif /* !SHARED */
 #else /* USE_IFUNC_TIME  */
-# include <time/time.c>
+# include <time.h>
+# include <time-clockid.h>
+# include <errno.h>
+
+/* Return the time now, and store it in *TIMER if not NULL.  */
+
+__time64_t
+__time64 (__time64_t *timer)
+{
+  struct __timespec64 ts;
+  __clock_gettime64 (TIME_CLOCK_GETTIME_CLOCKID, &ts);
+
+  if (timer != NULL)
+    *timer = ts.tv_sec;
+  return ts.tv_sec;
+}
+
+# if __TIMESIZE != 64
+libc_hidden_def (__time64)
+
+time_t
+__time (time_t *timer)
+{
+  __time64_t t = __time64 (NULL);
+
+  if (! in_time_t_range (t))
+    {
+      __set_errno (EOVERFLOW);
+      return -1;
+    }
+
+  if (timer != NULL)
+    *timer = t;
+  return t;
+}
+# endif
+weak_alias (__time, time)
 #endif

sysdeps/unix/sysv/linux/x86/time.c

@@ -16,5 +16,7 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
-#define USE_IFUNC_TIME
+#ifdef __x86_64__
+# define USE_IFUNC_TIME
+#endif
 #include <sysdeps/unix/sysv/linux/time.c>