y2038: linux: Provide __clock_getres64 implementation

This patch provides new __clock_getres64 explicit 64 bit function for
getting the resolution (precision) of specified clock ID. Moreover, a
32 bit version - __clock_getres has been refactored to internally use
__clock_getres64.

The __clock_getres is now supposed to be used on systems still supporting
32 bit time (__TIMESIZE != 64) - hence the necessary conversion from 64 bit
struct __timespec64 to struct timespec.

The new clock_getres_time64 syscall available from Linux 5.1+ has been used,
when applicable.
On systems which are not supporting clock_getres_time64 (as their
clock_getres supports 64 bit time ABI) the vDSO syscall is attempted.
On the contrary the non-vDSO syscall is used for clock_getres_time64 as
up till now the kernel is not providing such interface.

No additional checks (i.e. if tv_nsec value overflow) are performed on
values returned via clock_getres{_time64} syscall, as it is assumed that
the Linux kernel will either return 0 and provide correct value or error.

The check for tv_sec being out of range on systems still supporting 32 bit
time (__TIMESIZE != 64) without Y2038 time support is also omitted as it is
_very_ unlikely that we would have a timer with resolution which exceeds 32
bit time_t range.

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

- The glibc has been build tested (make PARALLELMFLAGS="-j8") for
x86 (i386), x86_64-x32, and armv7

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

- Use of cross-test-ssh.sh for ARM (armv7):
  make PARALLELMFLAGS="-j8" test-wrapper='./cross-test-ssh.sh root@192.168.7.2' xcheck

Linux kernel, headers and minimal kernel version for glibc build test
matrix:
- Linux v5.1 (with clock_getres_time64) and glibc build with v5.1 as
  minimal kernel version (--enable-kernel="5.1.0")
  The __ASSUME_TIME64_SYSCALLS flag defined.

- Linux v5.1 and default minimal kernel version
  The __ASSUME_TIME64_SYSCALLS not defined, but kernel supports
  clock_getres_time64 syscall.

- Linux v4.19 (no clock_getres_time64 support) with default minimal kernel
  version for contemporary glibc

  This kernel doesn't support clock_getres_time64 syscall, so the fallback
  to clock_getres is tested.

The above tests were performed with Y2038 redirection applied as well as
without (so the __TIMESIZE != 64 execution path is checked as well).

No regressions were observed.

include/time.h

@@ -133,6 +133,14 @@ extern int __clock_settime64 (clockid_t clock_id,
 libc_hidden_proto (__clock_settime64)
 #endif
 
+#if __TIMESIZE == 64
+# define __clock_getres64 __clock_getres
+#else
+extern int __clock_getres64 (clockid_t clock_id,
+                             struct __timespec64 *tp);
+libc_hidden_proto (__clock_getres64);
+#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.

sysdeps/unix/sysv/linux/clock_getres.c

@@ -19,21 +19,53 @@
 #include <sysdep.h>
 #include <errno.h>
 #include <time.h>
-#include "kernel-posix-cpu-timers.h"
 
 #ifdef HAVE_CLOCK_GETRES_VSYSCALL
 # define HAVE_VSYSCALL
 #endif
 #include <sysdep-vdso.h>
-
 #include <shlib-compat.h>
+#include <kernel-features.h>
 
 /* Get resolution of clock.  */
 int
+__clock_getres64 (clockid_t clock_id, struct __timespec64 *res)
+{
+#ifdef __ASSUME_TIME64_SYSCALLS
+# ifndef __NR_clock_getres_time64
+  return INLINE_VSYSCALL (clock_getres, 2, clock_id, res);
+# else
+  return INLINE_SYSCALL (clock_getres_time64, 2, clock_id, res);
+# endif
+#else
+# ifdef __NR_clock_getres_time64
+  int ret = INLINE_SYSCALL (clock_getres_time64, 2, clock_id, res);
+  if (ret == 0 || errno != ENOSYS)
+    return ret;
+# endif
+  struct timespec ts32;
+  int retval = INLINE_VSYSCALL (clock_getres, 2, clock_id, &ts32);
+  if (! retval && res)
+    *res = valid_timespec_to_timespec64 (ts32);
+
+  return retval;
+#endif
+}
+
+#if __TIMESIZE != 64
+int
 __clock_getres (clockid_t clock_id, struct timespec *res)
 {
-  return INLINE_VSYSCALL (clock_getres, 2, clock_id, res);
+  struct __timespec64 ts64;
+  int retval;
+
+  retval = __clock_getres64 (clock_id, &ts64);
+  if (! retval && res)
+    *res = valid_timespec64_to_timespec (ts64);
+
+  return retval;
 }
+#endif
 
 versioned_symbol (libc, __clock_getres, clock_getres, GLIBC_2_17);
 /* clock_getres moved to libc in version 2.17;