Y2038: provide kernel support indication

Introduce __ASSUME_KERNEL_Y2038_SUPPORT which means that
the underlying kernel *always* implements 64-bit-time Y2038-proof
syscalls.

If __ASSUME_KERNEL_Y2038_SUPPORT is not defined, glibc must
check dynamically whether the underlying kernel provides these
syscalls or not, and must fallback to 32-bit-time syscalls if
not.

Introduce function __y2038_get_kernel_support(), which returns
true if the underlying kernel supports 64-bit-time syscalls.

Also introduce function __y2038_set_kernel_support(), which
allows an implementation to notify glibc of the absence of
64-bit-time syscalls (ENOSYS) so that they are not needlessly
tried again.

* misc/Makefile: Add module y2038-support.
* misc/Versions: (__y2038_get_kernel_support): Add to GLIBC_PRIVATE.
* misc/Versions: (__y2038_set_kernel_support): Likewise.
* misc/y2038-support.c: New file.
* misc/y2038-support.h: Likewise.
* sysdeps/unix/sysv/linux/kernel-features.h (__ASSUME_KERNEL_Y2038_SUPPORT): Add.
* sysdeps/unix/sysv/linux/y2038-support.c: New file.
* sysdeps/unix/sysv/linux/y2038-support.h: New file.

3 files changed, 94 insertions, 0 deletions

diff --git a/sysdeps/unix/sysv/linux/kernel-features.h b/sysdeps/unix/sysv/linux/kernel-features.h
index 5543d92..b47755b 100644
--- a/sysdeps/unix/sysv/linux/kernel-features.h
+++ b/sysdeps/unix/sysv/linux/kernel-features.h
@@ -154,3 +154,17 @@
    */
 
 #define __ASSUME_CLONE_DEFAULT 1
+
+/* Support for Y2038-proof times.
+
+   If defined, the underlying kernel always provides a set of syscalls
+   (__NR_* names and numbers) which can handle times beyond Y2038.
+
+   If undefined, whether the underlying kernel provides this set or not
+   must be checked dynamically.
+
+   Definition is commented out until Y2038 syscalls are merged in the
+   kernel.
+   */
+
+/* #define __ASSUME_KERNEL_Y2038_SUPPORT 1 */
diff --git a/sysdeps/unix/sysv/linux/y2038-support.c b/sysdeps/unix/sysv/linux/y2038-support.c
new file mode 100644
index 0000000..adbcd8f
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/y2038-support.c
@@ -0,0 +1,46 @@
+/* y2038 Linux kernel support indication.
+   Copyright (C) 2018 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
+   <http://www.gnu.org/licenses/>.  */
+
+#ifndef __ASSUME_KERNEL_Y2038_SUPPORT
+
+#include <stdbool.h>
+
+/* By default the underlying Linux kernel is assumed not to support Y2038.
+ * Any Linux architecture may (test and) claim Y2038 kernel support by
+ * setting __y2038_linux_support.
+ */
+bool __y2038_linux_support = false;
+
+/* For Linux, Y2038 kernel support is determined by __y2038_linux_support.  */
+
+bool
+__linux_y2038_get_kernel_support (void)
+{
+  return __y2038_linux_support;
+}
+strong_alias (__linux_y2038_get_kernel_support, __y2038_get_kernel_support)
+
+void
+__linux_y2038_set_kernel_support (bool support)
+{
+  __y2038_linux_support = support;
+}
+strong_alias (__linux_y2038_set_kernel_support, __y2038_set_kernel_support)
+
+#endif
diff --git a/sysdeps/unix/sysv/linux/y2038-support.h b/sysdeps/unix/sysv/linux/y2038-support.h
new file mode 100644
index 0000000..b8b6b8e
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/y2038-support.h
@@ -0,0 +1,34 @@
+/* y2038 Linux kernel support indication.
+   Copyright (C) 2018 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
+   <http://www.gnu.org/licenses/>.  */
+
+#ifndef __ASSUME_KERNEL_Y2038_SUPPORT
+
+#include <stdbool.h>
+
+/* Indicates Y2038 support.
+ * Can be read directly from within libc linux-related files.
+ * Can be written non-zero to indicate support or lack thereof.
+ * Other libraries (e.g. librt) cannot access this variable and must
+ * call __y2038_get_kernel_support() and __y2038_set_kernel_support().  */
+extern bool __y2038_linux_support;
+
+#endif
+
+/* As a fallback, provide generic Y2038 support indication.  */
+#include <misc/y2038-support.h>