Re: [Y2038] kernel/libc uapi changes for y2038

In the patch series I posted recently [1], I introduce new system calls to deal
with modified data structures, but left the question open on how these should
be best accessed from libc. The patches introduce a new __kernel_time64_t type
and based on that five new data structured: struct __kernel_timespec,
struct __kernel_itimerspec, struct __kernel_stat, struct __kernel_rusage,
and struct __kernel_timex. This works fine for the case when all libc
implementations provide their own definitions to user space, but not for
the simplest case (e.g. klibc) where the user-visible structures come directly
from the kernel uapi headers.

I still don't know what model the various libc developers prefer, so here is
an alternative approach, as a patch on top of the previous series:

Now, we rename the original structures to struct __old_kernel_*, and use a
macro to define either the __old_kernel_* or the __kernel_* structure name
to the name we actually want in user space, based on a __KERNEL_TIME_BITS
macro that can be set to either 32 or 64 for 32-bit architectures by
the libc. Depending on that macro, the compiler will either see one
of these combinations (for each of the five structures):

a) __BITS_PER_LONG == 32 && __KERNEL_TIME_BITS == 32:

   struct timespec              based on 32-bit __kernel_time_t
   struct __kernel_timespec     based on 64-bit __kernel_time64_t

b) __BITS_PER_LONG == 64 && __KERNEL_TIME_BITS == 64:

   struct timespec              based on 64-bit __kernel_time_t
   struct __kernel_timespec     based on 64-bit __kernel_time64_t

c) __BITS_PER_LONG == 32 && __KERNEL_TIME_BITS == 64:

   struct __old_kernel_timespec based on 32-bit __kernel_time_t
   struct timespec              based on 64-bit __kernel_time64_t

Would this work for everyone?  Any alternative suggestions?

        Arnd

[1] http://git.kernel.org/cgit/linux/kernel/git/arnd/playground.git/log/?h=y2038-syscalls
    https://lwn.net/Articles/643407/

diff --git a/include/uapi/asm-generic/bitsperlong.h b/include/uapi/asm-generic/bitsperlong.h
index 23e6c416b85f..ecdaf4f77f35 100644
--- a/include/uapi/asm-generic/bitsperlong.h
+++ b/include/uapi/asm-generic/bitsperlong.h

@@ -12,4 +12,13 @@
 #define __BITS_PER_LONG 32
 #endif

+/*
+ * Traditionally we define defines 'time_t' as 'long', but we need to
+ * migrate to a 64-bit type until 2038. This one is designed to be
+ * overridden by user space if it's prepared to handle 64-bit time_t.
+ */
+#ifndef __KERNEL_TIME_BITS
+#define __KERNEL_TIME_BITS __BITS_PER_LONG
+#endif
+
 #endif /* _UAPI__ASM_GENERIC_BITS_PER_LONG */

diff --git a/include/uapi/asm-generic/kernel_stat.h b/include/uapi/asm-generic/kernel_stat.h
index d1db22583046..3693496c78aa 100644
--- a/include/uapi/asm-generic/kernel_stat.h
+++ b/include/uapi/asm-generic/kernel_stat.h

@@ -1,6 +1,14 @@
 #ifndef __ASM_GENERIC_KERNEL_STAT_H
 #define __ASM_GENERIC_KERNEL_STAT_H

+#include <asm/bitsperlong.h>
+
+#if __KERNEL_TIME_BITS == 32 || __BITS_PER_LONG == 64
+#define __old_kernel_stat2 stat
+#else
+#define __kernel_stat stat
+#endif
+
 /*
  * The new structure that works on both 32-bit and 64-bit and survives y2038
  * The layout matches 'struct stat' from asm-generic/stat.h on 64-bit

diff --git a/include/uapi/asm-generic/stat.h b/include/uapi/asm-generic/stat.h
index 64c32ba7c1a9..f66b28b96c8d 100644
--- a/include/uapi/asm-generic/stat.h
+++ b/include/uapi/asm-generic/stat.h

@@ -22,7 +22,7 @@

 #define STAT_HAVE_NSEC 1

-struct stat {
+struct __old_kernel_stat2 {
        unsigned long   st_dev;         /* Device.  */
        unsigned long   st_ino;         /* File serial number.  */
        unsigned int    st_mode;        /* File mode.  */

diff --git a/include/uapi/linux/resource.h b/include/uapi/linux/resource.h
index c4f3ba44db00..9a3876cc4436 100644
--- a/include/uapi/linux/resource.h
+++ b/include/uapi/linux/resource.h

@@ -3,10 +3,16 @@

 #include <linux/time.h>
 #include <linux/types.h>
+#include <asm/bitsperlong.h>

 /*
  * Resource control/accounting header file for linux
  */
+#if __KERNEL_TIME_BITS == 32 || __BITS_PER_LONG == 64
+#define __old_kernel_rusage rusage
+#else
+#define __kernel_rusage rusage
+#endif