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

On 05/18/2015 02:53 AM, Arnd Bergmann wrote:

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?

It seems to work, except I don't really understand why there is a
difference between (b) and (c).

I also have no problem just having klibc contain its own definitions of
these structures, as long as one can prevent the kernel from defining them.

	-hpa