[PATCH 0/9] posix_clocks: Prepare syscalls for 64 bit time_t conversion

[Arnd Bergmann]

On Thu, Nov 16, 2017 at 10:04 AM, Thomas Gleixner <tglx at linutronix.de> wrote:
> On Wed, 15 Nov 2017, Deepa Dinamani wrote:
>> > I had on concern about x32, maybe we should check
>> > for "COMPAT_USE_64BIT_TIME" before zeroing out the tv_nsec
>> > bits.
>>
>> Thanks, I think you are right. I had the check conditional on
>> CONFIG_64BIT_TIME and then removed as I forgot why I added it. :)
>>
>> > Regarding CONFIG_COMPAT_TIME/CONFIG_64BIT_TIME, would
>> > it help to just leave out that part for now and unconditionally
>> > define '__kernel_timespec' as 'timespec' until we are ready to
>> > convert the architectures?
>>
>> Another approach would be to use separate configs:
>>
>> 1. To indicate 64 bit time_t syscall support. This will be dependent
>> on architectures as CONFIG_64BIT_TIME.
>> We can delete this once all architectures have provided support for this.
>>
>> 2. Another config (maybe COMPAT_32BIT_TIME?) to be introduced later,
>> which will compile out all syscalls/ features that use 32 bit time_t.
>> This can help build a y2038 safe kernel later.
>>
>> Would this work for everyone?
>
> Having extra config switches which are selectable by architectures and
> removed when everything is converted is definitely the right way to go.
>
> That allows you to gradually convert stuff w/o inflicting wreckage all over
> the place.

The CONFIG_64BIT_TIME would do that nicely for the new stuff like
the conditional definition of __kernel_timespec, this one would get
removed after we convert all architectures.

A second issue is how to control the compilation of the compat syscalls.
CONFIG_COMPAT_32BIT_TIME handles that and could be defined
in Kconfig as 'def_bool (!64BIT && CONFIG_64BIT_TIME) || COMPAT',
this is then just a more readable way of expressing exactly when the
functions should be built.

For completeness, there may be a third category, depending on how
we handle things like sys_nanosleep(): Here, we want the native
sys_nanosleep on 64-bit architectures, and compat_sys_nanosleep()
to handle the 32-bit time_t variant on both 32-bit and 64-bit targets,
but our plan is to not have a native 32-bit sys_nanosleep on 32-bit
architectures any more, as new glibc should call clock_nanosleep()
with a new syscall number instead. Should we then enclose
sys_nanosleep in "#if !defined(CONFIG_64BIT_TIME) ||
defined(CONFIG_64BIT)", or should we try to come up with another
Kconfig symbol name that expresses this better?

       Arnd