Re: [RFC PATCH v2 1/4] y2038: add 64bit time_t support in timeval for 32bit architecture

[Arnd Bergmann <arnd@arndb.de>]

On Wednesday 15 July 2015 11:18:31 Bamvor Zhang Jian wrote:
> Hi, Arnd
> 
> On 07/09/2015 06:26 PM, Arnd Bergmann wrote:
> > On Thursday 09 July 2015 17:02:47 Bamvor Zhang Jian wrote:
> >> On 07/09/2015 04:09 AM, John Stultz wrote:
> >>> On Mon, Jun 29, 2015 at 7:23 AM, Bamvor Zhang Jian
> >>> <bamvor.zhangjian@linaro.org> wrote:
> >>>> +int get_timeval64(struct timeval64 *tv,
> >>>> +                  const struct __kernel_timeval __user *utv)
> >>>> +{
> >>>> +       struct __kernel_timeval ktv;
> >>>> +       int ret;
> >>>> +
> >>>> +       ret = copy_from_user(&ktv, utv, sizeof(ktv));
> >>>> +       if (ret)
> >>>> +               return -EFAULT;
> >>>> +
> >>>> +       tv->tv_sec = ktv.tv_sec;
> >>>> +       if (!IS_ENABLED(CONFIG_64BIT)
> >>>> +#ifdef CONFIG_COMPAT
> >>>> +          || is_compat_task()
> >>>> +#endif
> >>>
> >>> These sorts of ifdefs are to be avoided inside of functions.
> >>
> >>> Instead, it seems is_compat_task() should be defined to 0 in the
> >>> !CONFIG_COMPAT case, so you can avoid the ifdefs and the compiler can
> >>> still optimize it out.
> >> I add this ifdef because I got compile failure on arm platform. This
> >> file do not include the <linux/compat.h> directly. And in arm64,
> >> compat.h is included implicitily.
> >> So, I am not sure what I should do here. Include <linux/compat.h> in
> >> this file directly or add a this check at the beginning of this file?
> >>
> >> #ifndef is_compat_task
> >> #define is_compat_task() (0)
> >> #endif
> >>
> >
> > Actually I think we can completely skip this test here: Unlike
> > timespec, timeval is defined in a way that always lets user space
> > use a 64-bit type for the microsecond portion (suseconds_t tv_usec).
>
> I do not familar with this type. I grep the suseconds_t in glibc, it
> seems that suseconds_t(__SUSECONDS_T_TYPE) is defined as
> __SYSCALL_SLONG_TYPE which is __SLONGWORD_TYPE(32bit on 32bit
> architecture).

Correct, but POSIX allows it to be redefined along with time_t, so
timeval can be a pair of 64-bit values. In contrast, timespec is
required by POSIX (and C11) to be a time_t and a 'long', which is
why we need a hack to check the size of the second word of the
timespec structure.

	Arnd