linux-kernel - Re: [Y2038] [PATCH v2 07/10] include: Add new y2038 safe __kernel

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Date:   Fri, 15 Dec 2017 11:36:01 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Ben Hutchings <ben.hutchings@...ethink.co.uk>
Cc:     Deepa Dinamani <deepa.kernel@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        John Stultz <john.stultz@...aro.org>,
        y2038 Mailman List <y2038@...ts.linaro.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Albert ARIBAUD <albert.aribaud@...ev.fr>,
        GNU C Library <libc-alpha@...rceware.org>
Subject: Re: [Y2038] [PATCH v2 07/10] include: Add new y2038 safe __kernel_timespec

On Fri, Dec 15, 2017 at 1:11 AM, Ben Hutchings
<ben.hutchings@...ethink.co.uk> wrote:
> On Mon, 2017-11-27 at 11:30 -0800, Deepa Dinamani wrote:
>> --- a/include/uapi/linux/time.h
>> +++ b/include/uapi/linux/time.h
>> @@ -42,6 +42,13 @@ struct itimerval {
>> > >   struct timeval it_value;        /* current value */
>>  };
>>
>> +#ifndef __kernel_timespec
>> +struct __kernel_timespec {
>> +     __kernel_time64_t       tv_sec;                 /* seconds */
>> +     long long               tv_nsec;                /* nanoseconds */
>> +};
>> +#endif
>
> I wonder if it makes sense to override the alignment of this structure?
>  (64-bit types are aligned differently on 32-bit vs 64-bit x86, but not
> other compat cases.)  It might reduce the need for conversions in
> compat code elsewhere later.

I think the alignment here should be agreed with glibc so they use the
same alignment for their new timespec. I don't see a specific mention
of this at
https://sourceware.org/glibc/wiki/Y2038ProofnessDesign#struct___timespec64
so I'm adding Albert and libc-alpha to Cc for clarification.

I agree it would simplify things a bit to ensure that time64_t and
__timespec64/__kernel_timespec have a 64-bit alignment even on
x86-32, but I don't know if there is precedent for using gcc
__attribute__((aligned(8)) on POSIX data types, of if that might
cause problems e.g. for non-gnu compilers.

       Arnd