linux-kernel - Re: [PATCH v2 3/4] input: Deprecate real timestamps beyond year 2106

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Date:   Fri, 28 Oct 2016 23:47:39 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Deepa Dinamani <deepa.kernel@...il.com>
Cc:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        linux-input@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        y2038 Mailman List <y2038@...ts.linaro.org>
Subject: Re: [PATCH v2 3/4] input: Deprecate real timestamps beyond year 2106

On Friday, October 28, 2016 2:39:35 PM CEST Deepa Dinamani wrote:
> >> >> @@ -55,24 +60,24 @@ struct ff_effect_compat {
> >> >>
> >> >>  static inline size_t input_event_size(void)
> >> >>  {
> >> >> -       return (in_compat_syscall() && !COMPAT_USE_64BIT_TIME) ?
> >> >> -               sizeof(struct input_event_compat) : sizeof(struct input_event);
> >> >> +       return in_compat_syscall() ? sizeof(struct raw_input_event_compat) :
> >> >> +                                    sizeof(struct raw_input_event);
> >> >>  }
> >> >
> >> > I think the COMPAT_USE_64BIT_TIME check has to stay here,
> >> > it's needed for x32 mode on x86-64.
> >>
> >> There is no time_t anymore in the raw_input_event structure.
> >> The struct uses __kernel_ulong_t type.
> >> This should take care of x32 support.
> >
> > I don't think it does.
> >
> >> From this cover letter:
> >> https://www.spinics.net/lists/linux-arch/msg16356.html
> >>
> >> I see that that the __kernel types were introduced to address the ABI
> >> issues for x32.
> >
> > This is a variation of the problem we are trying to solve for
> > the other architectures in your patch set:
> >
> > On x32, the kernel uses produces a structure with the 64-bit
> > layout, using __u64 tv_sec, to match the current user space
> > that has 64-bit __kernel_ulong_t and 64-bit time_t, but
> > in_compat_syscall() also returns 'true' here, as this is
> > mostly a 32-bit ABI (time_t being one of the exceptions).
> 
> Yes, I missed this.
> 
> in_compat_syscall() is true for x32, this would mean we end up here
> even if it is a x32 syscall.
> But, wouldn't it be better to use in_x32_syscall() here since there is
> no timeval any more?

We have to distinguish four cases on x86:

- native 32-bit, input_event with 32-bit time_t
- compat 32-bit, input_event_compat with 32-bit time_t
- native 64-bit, input_event with 64-bit time_t
- compat x32, input_event with 64-bit time_t

The first three can happen on other architectures too,
the last one is x86 specific. There are probably other ways
to express the condition above, but I can't think of one
that is better than the one we have today.

	Arnd