linux-kernel - Re: [PATCH 2/9] y2038: asm-generic: extend sysvipc data structures

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Message-ID: <CAMuHMdUtJ2x+axBLj5qdsRwod=XdRkmeYJm1N7H5V_CXLo5J0g@mail.gmail.com>
Date:	Thu, 21 May 2015 10:56:34 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	y2038@...ts.linaro.org, baolin.wang@...aro.org,
	albert.aribaud@...ev.fr, John Stultz <john.stultz@...aro.org>,
	bamvor.zhangjian@...aro.org, ruchandani.tina@...il.com,
	"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	libc-alpha@...rceware.org, Linux-Arch <linux-arch@...r.kernel.org>,
	Manfred Spraul <manfred@...orfullife.com>,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [PATCH 2/9] y2038: asm-generic: extend sysvipc data structures

Hi Arnd,

On Wed, May 20, 2015 at 5:07 PM, Arnd Bergmann <arnd@...db.de> wrote:
> --- a/include/uapi/asm-generic/sembuf.h
> +++ b/include/uapi/asm-generic/sembuf.h
> @@ -12,23 +12,29 @@
>   * everyone just ended up making identical copies without specific
>   * optimizations, so we may just as well all use the same one.
>   *
> - * 64 bit architectures typically define a 64 bit __kernel_time_t,
> + * 64 bit architectures use a 64-bit __kernel_time_t here, while
> + * 32 bit architectures have a pair of unsigned long values.
>   * so they do not need the first two padding words.
> - * On big-endian systems, the padding is in the wrong place.
>   *
> - * Pad space is left for:
> - * - 64-bit time_t to solve y2038 problem
> - * - 2 miscellaneous 32-bit values
> + * On big-endian systems, the padding is in the wrong place for
> + * historic reasons, so user space has to reconstruct a time_t
> + * value using
> + *
> + * user_semid_ds.sem_otime = kernel_semid64_ds.sem_otime +
> + *             (long long)(kernel_semid64_ds.sem_otime_high << 32)

The cast to "long long" should be inside the parentheses, to promote the
(32-bit) sem_otime_high to a first 64-bit integer first.
Else it will be shifted into oblivion (oh no, C undefined behavior).

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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