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Re: [RFC v6 06/23] sysdeps: Use long types with 64-bit time_t on 32-bit archs
- From: Arnd Bergmann <arnd at arndb dot de>
- To: Alistair Francis <alistair dot francis at wdc dot com>
- Cc: GNU C Library <libc-alpha at sourceware dot org>, Adhemerval Zanella <adhemerval dot zanella at linaro dot org>, Florian Weimer <fweimer at redhat dot com>, Joseph Myers <joseph at codesourcery dot com>, palmerdabbelt at google dot com, macro at wdc dot com, Zong Li <zongbox at gmail dot com>, Alistair Francis <alistair23 at gmail dot com>
- Date: Sun, 12 Jan 2020 22:41:02 +0100
- Subject: Re: [RFC v6 06/23] sysdeps: Use long types with 64-bit time_t on 32-bit archs
- References: <cover.1578824547.git.alistair.francis@wdc.com> <96c5552629ae8b306cff4b5cef44ba00634276d2.1578824547.git.alistair.francis@wdc.com>
On Sun, Jan 12, 2020 at 11:40 AM Alistair Francis
<alistair.francis@wdc.com> wrote:
>
> If we are using a 32-bit architecture with a 64-bit time_t let's
> define __SEM_PAD_BEFORE_TIME as 1. This is because the kernel is
> expecting a type __kernel_old_time_t which is 32-bit even with a
> 64-bit time_t. Instead of adding another check, let's just set
> __SEM_PAD_BEFORE_TIME as that will use a long type instead of
> long long.
I think you need to do this differently to avoid truncating the timestamp
to 32 bit. The type in the generic kernel headers for 32-bit targets is not
'__kernel_old_time_t sem_otime; __kernel_long_t pad' but it is
'unsigned long sem_otime; unsigned long sem_otime_high;'.
Both need to be combined to read the 64-bit time_t value. It should
be possible to do this in an architecture independent way, but note
that the two halves are not always adjacent or in the correct order,
and in one case (mips shmbuf) the high word is only 16 bit instead
of 32.
Arnd
> ---
> sysdeps/unix/sysv/linux/bits/sem-pad.h | 13 ++++++++++++-
> 1 file changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/sysdeps/unix/sysv/linux/bits/sem-pad.h b/sysdeps/unix/sysv/linux/bits/sem-pad.h
> index 566ce039cc..4b419a6100 100644
> --- a/sysdeps/unix/sysv/linux/bits/sem-pad.h
> +++ b/sysdeps/unix/sysv/linux/bits/sem-pad.h
> @@ -30,4 +30,15 @@
> layout conversions for this structure. */
>
> #define __SEM_PAD_AFTER_TIME (__TIMESIZE == 32)
> -#define __SEM_PAD_BEFORE_TIME 0
> +
> +/* If we are using a 32-bit architecture with a 64-bit time_t let's
> + define __SEM_PAD_BEFORE_TIME as 1. This is because the kernel is
> + expecting a type __kernel_old_time_t which is 32-bit even with a
> + 64-bit time_t. Instead of adding another check, let's just set
> + __SEM_PAD_BEFORE_TIME as that will use a long type instead of
> + long long. */
> +#if __WORDSIZE == 32 && __TIMESIZE == 64
> +# define __SEM_PAD_BEFORE_TIME 1
> +#else
> +# define __SEM_PAD_BEFORE_TIME 0
> +#endif
> --
> 2.24.1
>