linux-kernel - Re: [Y2038] [PATCH 5/9] y2038: introduce CONFIG_64BIT

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Date:   Fri, 08 Dec 2017 16:53:27 +0000
From:   Ben Hutchings <ben.hutchings@...ethink.co.uk>
To:     Deepa Dinamani <deepa.kernel@...il.com>, tglx@...utronix.de,
        john.stultz@...aro.org
Cc:     y2038@...ts.linaro.org, linux-kernel@...r.kernel.org, arnd@...db.de
Subject: Re: [Y2038] [PATCH 5/9] y2038: introduce CONFIG_64BIT_TIME

On Fri, 2017-11-10 at 14:42 -0800, Deepa Dinamani wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> There are a total of 53 system calls (aside from ioctl) that pass a time_t
> or derived data structure as an argument, and in order to extend time_t
> to 64-bit, we have to replace them with new system calls and keep providing
> backwards compatibility.
> 
> To avoid adding completely new and untested code for this purpose, we
> introduce a new CONFIG_64BIT_TIME symbol. Every architecture that supports
> new 64 bit time_t syscalls enables this config via ARCH_HAS_64BIT_TIME.
> 
> After this is done for all architectures, the CONFIG_64BIT_TIME symbol
> can be made a user-selected option, to enable users to build a kernel
> that only provides y2038-safe system calls by making 32 time_t syscalls
> conditionally included based on the above config.

I don't understand why we would want to change the semantics of
CONFIG_64BIT_TIME symbol from "enable 64-bit time support" to "disable
32-bit time support".

Why not add two config symbols:

config 32BIT_TIME
	def_bool COMPAT || !64BIT

config 64BIT_TIME
	def_bool ARCH_HAS_64BIT_TIME

and then make 32BIT_TIME user-configurable later?

> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> Signed-off-by: Deepa Dinamani <deepa.kernel@...il.com>
> ---
>  arch/Kconfig | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 8911ff37335a..3266ac1a4ff7 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -875,6 +875,17 @@ config OLD_SIGACTION
>  config COMPAT_OLD_SIGACTION
>  	bool
>  
> +config ARCH_HAS_64BIT_TIME
> +	def_bool n
> +
> +config CONFIG_64BIT_TIME

The CONFIG_ prefix is added by kconfig scripts and shouldn't be used in
the Kconfig file.

Ben.

> +	def_bool ARCH_HAS_64BIT_TIME
> +	help
> +	  This should be selected by all architectures that need to support
> +	  new system calls with a 64-bit time_t. This is relevant on all 32-bit
> +	  architectures, and 64-bit architectures as part of compat syscall
> +	  handling.
> +
>  config ARCH_NO_COHERENT_DMA_MMAP
>  	bool
>  

-- 
Ben Hutchings
Software Developer, Codethink Ltd.