linux-kernel - Re: [PATCH] LoongArch: Define __ARCH_WANT_NEW

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Date: Wed, 15 May 2024 14:25:28 +0000
From: "Arnd Bergmann" <arnd@...db.de>
To: "Bibo Mao" <maobibo@...ngson.cn>, "Huacai Chen" <chenhuacai@...ngson.cn>,
 "Huacai Chen" <chenhuacai@...nel.org>
Cc: loongarch@...ts.linux.dev, Linux-Arch <linux-arch@...r.kernel.org>,
 "Xuefeng Li" <lixuefeng@...ngson.cn>, guoren <guoren@...nel.org>,
 "WANG Xuerui" <kernel@...0n.name>, "Jiaxun Yang" <jiaxun.yang@...goat.com>,
 linux-kernel@...r.kernel.org, loongson-kernel@...ts.loongnix.cn,
 stable@...r.kernel.org
Subject: Re: [PATCH] LoongArch: Define __ARCH_WANT_NEW_STAT in unistd.h

On Wed, May 15, 2024, at 09:30, maobibo wrote:
> On 2024/5/11 下午8:17, Arnd Bergmann wrote:
>> On Sat, May 11, 2024, at 12:01, Huacai Chen wrote:
>> 
>> Importantly, we can't just add fstatat64() on riscv32 because
>> there is no time64 version for it other than statx(), and I don't
>> want the architectures to diverge more than necessary.
> yes, I agree. Normally there is newfstatat() on 64-bit architectures but 
> fstatat64() on 32-bit ones.
>
> I do not understand why fstatat64() can be added for riscv32 still.
> 32bit timestamp seems works well for the present, it is valid until
> (0x1UL << 32) / 365 / 24 / 3600 + 1970 == 2106 year. Year 2106 should
> be enough for 32bit system.

There is a very small number of interfaces for which we ended up
not using a 64-bit time_t replacement, but those are only for
relative times, not epoch based offsets. The main problems
here are:

- time_t is defined to be a signed value in posix, and we need
  to handle file timestamps before 1970 in stat(), so changing
  this one to be unsigned is not an option.

- A lot of products have already shipped that will have to
  be supported past 2038 on existing 32-bit hardware. We
  cannot regress on architectures that have already been
  fixed to support this. 

- file timestamps can also be set into the future, so applications
  relying on this are broken before 2038.

      Arnd