linux-kernel - Re: [PATCH] efi: cper: avoid using get

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Date:   Mon, 18 Jun 2018 17:54:06 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     y2038 Mailman List <y2038@...ts.linaro.org>,
        Tyler Baicar <tbaicar@...eaurora.org>,
        Will Deacon <will.deacon@....com>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...e.de>,
        Yazen Ghannam <yazen.ghannam@....com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        gengdongjiu <gengdongjiu@...wei.com>,
        linux-efi <linux-efi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] efi: cper: avoid using get_seconds()

On Mon, Jun 18, 2018 at 5:50 PM, Ard Biesheuvel
<ard.biesheuvel@...aro.org> wrote:
> On 18 June 2018 at 17:49, Arnd Bergmann <arnd@...db.de> wrote:
>> On Mon, Jun 18, 2018 at 5:47 PM, Ard Biesheuvel
>> <ard.biesheuvel@...aro.org> wrote:
>>> On 18 June 2018 at 16:17, Arnd Bergmann <arnd@...db.de> wrote:
>>
>>>> -               atomic64_set(&seq, ((u64)get_seconds()) << 32);
>>>> +       if (!atomic64_read(&seq)) {
>>>> +               time64_t time = ktime_get_real_seconds();
>>>> +
>>>> +               /*
>>>> +                * This code is unlikely to still be needed in year 2106,
>>>> +                * but just in case, let's use a few more bits for timestamps
>>>> +                * after y2038 to be sure they keep increasing monotonically
>>>> +                * for the next few hundred years...
>>>> +                */
>>>> +               if (time < 0x80000000)
>>>> +                       atomic64_set(&seq, (ktime_get_real_seconds()) << 32);
>>>> +               else
>>>> +                       atomic64_set(&seq, 0x8000000000000000ull |
>>>> +                                          ktime_get_real_seconds() << 24);
>>>> +       }
>>>
>>> Given that these values are never decoded and interpreted as
>>> timestamps, can't we simply switch to the second flavour immediately?
>>
>> I considered that, but the downside would be that all future filenames would
>> come before all past file names.
>
> Won't we have that same problem in 2038?

No, it goes from 0x7fffffff00000000 to 0x8000000000000000, followed
by 0x8000000001000000.

       Arnd