netdev - Re: [PATCH 2/2] [RFC] packet: experimental support for 64-bit timestamps

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Message-ID: <CAK8P3a0GLC7gatA5RgWomwq5i2dqd_bcVJqWaYUNZz=mgWWbbQ@mail.gmail.com>
Date:   Tue, 28 Nov 2017 15:22:16 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Björn Töpel <bjorn.topel@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        "David S. Miller" <davem@...emloft.net>,
        y2038 Mailman List <y2038@...ts.linaro.org>,
        Shuah Khan <shuah@...nel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        Mike Maloney <maloney@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        "Rosen, Rami" <rami.rosen@...el.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        Sowmini Varadhan <sowmini.varadhan@...cle.com>,
        Network Development <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-kselftest@...r.kernel.org,
        Björn Töpel <bjorn.topel@...el.com>
Subject: Re: [PATCH 2/2] [RFC] packet: experimental support for 64-bit timestamps

On Tue, Nov 28, 2017 at 3:08 PM, Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
> On Tue, Nov 28, 2017 at 3:46 AM, Arnd Bergmann <arnd@...db.de> wrote:
>> On Tue, Nov 28, 2017 at 8:04 AM, Björn Töpel <bjorn.topel@...il.com> wrote:
>>> 2017-11-27 21:51 GMT+01:00 Arnd Bergmann <arnd@...db.de>:
>>> [...]
>>>>> There already is an effort to come up with a new AF_PACKET V4 [1].
>>>>> We should make sure that any new interface does not have the
>>>>> Y2038/Y2106 issue. But, if a new version is being developed and
>>>>> that subsumes all existing use cases, then there probably is no need
>>>>> for another version that is a very small diff to V3.
>>>>
>>>> Ah, perfect, that's good timing. Adding Björn to Cc here.
>>>>
>>>
>>> Unfortunately, for the Y2038/Y2106 cases, we'll be (as a result of
>>> netdevconf discussions) moving the AF_PACKET V4 implementation to a
>>> separate, new, address/packet family.
>>
>> Ok, I see.
>
> Does it matter whether the replacement is a new version or a
> new packet family?

It depends on whether the new packet family provides a superset of
the AF_PACKET features or not. If we can expect that all users of
AF_PACKET can migrate to the replacement over time, then doing
it there is sufficient, otherwise adding 64-bit timestamps into AF_PACKET
may be a better way to upgrade existing users.

>>>>> If adding support for existing applications is useful, another approach
>>>>> would be to add a new socket option that changes the semantics for
>>>>> the two u32 fields in each of V1, V2 and V3 to hold nsec. Add a single
>>>>> check after filling in those structs whether the option is set and, if so,
>>>>> overwrite the two fields.
>>>>>
>>>>> [1] https://lwn.net/Articles/737947/
>>>>
>>>> I don't think that's necessary. As long as the V4 capabilities are a
>>>> superset of V1-V3, we should be able to just require all users to
>>>> move to V4 (or later) in the next 89 years, and make sure that they
>>>> use unsigned seconds if they care about 2038.
>>>>
>>>
>>> Given that V4 wont be around for AF_PACKET -- at least not in the
>>> shape of our patches -- Willem's suggestion is probably a good way
>>> forward.
>>
>> That leaves one question: should we do that now, or wait until some
>> other reason for a V4 comes up? I don't mind creating another
>> patch for this, just want to get a feeling of whether the API clutter
>> is worth it when we have a way out that works until y2106 (at
>> which point we run into other problems as well).
>
> I don't expect that we'll have another packet version independent
> from the work that Björn is doing. The choice to implement using
> a new packet family is given by the complexity of the existing code,
> especially the various locking mechanisms.

Ok.

> From that point of view, and if we want to offer a Y2106 proof
> AF_PACKET independent from the above, no reason to wait.

Agreed.

       Arnd