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

[Willem de Bruijn]

On Tue, Nov 28, 2017 at 3:46 AM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Tue, Nov 28, 2017 at 8:04 AM, BjÃrn TÃpel <bjorn.topel@xxxxxxxxx> wrote:
>> 2017-11-27 21:51 GMT+01:00 Arnd Bergmann <arnd@xxxxxxxx>:
>> [...]
>>>> 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 BjÃrn 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?

>>>> 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 BjÃrn 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.

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