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

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Date:   Mon, 27 Nov 2017 15:35:45 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Jiri Pirko <jiri@...nulli.us>
Cc:     Arnd Bergmann <arnd@...db.de>,
        "David S. Miller" <davem@...emloft.net>, 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
Subject: Re: [PATCH 2/2] [RFC] packet: experimental support for 64-bit timestamps

On Mon, Nov 27, 2017 at 11:59 AM, Jiri Pirko <jiri@...nulli.us> wrote:
> Mon, Nov 27, 2017 at 05:19:25PM CET, arnd@...db.de wrote:
>>I tried to figure out what it would take to do a version 4 mmap packet
>>socket interface to completely avoid the y2106 overflow problem. This is
>>what I came up with, reusing most of the v3 code, except for the parts
>>where we access the timestamps.
>>
>>For kselftest, I'm adding support for testing v4 in addition to v1-v3,
>>but the test currently does not look at the timestamps, so it won't
>>check that the timestamp format actually works as intended, only that
>>I didn't break the parts that worked in the v3 selftest.
>>
>>Overall, this is more of a mess than I expected, so it's probably not
>>worth doing a v4 format just for the timestamp, but the patch can serve
>>as a reference for anyone that needs a new format for other reasons and
>>fixes this along with the other changes.
>>
>>Signed-off-by: Arnd Bergmann <arnd@...db.de>
>>---
>
> [...]
>
>
>>@@ -250,7 +269,8 @@ struct tpacket_block_desc {
>> enum tpacket_versions {
>>       TPACKET_V1,
>>       TPACKET_V2,
>>-      TPACKET_V3
>>+      TPACKET_V3,
>>+      TPACKET_V4,
>
> I wonder with how many versions are we going to eventually end up with :O

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.

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/