netdev - Re: [PATCH net,v2] uapi: linux: restore IPPROTO_MAX to 256 and add IPPROTO_UAPI

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Date:   Wed, 12 Apr 2023 18:14:25 +0200 (CEST)
From:   Jan Engelhardt <jengelh@...i.de>
To:     Matthieu Baerts <matthieu.baerts@...sares.net>
cc:     Jakub Kicinski <kuba@...nel.org>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        netfilter-devel@...r.kernel.org, davem@...emloft.net,
        netdev@...r.kernel.org, pabeni@...hat.com, edumazet@...gle.com,
        mathew.j.martineau@...ux.intel.com, mptcp@...ts.linux.dev
Subject: Re: [PATCH net,v2] uapi: linux: restore IPPROTO_MAX to 256 and add
 IPPROTO_UAPI_MAX


On Wednesday 2023-04-12 17:22, Matthieu Baerts wrote:
>>> But I don't know how to
>>> make sure this will not have any impact on MPTCP on the userspace side,
>>> e.g. somewhere before calling the socket syscall, a check could be done
>>> to restrict the protocol number to IPPROTO_MAX and then breaking MPTCP
>>> support.
>> 
>> Then again any code which stores the ipproto in an unsigned char will 
>> be broken. A perfect solution is unlikely to exist.

IPPROTO_MPTCP (262) is new, anything using MPTCP is practically new code
for the purposes of discussion, and when MPTCP support is added to some
application, you simply *have to* update any potential uint8 check.

>I wonder if the root cause is not the fact we mix the usage of the
>protocol parameter from the socket syscall (int/s32) and the protocol
>field from the IP header (u8).
>
>To me, the enum is linked to the socket syscall, not the IP header. In
>this case, it would make sense to have a dedicated "MAX" macro for the
>IP header, no?

IPPROTO_MAX (256) was the sensible maximum value [array size]
for both the IP header *and* the socket interface.

Then the socket interface was extended, so IPPROTO_MAX, at the very
least, keeps the meanings it can keep, which is for the one for the
IP header.
Makes me wonder why MPTCP got 262 instead of just 257.