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

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Message-ID: <4fa60957-2718-cac2-4b01-12aaf48b76b4@tessares.net>
Date:   Wed, 12 Apr 2023 18:44:48 +0200
From:   Matthieu Baerts <matthieu.baerts@...sares.net>
To:     Jan Engelhardt <jengelh@...i.de>
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 12/04/2023 18:14, Jan Engelhardt wrote:
> 
> 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 hope such check doesn't exist :)

IPPROTO_MPTCP is only used when creating the socket, with the "protocol"
parameter which accepts an integer.

>> 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.

Just in case a uint8 is used somewhere, we fallback to TCP (6):

  IPPROTO_MPTCP & 0xff = IPPROTO_TCP

Instead of IPPROTO_ICMP (1).

We did that to be on the safe side, not knowing all the different
userspace implementations :)

Cheers,
Matt
-- 
Tessares | Belgium | Hybrid Access Solutions
www.tessares.net