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

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Message-ID: <4r3sqop-o651-6o1q-578-o4p519668073@vanv.qr>
Date:   Wed, 12 Apr 2023 20:24:59 +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 18:44, Matthieu Baerts wrote:
>
>> 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 :)

Silent failure? That's terrible.

	int IPPROTO_MPTCP = 257;
	socket(AF_INET, SOCK_STREAM, (uint8_t)IPPROTO_MPTCP);

on the other hand would immediately fail with EPROTONOSUPP
and make hidden uint8 truncation readily visible.