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Date:   Fri, 8 Dec 2023 14:17:12 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     David Laight <David.Laight@aculab.com>
Cc:     "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
        gus Gusenleitner Klaus <gus@keba.com>,
        Al Viro <viro@ftp.linux.org.uk>,
        Thomas Gleixner <tglx@linutronix.de>,
        lkml <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>, "bp@alien8.de" <bp@alien8.de>,
        "dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
        "x86@kernel.org" <x86@kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        "dsahern@kernel.org" <dsahern@kernel.org>,
        "kuba@kernel.org" <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Eric Dumazet <edumazet@google.com>
Subject: Re: [RFC][PATCHES v2] checksum stuff
Message-ID: <20231208141712.GA1674809@ZenIV>
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On Fri, Dec 08, 2023 at 12:04:24PM +0000, David Laight wrote:
> I've just read RFC 792 and done some experiments.
> The kernel ICMP checksum code is just plain broken.
> 
> RFC 792 is quite clear that the checksum is the 16-bit ones's
> complement of the one's complement sum of the ICMP message
> starting with the ICMP Type.
> 
> The one's complement sum of 0xfffe and 0x0001 is zero (not the 0xffff

It is not.  FYI, N-bit one's complement sum is defined as

X + Y <= MAX_N_BIT ? X + Y : X + Y - MAX_N_BIT,

where MAX_N_BIT is 2^N - 1.

You add them as natural numbers.  If there is no carry and result
fits into N bits, that's it.  If there is carry, you add it to
the lower N bits of sum.

Discussion of properties of that operation is present e.g. in
RFC1071, titled "Computing the Internet Checksum".

May I politely suggest that some basic understanding of the
arithmetics involved might be useful for this discussion?