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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Al Viro' <viro@zeniv.linux.org.uk>,
        "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>
CC:     gus Gusenleitner Klaus <gus@keba.com>,
        Al Viro <viro@ftp.linux.org.uk>,
        Thomas Gleixner <tglx@linutronix.de>,
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        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>,
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        "kuba@kernel.org" <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Eric Dumazet <edumazet@google.com>
Subject: RE: [RFC][PATCHES v2] checksum stuff
Thread-Topic: [RFC][PATCHES v2] checksum stuff
Thread-Index: AQHaJyGyb9WP/snuGU2gdqpp5IxoMLCcGsYA
Date:   Wed, 6 Dec 2023 11:10:45 +0000
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From: Al Viro
> Sent: 05 December 2023 02:21
> 
> We need a way for csum_and_copy_{from,to}_user() to report faults.
> The approach taken back in 2020 (avoid 0 as return value by starting
> summing from ~0U, use 0 to report faults) had been broken; it does
> yield the right value modulo 2^16-1, but the case when data is
> entirely zero-filled is not handled right.  It almost works, since
> for most of the codepaths we have a non-zero value added in
> and there 0 is not different from anything divisible by 0xffff.
> However, there are cases (ICMPv4 replies, for example) where we
> are not guaranteed that.
> 
> In other words, we really need to have those primitives return 0
> on filled-with-zeroes input.

Do we?
I've not seen any justification for this at all.
IIRC the ICMPv4 reply code needs the checksum function return 0xffff
for all-zero input.

So the correct and simple fix is to initialise the sum to 0xffff
in the checksum function.

	David

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