linux-kernel - Re: [PATCH] amd64: Fix csum_partial_copy

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Message-ID: <20231018154205.GT800259@ZenIV>
Date:   Wed, 18 Oct 2023 16:42:05 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     gus Gusenleitner Klaus <gus@...a.com>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
        "David S. Miller" <davem@...emloft.net>,
        "dsahern@...nel.org" <dsahern@...nel.org>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "kuba@...nel.org" <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH] amd64: Fix csum_partial_copy_generic()

On Wed, Oct 18, 2023 at 06:18:05AM +0000, gus Gusenleitner Klaus wrote:
> The checksum calculation is wrong in case of an source buffer
> containing zero bytes only. The expected return value is 0, the
> actual return value is 0xfffffff.

Expected where?  The actual checksum is defined modulo 0xffff, so
0 and 0xffffffff represent the same final value.

The only twist is that in some situations we internally use 0 for
"not calculated yet".

> This problem occurs when a ICMP echo reply is sent that has set
> zero identifier, sequence number and data.

What problem?  Could you please either point to specific RFC or
show that packets are rejected by some existing system, or...?