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

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Date:   Thu, 19 Oct 2023 06:02:50 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     gus Gusenleitner Klaus <gus@...a.com>
Cc:     Al Viro <viro@....linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        lkml <linux-kernel@...r.kernel.org>,
        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>,
        "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: AW: [PATCH] amd64: Fix csum_partial_copy_generic()

On Thu, Oct 19, 2023 at 04:44:04AM +0000, gus Gusenleitner Klaus wrote:
> > 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...?
> 
> Here's our situation:
> Our device gets pinged by a third party manufacturer robot controller.
> We have updated the kernel in our device to 5.15 from 4.9, the robot
> controller is kept unchanged. At 4.9, our device's ping reply is accepted
> by the robot controller, at 5.15 it's not.
> 
> Wireshark shows a bad checksum warning:
>  'Checksum: 0x0000 incorrect, should be 0xffff' 
> 

Lovely.  I think I see what's going on, give me a few to think about it...