netdev - Re: [Patch net] net: invert the check of detecting hardware RX checksum fault

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Message-ID: <20181116045019.kmvo36uk3o6bahxu@gondor.apana.org.au>
Date:   Fri, 16 Nov 2018 12:50:19 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Tom Herbert <tom@...bertland.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [Patch net] net: invert the check of detecting hardware RX
 checksum fault

On Thu, Nov 15, 2018 at 06:23:38PM -0800, Cong Wang wrote:
>
> > Normally if the hardware's partial checksum is valid then we just
> > trust it and send the packet along.  However, if the partial
> > checksum is invalid we don't trust it and we will compute the
> > whole checksum manually which is what ends up in sum.
> 
> Not sure if I understand partial checksum here, but it is the
> CHECKSUM_COMPLETE case which I am trying to fix, not
> CHECKSUM_PARTIAL.

What I meant by partial checksum is the checksum produced by the
hardware on RX.  In the kernel we call that CHECKSUM_COMPLETE.
CHECKSUM_PARTIAL is the absence of the substantial part of the
checksum which is something we use in the kernel primarily for TX.

Yes the names are confusing :)

> So, in other word, a checksum *match* is the intended to detect
> this HW RX checksum fault?

Correct.  Or more likely it's probably a bug in either the driver
or if there are overlaying code such as VLAN then in that code.

Basically if the RX checksum is buggy, it's much more likely to
cause a valid packet to be rejected than to cause an invalid packet
to be accepted, because we still verify that checksum against the
pseudoheader.  So we only attempt to catch buggy hardware/drivers
by doing a second manual verification for the case where the packet
is flagged as invalid.

> Sure, my case is nearly same with Pawel's, except I have no vlan:
> https://marc.info/?l=linux-netdev&m=154086647601721&w=2

Can you please provide your backtrace?

Thanks,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt