netdev - Re: [PATCH 9/9 v2] net: Add sysctl to trust checksum

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Date:	Sat, 26 Apr 2014 19:31:35 -0700
From:	Stephen Hemminger <stephen@...workplumber.org>
To:	Tom Herbert <therbert@...gle.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH 9/9 v2] net: Add sysctl to trust checksum_complete

On Sat, 26 Apr 2014 14:26:35 -0700 (PDT)
Tom Herbert <therbert@...gle.com> wrote:

> Currently if a device provides CHECKSUM_COMPLETE but the checksum
> is calculated to be invalid we recompute the checksum and try
> again in software. On the other hand, if device returns
> CHECKSUM_UNNECESSARY we implicitly trust it and don't verify what it
> did. This seems backwards!
> 
> Add a sysctl to trust the device and report an invalid checksum when
> CHECKSUM_COMPLETE shows it is incorrect. sysctl defaults to enabled.
> 
> Signed-off-by: Tom Herbert <therbert@...gle.com>
> ---

NO. Make one choice and do it consistently.

Papering over driver bugs or design confusion with a sysctl is not a
reasonable choice.

If some device (or code path) has invalid checksum logic, it should
be reported once and go ahead and fix it in software. The problem with
CHECKSUM_UNNECESSARY is that there is no way to check that the device
is broken without computing the checksum (catch-22).

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