netdev - Re: [RFC PATCH] Fix false positives in can_checksum

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Message-ID: <1444076559.4674.205.camel@infradead.org>
Date:	Mon, 05 Oct 2015 21:22:39 +0100
From:	David Woodhouse <dwmw2@...radead.org>
To:	Tom Herbert <tom@...bertland.com>
Cc:	Jesse Brandeburg <jesse.brandeburg@...il.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [RFC PATCH] Fix false positives in can_checksum_protocol()

On Mon, 2015-10-05 at 09:23 -0700, Tom Herbert wrote:
> 
> 1) Drivers may advertise NETIF_F_HW_CSUM. The stack will indicate
> checksum offload exclusively using the
> CHECKSUM_PARTIAL/csum_start/csum_offset interface. No additional
> interfaces (bits in skbuff should not be needed)
> 2) A driver may inspect packets via ndo_check to decide if it wants to
> offload the checksum, if not cancels NETIF_F_HW_CSUM in the packet.
> 3) In driver xmit when CHECKSUM_PARTIAL is set the driver MUST
> correctly resolve the checksum-- either by properly offloading to the
> device or calling skb_checksum_help.

In cases where they haven't used .ndo_features_check() to ensure that
they don't *see* such packets, sure. But using .ndo_features_check()
should probably be the preferred method.

> 4) To help drivers for devices with limited offload capabilities we'll
> define a helper function to check for typical restrictions (.e.g. IPv4
> only, TCP/UDP only. no encapsulation, no IPv6 extension headers,
> etc.). I am working on this helper function and will send RFC shortly.

I do suspect that helper function would benefit from seeing TCP/UDP
flags in the high bits of csum_offset, rather than grubbing around in
the packet for itself to see if it's really TCP/UDP. After all, it's
almost free to set those in the first place — at least for locally
-generated packets. And not *so* hard to add them in
skb_partial_csum_set(). But hey, if you can push an implementation
which is grubbing around in the packet then at least *I* don't have to
feel dirty for it... :)

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@...el.com                              Intel Corporation


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