Re: [RFC PATCH] Fix false positives in can_checksum_protocol()

[David Woodhouse <dwmw2@infradead.org>]

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On Mon, 2015-09-28 at 12:13 -0700, Tom Herbert wrote:
> 
> > Perhaps a better solution would be a bit in the skbuff which indicates
> > that it *is* a TCP or UDP checksum. That would be set by our UDP and
> > TCP sockets, cleared by encapsulation, also set if appropriate by
> > skb_partial_csum_set().
> >
> Yes I agree. What I have been thinking to do is steal two bits from
> csum_offset that would indicate that the checksum is IPv4 or IPv6
> (specifically that the checksum value is seeded with an IPv4 or IPv6
> pseudo header). This information plus the csum_offset would be
> sufficient for drivers to identify the checksum as UDP/TCP-IPv4/IPv6.

> The other case that needs special handling is inner vs. outer
> checksum, but that can be deduced by comparing (inner of outer)
> transport offset to checksum start. With this and a couple of utility
> functions we should be able to start deprecating NETIF_F_IP_CSUM and
> NETIF_F_IPV6_CSUM.

You mean drivers which currently set NETIF_F_IP_CSUM would need to
provide a .ndo_features_check() which tolerates only the packets they
can actually handle? And we'd just ensure that the bits are there for
them to use, in the skbuff? That seems reasonable.

Note that 'seeded with an IPv[46] pseudo header' isn't quite
sufficient. Some hardware like 8139cp is explicitly told to do a UDP or
a TCP checksum with a bit in the descriptor, so any UDP-like or TCP
-like checksum works out fine.

Other hardware works out whether to do a UDP or a TCP checksum for
*itself*, so it *can't* cope with other protocols which just happen to
look the same. For those it really *must* be IPPROTO_TCP or IPPROTO_UDP
and they're going to be looking in the IP header for it.

I do suspect we'll want a bit which says it's *actually* TCP or UDP,
not just 'seeded with a pseudo-header'. That's the important
distinction for NETIF_F_IP_CSUM vs. NETIF_F_HW_CSUM.

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


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