Re: [Qemu-devel] [PATCH v3] net: Transmit zero UDP checksum as 0xFFFF

[Eric Blake <eblake@redhat.com>]

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On 11/15/2017 09:07 PM, Ed Swierk via Qemu-devel wrote:

[meta-comment]

> The checksum algorithm used by IPv4, TCP and UDP allows a zero value
> to be represented by either 0x0000 and 0xFFFF. But per RFC 768, a zero
> UDP checksum must be transmitted as 0xFFFF, as 0x0000 is a special
> value meaning no checksum.
> 
> Substitute 0xFFFF whenever a checksum is computed as zero when
> modifying a UDP datagram header. Doing this on IPv4 packets and TCP
> segments is unnecessary but legal. Add a wrapper for
> net_checksum_finish() that makes the substitution.
> 
> (We can't just change net_checksum_finish(), as that function is also
> used by receivers to verify checksums, and in that case the expected
> value is always 0x0000.)

This part below...

> 
> v3:
> 
> Leave net_tx_pkt_update_ip_checksums() alone since it's only computing
> a partial sum of the IP pseudo-header.
> 
> Rename wrapper to net_checksum_finish_nozero() for clarity.
> 
> v2:
> 
> Add a wrapper net_checksum_finish_hdr() rather than duplicating the
> logic at every caller.

...through here, belongs...

> 
> Signed-off-by: Ed Swierk <eswierk@skyportsystems.com>
> ---

...here, after the --- separator (you can have more than one ---; 'git
am' stops parsing at the first).  It is data that is useful to
reviewers, but will be meaningless a year from now when browsing git
history (at which point we won't care how many revisions the patch went
through on the list, but only what was checked in).

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


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