[ovs-dev] [PATCH v2] datapath: Fix checksum update for actions on UDP packets.

[Ben Pfaff]

On Wed, Mar 07, 2012 at 02:26:46PM -0800, Jesse Gross wrote:
> On Wed, Mar 7, 2012 at 10:46 AM, Ben Pfaff <blp at nicira.com> wrote:
> > On Tue, Mar 06, 2012 at 03:40:51PM -0800, Jesse Gross wrote:
> >> When modifying IP addresses or ports on a UDP packet we don't
> >> correctly follow the rules for unchecksummed packets.  This meant
> >> that packets without a checksum can be given a incorrect new checksum
> >> and packets with a checksum can become marked as being unchecksummed.
> >> This fixes it to handle those requirements.
> >>
> >> Bug #8937
> >>
> >> Signed-off-by: Jesse Gross <jesse at nicira.com>
> >> ---
> >> v2: CSUM_MANGLED_0 was not introduced until 2.6.20
> >
> > It's not obvious to me why the UDP cases in this file need a check for
> > OVS_CSUM_PARTIAL but the TCP cases do not.
> 
> There are two things:
>  * inet_proto_csum_replaceX includes a check for CHECKSUM_PARTIAL and
> it is no-op if it's an offloaded checksum and the modified part is not
> in the pseudoheader, so both TCP and UDP do handle this (and did
> before this patch).
>  * When using CHECKSUM_PARTIAL with UDP the checksum of the
> pseudoheader is stored in the checksum field without using
> CSUM_MANGLED_0 so it's possible for it to have the value 0 and be
> interpreted as an unchecksummed packet.  Actual unchecksummed packets
> won't use CHECKSUM_PARTIAL so we know the we always need to update
> these.

Thank you for the explanations.  After staring at the code and the
changes for a while longer, it all makes sense.

Acked-by: Ben Pfaff <blp at nicira.com>
(again)