Re: [PATCH 3.2 085/115] veth: donât modify ip_summed; doing so treats packets with bad checksums as good.

[Ben Greear]

On 04/28/2016 03:29 AM, Sabrina Dubroca wrote:
> Hello,
>
> 2016-04-27, 17:14:44 -0700, Ben Greear wrote:
> > On 04/27/2016 05:00 PM, Hannes Frederic Sowa wrote:
> > > Hi Ben,
> > >
> > > On Wed, Apr 27, 2016, at 20:07, Ben Hutchings wrote:
> > > > On Wed, 2016-04-27 at 08:59 -0700, Ben Greear wrote:
> > > > > On 04/26/2016 04:02 PM, Ben Hutchings wrote:
> > > > > > 3.2.80-rc1 review patch.  If anyone has any objections, please let me know.
> > > > > I would be careful about this.  It causes regressions when sending
> > > > > PACKET_SOCKET buffers from user-space to veth devices.
> > > > >
> > > > > There was a proposed upstream fix for the regression, but it has not gone
> > > > > into the tree as far as I know.
> > > > >
> > > > > http://www.spinics.net/lists/netdev/msg370436.html
> > > > [...]
> > > >
> > > > OK, I'll drop this for now.
> > >
> > > The fall out from not having this patch is in my opinion a bigger
> > > fallout than not having this patch. This patch fixes silent data
> > > corruption vs. the problem Ben Greear is talking about, which might not
> > > be that a common usage.
> > >
> > > What do others think?
> > >
> > > Bye,
> > > Hannes
> >
> > This patch from Cong Wang seems to fix the regression for me, I think it should be added and
> > tested in the main tree, and then apply them to stable as a pair.
> >
> > http://dmz2.candelatech.com/?p=linux-4.4.dev.y/.git;a=commitdiff;h=8153e983c0e5eba1aafe1fc296248ed2a553f1ac;hp=454b07405d694dad52e7f41af5816eed0190da8a
>
> Actually, no, this is not really a regression.
>
> If you capture packets on a device with checksum offloading enabled,
> the TCP/UDP checksum isn't filled.  veth also behaves that way.  What
> the "veth: don't modify ip_summed" patch does is enable proper
> checksum validation on veth.  This really was a bug in veth.
>
> Cong's patch would also break cases where we choose to inject packets
> with invalid checksums, and they would now be accepted as correct.
>
> Your use case is invalid, it just happened to work because of a
> bug.  If you want the stack to fill checksums so that you want capture
> and reinject packets, you have to disable checksum offloading (or
> compute the checksum yourself in userspace).

Disabling checksum offloading or computing in user-space (and then
recomputing in veth to verify the checksum?) is a huge performance loss.

Maybe we could add a socket option to enable Cong's patch on a per-socket
basis?  That way my use-case can still work and you can have this new
behaviour by default?

Thanks,
Ben


--
Ben Greear <greearb@xxxxxxxxxxxxxxx>
Candela Technologies Inc  http://www.candelatech.com