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    SubjectRe: [PATCH 3.2 085/115] veth: don’t modify ip summ ed; doing so treats packets with bad checksums as good.
    [oops – resending this because I was using gmail in HTML mode before
    by accident]

    There was a discussion on a separate thread about this. I agree with
    Sabrina fully. I believe veth should provide an abstraction layer that
    correctly emulates a physical network in all ways.

    Consider an environment where we have multiple physical computers.
    Each computer runs one or more containers, each of which has a
    publicly routable ip address. When adding a new app to the cluster, a
    scheduler might decide to run this container on any physical machine
    of its choice, assuming that apps have a way of routing traffic to
    their backends (we did something similar Google >10 years ago). This
    is something we might imagine happening with docker and ipv6 for
    instance.

    If you have an app, A, which sends raw ethernet frames (the simplest
    case I could imagine) with TCP data that has invalid checksums to app
    B, which is receiving it, the behaviour of the system _will be
    different_ depending upon whether app B is scheduled to run on the
    same machine as app A or not. This seems like a clear bug and a broken
    abstraction (especially as the default case), and something we should
    endeavour to avoid.

    I do see Ben's point about enabling sw checksum verification as
    potentially incurring a huge performance penalty (I haven't had a
    chance to measure it) that is completely wasteful in the vast majority
    of cases.

    Unfortunately I just don't see how we can solve this problem in a way
    that preserves a correct abstraction layer while also avoiding excess
    work. I guess the first piece of data that would be helpful is to
    determine just how big of a performance penalty this is. If it's
    small, then maybe it doesn't matter.




    On Thu, Apr 28, 2016 at 6:29 AM, Sabrina Dubroca <sd@queasysnail.net> 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).
    >
    > Thanks.
    >
    > --
    > Sabrina

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