Date:   Tue, 11 Apr 2017 09:46:56 -0700
From:   Tom Herbert <tom@...bertland.com>
To:     Edward Cree <ecree@...arflare.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>
Subject: Re: RFC: Checksum offload and XDP

On Tue, Apr 11, 2017 at 8:55 AM, Edward Cree <ecree@...arflare.com> wrote:
> On 10/04/17 19:26, Tom Herbert wrote:
>> Not having checksum offload in XDP is going to get more painful once
>> we start seeing a lot programs doing packet modifications. One nice
>> thing we do for ILA router is pre-compute the checksum delta necessary
>> to maintain checksum neutral property in the packet. So that after
>> doing ILA routing in XDP the checksum complete value is still valid as
>> is the transport layer checksum.
>>
>> It's conceivable we could generalize this by having a u16 checksum
>> delta returned from XDP program. If the checksum diff can be
>> pre-computed in a structure for doing the translation, then there
>> should be little cost other than making API a little more complex. On
>> return the checksum_complete value is updated jusy by adding in the
>> diff value.
>>
>> Tom
>
> AIUI you're suggesting to have the user's BPF program do this calculation  and (somehow) feed the diff back to the caller.  As well as adding work
>  for the XDP program writer, it also means trusting they got it right.
> My suggestion was that the eBPF verifier could insert instructions around
>  every write that update the diff value automagically, and not allow the
>  user's program to touch it directly.
> There would be some cleverness required, for instance to determine which
>  byte of the checksum to add to (and thus sometimes shift the byte diff
>  by 8 before adding it in) - which might require a runtime check when the
>  load is indexed.  Or might not, if the verifier sees something like
>  packet[ETH_HLEN + (ihl << 2)] and can deduce the low bit of the offset.
> For the simple case, we would translate "write byte to packet+1" into:
>         diff -= packet[1];
>         write byte to packet+1;
>         diff += packet[1];
> Alexei, does this sound reasonable?
>
> The counter-argument, of course, is that if the XDP program edits fields
>  that are protected by an Internet checksum (which in practice usually
>  means anything but the Ethernet header) and then fixes up the checksums
>  itself, we will edit our diff value twice just to conclude that diff==0.
>  And maybe we are willing to trust root with the ability to lie to the
>  kernel about incoming packets' checksum_complete values.
>
Works for modifying IPv4 or when we are expressly doing a checksum
neutral operation, but that doesn't cover the general case. Suppose we
simply want to pop an IPv6 header in IPv6/IPv6 encpasulation (there is
some push in IETF for expanded use of IPIP tunneling with IPv6 to deal
with the inserting EH into a packet problem). In that case the bits
being chucked won't sum to zero and they're probably not also the same
so the trick for ILA won't work so we need something to rectify the
checksum if we're doing XDP_PASS for instance.

As far as trust is concerned I don't think that is a major issue. The
XDP program can already modify the packet in arbitrary ways so if the
checksum is messed up this likely results in unnecessarily dropping
packets that actually a good checksum (as opposed to allowing packets
with bad checksum to pass). Note that this only applies to
checksum_complete, if we were to allow XDP program to return
checksum_unnecessary for instance then it's more a leap of faith that
things are always correct.

Tom

> -Ed
>

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