Re: [PATCH 1/3] ipv6: Select fragment id during UFO/GSO segmentation if not set.

[Hannes Frederic Sowa <hannes@stressinduktion.org>]

On Di, 2015-01-27 at 09:26 -0500, Vlad Yasevich wrote:
> On 01/27/2015 08:47 AM, Hannes Frederic Sowa wrote:
> > On Di, 2015-01-27 at 10:42 +0200, Michael S. Tsirkin wrote:
> >> On Tue, Jan 27, 2015 at 02:47:54AM +0000, Ben Hutchings wrote:
> >>> On Mon, 2015-01-26 at 09:37 -0500, Vladislav Yasevich wrote:
> >>>> If the IPv6 fragment id has not been set and we perform
> >>>> fragmentation due to UFO, select a new fragment id.
> >>>> When we store the fragment id into skb_shinfo, set the bit
> >>>> in the skb so we can re-use the selected id.
> >>>> This preserves the behavior of UFO packets generated on the
> >>>> host and solves the issue of id generation for packet sockets
> >>>> and tap/macvtap devices.
> >>>>
> >>>> This patch moves ipv6_select_ident() back in to the header file.  
> >>>> It also provides the helper function that sets skb_shinfo() frag
> >>>> id and sets the bit.
> >>>>
> >>>> It also makes sure that we select the fragment id when doing
> >>>> just gso validation, since it's possible for the packet to
> >>>> come from an untrusted source (VM) and be forwarded through
> >>>> a UFO enabled device which will expect the fragment id.
> >>>>
> >>>> CC: Eric Dumazet <edumazet@google.com>
> >>>> Signed-off-by: Vladislav Yasevich <vyasevic@redhat.com>
> >>>> ---
> >>>>  include/linux/skbuff.h |  3 ++-
> >>>>  include/net/ipv6.h     |  2 ++
> >>>>  net/ipv6/ip6_output.c  |  4 ++--
> >>>>  net/ipv6/output_core.c |  9 ++++++++-
> >>>>  net/ipv6/udp_offload.c | 10 +++++++++-
> >>>>  5 files changed, 23 insertions(+), 5 deletions(-)
> >>>>
> >>>> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> >>>> index 85ab7d7..3ad5203 100644
> >>>> --- a/include/linux/skbuff.h
> >>>> +++ b/include/linux/skbuff.h
> >>>> @@ -605,7 +605,8 @@ struct sk_buff {
> >>>>  	__u8			ipvs_property:1;
> >>>>  	__u8			inner_protocol_type:1;
> >>>>  	__u8			remcsum_offload:1;
> >>>> -	/* 3 or 5 bit hole */
> >>>> +	__u8			ufo_fragid_set:1;
> >>> [...]
> >>>
> >>> Doesn't the flag belong in struct skb_shared_info, rather than struct
> >>> sk_buff?  Otherwise this looks fine.
> >>>
> >>> Ben.
> >>
> >> Hmm we seem to be out of tx flags.
> >> Maybe ip6_frag_id == 0 should mean "not set".
> > 
> > Maybe that is the best idea. Definitely the ufo_fragid_set bit should
> > move into the skb_shared_info area.
> 
> That's what I originally wanted to do, but had to move and grow txflags thus
> skb_shinfo ended up growing.  I wanted to avoid that, so stole an skb flag.
> 
> I considered treating fragid == 0 as unset, but a 0 fragid is perfectly valid
> from the protocol perspective and could actually be generated by the id generator
> functions.  This may cause us to call the id generation multiple times.

Are there plans in the long run to let virtio_net transmit auxiliary
data to the other end so we can clean all of this this up one day?

I don't like the whole situation: looking into the virtio_net headers
just adding a field for ipv6 fragmentation ids to those small structs
seems bloated, not doing it feels incorrect. :/

Thoughts?

Bye,
Hannes