Subject: Re: [PATCH net-next v2] net: openvswitch: fix uAPI incompatibility with existing user space

From: Vlad Buslov <[email-masked]>
Date: Thu, 12 May 2022 13:08:14 +0300

On Thu 12 May 2022 at 12:19, Eelco Chaudron <[email-masked]> wrote:
> On 7 Apr 2022, at 12:22, Ilya Maximets wrote:
>
>> On 4/7/22 10:02, Vlad Buslov wrote:
>>> On Mon 14 Mar 2022 at 20:40, Ilya Maximets <[email-masked]> wrote:
>>>> On 3/14/22 19:33, Roi Dayan wrote:
>>>>>
>>>>>
>>>>> On 2022-03-10 8:44 PM, Aaron Conole wrote:
>>>>>> Ilya Maximets <[email-masked]> writes:
>>>>>>
>>>>>>> Few years ago OVS user space made a strange choice in the commit [1]
>>>>>>> to define types only valid for the user space inside the copy of a
>>>>>>> kernel uAPI header.  '#ifndef __KERNEL__' and another attribute was
>>>>>>> added later.
>>>>>>>
>>>>>>> This leads to the inevitable clash between user space and kernel types
>>>>>>> when the kernel uAPI is extended.  The issue was unveiled with the
>>>>>>> addition of a new type for IPv6 extension header in kernel uAPI.
>>>>>>>
>>>>>>> When kernel provides the OVS_KEY_ATTR_IPV6_EXTHDRS attribute to the
>>>>>>> older user space application, application tries to parse it as
>>>>>>> OVS_KEY_ATTR_PACKET_TYPE and discards the whole netlink message as
>>>>>>> malformed.  Since OVS_KEY_ATTR_IPV6_EXTHDRS is supplied along with
>>>>>>> every IPv6 packet that goes to the user space, IPv6 support is fully
>>>>>>> broken.
>>>>>>>
>>>>>>> Fixing that by bringing these user space attributes to the kernel
>>>>>>> uAPI to avoid the clash.  Strictly speaking this is not the problem
>>>>>>> of the kernel uAPI, but changing it is the only way to avoid breakage
>>>>>>> of the older user space applications at this point.
>>>>>>>
>>>>>>> These 2 types are explicitly rejected now since they should not be
>>>>>>> passed to the kernel.  Additionally, OVS_KEY_ATTR_TUNNEL_INFO moved
>>>>>>> out from the '#ifdef __KERNEL__' as there is no good reason to hide
>>>>>>> it from the userspace.  And it's also explicitly rejected now, because
>>>>>>> it's for in-kernel use only.
>>>>>>>
>>>>>>> Comments with warnings were added to avoid the problem coming back.
>>>>>>>
>>>>>>> (1 << type) converted to (1ULL << type) to avoid integer overflow on
>>>>>>> OVS_KEY_ATTR_IPV6_EXTHDRS, since it equals 32 now.
>>>>>>>
>>>>>>>   [1] beb75a40fdc2 ("userspace: Switching of L3 packets in L2 pipeline")
>>>>>>>
>>>>>>> Fixes: 28a3f0601727 ("net: openvswitch: IPv6: Add IPv6 extension header support")
>>>>>>> Link: https://lore.kernel.org/netdev/[email-masked]
>>>>>>> Link: https://github.com/openvswitch/ovs/commit/beb75a40fdc295bfd6521b0068b4cd12f6de507c
>>>>>>> Reported-by: Roi Dayan <[email-masked]>
>>>>>>> Signed-off-by: Ilya Maximets <[email-masked]>
>>>>>>> ---
>>>>>>
>>>>>> Acked-by: Aaron Conole <[email-masked]>
>>>>>>
>>>>>
>>>>>
>>>>>
>>>>> I got to check traffic with the fix and I do get some traffic
>>>>> but something is broken. I didn't investigate much but the quick
>>>>> test shows me rules are not offloaded and dumping ovs rules gives
>>>>> error like this
>>>>>
>>>>> recirc_id(0),in_port(enp8s0f0_1),ct_state(-trk),eth(),eth_type(0x86dd),ipv6(frag=no)(bad
>>>>> key length 2, expected -1)(00 00/(bad mask length 2, expected -1)(00 00),
>>>>> packets:2453, bytes:211594, used:0.004s, flags:S., actions:ct,recirc(0x2)
>>>>
>>>> Such a dump is expected, because kernel parses fields that current
>>>> userspace doesn't understand, and at the same time OVS by design is
>>>> using kernel provided key/mask while installing datapath rules, IIRC.
>>>> It should be possible to make these dumps a bit more friendly though.
>>>>
>>>> For the offloading not working, see my comment in the v2 patch email
>>>> I sent (top email of this thread).  In short, it's a problem in user
>>>> space and it can not be fixed from the kernel side, unless we revert
>>>> IPv6 extension header support and never add any new types, which is
>>>> unreasonable.  I didn't test any actual offloading, but I had a
>>>> successful run of 'make check-offloads' with my quick'n'dirty fix from
>>>> the top email.
>>>
>>> Hi Ilya,
>>>
>>> I can confirm that with latest OvS master IPv6 rules offload still fails
>>> without your pastebin code applied.
>>>
>>>>
>>>> Since we're here:
>>>>
>>>> Toms, do you plan to submit user space patches for this feature?
>>>
>>> I see there is a patch from you that is supposed to fix compatibility
>>> issues caused by this change in OvS d96d14b14733 ("openvswitch.h: Align
>>> uAPI definition with the kernel."), but it doesn't fix offload for me
>>> without pastebin patch.
>>
>> Yes.  OVS commit d96d14b14733 is intended to only fix the uAPI.
>> Issue with offload is an OVS bug that should be fixed separately.
>> The fix will also need to be backported to OVS stable branches.
>>
>>> Do you plan to merge that code into OvS or you
>>> require some help from our side?
>>
>> I could do that, but I don't really have enough time.  So, if you
>> can work on that fix, it would be great.  Note that comments inside
>> the OVS's lib/odp-util.c:parse_key_and_mask_to_match() was blindly
>> copied from the userspace datapath and are incorrect for the general
>> case, so has to be fixed alongside the logic of that function.
>
> Tom or Vlad, are you working on this? Asking, as the release of a kernel with
> Tom’s “net: openvswitch: IPv6: Add IPv6 extension header support” patch will
> break OVS.
>
> //Eelco

Hi Eelco,

My simple fix for OvS was rejected and I don't have time to rework it at
the moment.

Regards,
Vlad