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

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Message-ID: <1a185332-3693-2750-fef2-f6938bbc8500@ovn.org>
Date:   Mon, 14 Mar 2022 20:40:43 +0100
From:   Ilya Maximets <i.maximets@....org>
To:     Roi Dayan <roid@...dia.com>, Aaron Conole <aconole@...hat.com>
Cc:     i.maximets@....org, Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Pravin B Shelar <pshelar@....org>,
        Toms Atteka <cpp.code.lv@...il.com>, netdev@...r.kernel.org,
        dev@...nvswitch.org, linux-kernel@...r.kernel.org,
        Johannes Berg <johannes@...solutions.net>,
        Maor Dickman <maord@...dia.com>
Subject: Re: [PATCH net-next v2] net: openvswitch: fix uAPI incompatibility
 with existing user space

On 3/14/22 19:33, Roi Dayan wrote:
> 
> 
> On 2022-03-10 8:44 PM, Aaron Conole wrote:
>> Ilya Maximets <i.maximets@....org> 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/3adf00c7-fe65-3ef4-b6d7-6d8a0cad8a5f@nvidia.com
>>> Link: https://github.com/openvswitch/ovs/commit/beb75a40fdc295bfd6521b0068b4cd12f6de507c
>>> Reported-by: Roi Dayan <roid@...dia.com>
>>> Signed-off-by: Ilya Maximets <i.maximets@....org>
>>> ---
>>
>> Acked-by: Aaron Conole <aconole@...hat.com>
>>
> 
> 
> 
> 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.

Since we're here:

Toms, do you plan to submit user space patches for this feature?

Best regards, Ilya Maximets.