netdev - Re: [net PATCH v5 6/6] virtio_net: XDP support for adjust

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Fri, 20 Jan 2017 11:26:46 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     John Fastabend <john.fastabend@...il.com>,
        john.r.fastabend@...el.com, netdev@...r.kernel.org,
        alexei.starovoitov@...il.com, daniel@...earbox.net
Subject: Re: [net PATCH v5 6/6] virtio_net: XDP support for adjust_head



On 2017年01月20日 05:11, Michael S. Tsirkin wrote:
> On Thu, Jan 19, 2017 at 11:05:40AM +0800, Jason Wang wrote:
>>
>> On 2017年01月18日 23:15, Michael S. Tsirkin wrote:
>>> On Tue, Jan 17, 2017 at 02:22:59PM -0800, John Fastabend wrote:
>>>> Add support for XDP adjust head by allocating a 256B header region
>>>> that XDP programs can grow into. This is only enabled when a XDP
>>>> program is loaded.
>>>>
>>>> In order to ensure that we do not have to unwind queue headroom push
>>>> queue setup below bpf_prog_add. It reads better to do a prog ref
>>>> unwind vs another queue setup call.
>>>>
>>>> At the moment this code must do a full reset to ensure old buffers
>>>> without headroom on program add or with headroom on program removal
>>>> are not used incorrectly in the datapath. Ideally we would only
>>>> have to disable/enable the RX queues being updated but there is no
>>>> API to do this at the moment in virtio so use the big hammer. In
>>>> practice it is likely not that big of a problem as this will only
>>>> happen when XDP is enabled/disabled changing programs does not
>>>> require the reset. There is some risk that the driver may either
>>>> have an allocation failure or for some reason fail to correctly
>>>> negotiate with the underlying backend in this case the driver will
>>>> be left uninitialized. I have not seen this ever happen on my test
>>>> systems and for what its worth this same failure case can occur
>>>> from probe and other contexts in virtio framework.
>>>>
>>>> Signed-off-by: John Fastabend<john.r.fastabend@...el.com>
>>> I've been thinking about it - can't we drop
>>> old buffers without the head room which were posted before
>>> xdp attached?
>>>
>>> Avoiding the reset would be much nicer.
>>>
>>> Thoughts?
>>>
>> As been discussed before, device may use them in the same time so it's not
>> safe. Or do you mean detect them after xdp were set and drop the buffer
>> without head room, this looks sub-optimal.
>>
>> Thanks
> Yes, this is what I mean.  Why is this suboptimal? It's a single branch
> in code. Yes we might lose some packets but the big hammer of device
> reset will likely lose more.
>

Maybe I was wrong but I think driver should try their best to avoid 
dropping packets. (And look at mlx4, it did something similar to this 
patch).

Thanks