netdev - Re: [patch net-next v2] net: virtio_net: implement exact header length guest feature

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Date:   Tue, 21 Feb 2023 16:39:32 +0100
From:   Jiri Pirko <jiri@...nulli.us>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        pabeni@...hat.com, edumazet@...gle.com, mst@...hat.com,
        jasowang@...hat.com, virtualization@...ts.linux-foundation.org,
        alvaro.karsz@...id-run.com, vmireyno@...vell.com, parav@...dia.com
Subject: Re: [patch net-next v2] net: virtio_net: implement exact header
 length guest feature

Tue, Feb 21, 2023 at 04:11:53PM CET, willemdebruijn.kernel@...il.com wrote:
>Jiri Pirko wrote:
>> From: Jiri Pirko <jiri@...dia.com>
>> 
>> Virtio spec introduced a feature VIRTIO_NET_F_GUEST_HDRLEN which when
>> set implicates that the driver provides the exact size of the header.
>> 
>> Quoting the original virtio spec:
>> "hdr_len is a hint to the device as to how much of the header needs to
>>  be kept to copy into each packet"
>> 
>> "a hint" might not be clear for the reader what does it mean, if it is
>> "maybe like that" of "exactly like that". This feature just makes it
>> crystal clear and let the device count on the hdr_len being filled up
>> by the exact length of header.
>> 
>> Also note the spec already has following note about hdr_len:
>> "Due to various bugs in implementations, this field is not useful
>>  as a guarantee of the transport header size."
>> 
>> Without this feature the device needs to parse the header in core
>> data path handling. Accurate information helps the device to eliminate
>> such header parsing and directly use the hardware accelerators
>> for GSO operation.
>> 
>> virtio_net_hdr_from_skb() fills up hdr_len to skb_headlen(skb).
>> The driver already complies to fill the correct value. Introduce the
>> feature and advertise it.
>> 
>> Note that virtio spec also includes following note for device
>> implementation:
>> "Caution should be taken by the implementation so as to prevent
>>  a malicious driver from attacking the device by setting
>>  an incorrect hdr_len."
>> 
>> There is a plan to support this feature in our emulated device.
>> A device of SolidRun offers this feature bit. They claim this feature
>> will save the device a few cycles for every GSO packet.
>> 
>> Signed-off-by: Jiri Pirko <jiri@...dia.com>
>> ---
>> v1->v2:
>> - extended patch description
>
>Is the expectation that in-kernel devices support this feature, and
>if so how would it affect them? If I read the spec correctly, devices

Well, the tap driver actually trusts the hdr_len to be of correct header
size nowadays.


>still need to be careful against malicious drivers, so cannot assume
>much beyond what they do today (i.e., a hint).

Malicious how? There is upper limit of size in tap which is checked.
I assume that for hw implementation, that would be the same.

But anyway, this discussion would be rather part of the spec/device
patch, don't you think?


>
>Might be good to point to the definition commit:
>https://github.com/oasis-tcs/virtio-spec/commit/4f1981a1ff46b7aeb801c4c524ff76e93d9ce022

There were couple of fixes to the spec since then, that's why I didn't
include it. It is trivial to look it up in the spec.