UDP GRO do not restore ervery UDP Packet

UDP GRO do not restore ervery UDP Packet

[Sean Tong]

I find that I do not restore every UDP paket from recvmsg when I
programming with  UDP_GRO in kernel version 5.0
I can get gso_size, but I do not know the last UDP packet size in
aggregate datagram.
For example, recvmsg returns  a packet size 50  of  and gso_size of
24, so the UDP packets is restored without knowing whether it is
24+24+2 split into 3 packets or 24+26 split into 2 pakcets.
Is there any way to get the last UDP packet size or number of merged
UDP packets?

Re: UDP GRO do not restore ervery UDP Packet

[Paolo Abeni]

Hi,

On Tue, 2019-04-09 at 14:09 +0800, Sean Tong wrote:
> For example, recvmsg returns  a packet size 50  of  and gso_size of
> 24, so the UDP packets is restored without knowing whether it is
> 24+24+2 split into 3 packets or 24+26 split into 2 pakcets.
> Is there any way to get the last UDP packet size or number of merged
> UDP packets?

In the described scenario, the ingress packets lengths are: 24, 24, 2.

The sequence 24, 26 is not possible.

Generally speaking, if the aggregated packet length is not a multiple
of gro_size, the length of the last UDP packet aggregated in the GRO
one is: 

<GRO packet size> % gso_size.

(2, in the above example).

If you observe something differnt, it's a bug and must be fixed -
please provide detailed setup information, thanks!

Paolo