Re: [PATCH net v2 1/2] virtio-net: xsk: rx: fix the frame's length check

[Bui Quang Minh]

On 6/24/25 17:02, Paolo Abeni wrote:
> On 6/21/25 4:49 PM, Bui Quang Minh wrote:
> > When calling buf_to_xdp, the len argument is the frame data's length
> > without virtio header's length (vi->hdr_len). We check that len with
> >
> > 	xsk_pool_get_rx_frame_size() + vi->hdr_len
> >
> > to ensure the provided len does not larger than the allocated chunk
> > size. The additional vi->hdr_len is because in virtnet_add_recvbuf_xsk,
> > we use part of XDP_PACKET_HEADROOM for virtio header and ask the vhost
> > to start placing data from
> >
> > 	hard_start + XDP_PACKET_HEADROOM - vi->hdr_len
> > not
> > 	hard_start + XDP_PACKET_HEADROOM
> >
> > But the first buffer has virtio_header, so the maximum frame's length in
> > the first buffer can only be
> >
> > 	xsk_pool_get_rx_frame_size()
> > not
> > 	xsk_pool_get_rx_frame_size() + vi->hdr_len
> >
> > like in the current check.
> >
> > This commit adds an additional argument to buf_to_xdp differentiate
> > between the first buffer and other ones to correctly calculate the maximum
> > frame's length.
> >
> > Fixes: a4e7ba702701 ("virtio_net: xsk: rx: support recv small mode")
> It looks like the checks in the blamed commit above are correct and the
> bug has been added with commit 99c861b44eb1f ("virtio_net: xsk: rx:
> support recv merge mode")???

AFAICS, the small mode has only 1 buffer per frame and that buffer is 
quite the same as first buffer in mergeable mode. That buffer still has 
virtio header (though it's smaller than in mergeable case), so the 
remaining space for data is only xsk_pool_get_rx_frame_size() not 
xsk_pool_get_rx_frame_size() + vi->hdr_len.

Thanks,
Quang Minh.