netdev - Re: [PATCH 1/1] vhost: Added pad cleanup if vnet

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Date: Mon, 15 Jan 2024 23:44:32 +0200
From: Yuri Benditovich <yuri.benditovich@...nix.com>
To: linux-kernel@...r.kernel.org
Cc: mst@...hat.com, jasowang@...hat.com, kvm@...r.kernel.org, 
	virtualization@...ts.linux.dev, netdev@...r.kernel.org, yan@...nix.com, 
	Andrew Melnychenko <andrew@...nix.com>
Subject: Re: [PATCH 1/1] vhost: Added pad cleanup if vnet_hdr is not present.

See also https://issues.redhat.com/browse/RHEL-1303


On Mon, Jan 15, 2024 at 9:48 PM Andrew Melnychenko <andrew@...nix.com> wrote:
>
> When the Qemu launched with vhost but without tap vnet_hdr,
> vhost tries to copy vnet_hdr from socket iter with size 0
> to the page that may contain some trash.
> That trash can be interpreted as unpredictable values for
> vnet_hdr.
> That leads to dropping some packets and in some cases to
> stalling vhost routine when the vhost_net tries to process
> packets and fails in a loop.
>
> Qemu options:
>   -netdev tap,vhost=on,vnet_hdr=off,...
>
> Signed-off-by: Andrew Melnychenko <andrew@...nix.com>
> ---
>  drivers/vhost/net.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index f2ed7167c848..57411ac2d08b 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -735,6 +735,9 @@ static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq,
>         hdr = buf;
>         gso = &hdr->gso;
>
> +       if (!sock_hlen)
> +               memset(buf, 0, pad);
> +
>         if ((gso->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) &&
>             vhost16_to_cpu(vq, gso->csum_start) +
>             vhost16_to_cpu(vq, gso->csum_offset) + 2 >
> --
> 2.43.0
>