Re: [PATCHv2] vhost-net: add dhclient work-around from userspace

Sridhar Samudrala
2010-06-28T22:19:41Z

On Mon, 2010-06-28 at 13:08 +0300, Michael S. Tsirkin wrote:
> Userspace virtio server has the following hack
> so guests rely on it, and we have to replicate it, too:
>
> Use port number to detect incoming IPv4 DHCP response packets,
> and fill in the checksum for these.
>
> The issue we are solving is that on linux guests, some apps
> that use recvmsg with AF_PACKET sockets, don't know how to
> handle CHECKSUM_PARTIAL;
> The interface to return the relevant information was added
> in 8dc4194474159660d7f37c495e3fc3f10d0db8cc,
> and older userspace does not use it.
> One important user of recvmsg with AF_PACKET is dhclient,
> so we add a work-around just for DHCP.
>
> Don't bother applying the hack to IPv6 as userspace virtio does not
> have a work-around for that - let's hope guests will do the right
> thing wrt IPv6.
>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>
> Dave, I'm going to put this patch on the vhost tree,
> no need for you to bother merging it - you'll get
> it with a pull request.
>
>
> drivers/vhost/net.c | 44 +++++++++++++++++++++++++++++++++++++++++++-
> 1 files changed, 43 insertions(+), 1 deletions(-)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index cc19595..03bba6a 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -24,6 +24,10 @@
> #include <linux/if_tun.h>
> #include <linux/if_macvlan.h>
>
> +#include <linux/ip.h>
> +#include <linux/udp.h>
> +#include <linux/netdevice.h>
> +
> #include <net/sock.h>
>
> #include "vhost.h"
> @@ -186,6 +190,44 @@ static void handle_tx(struct vhost_net *net)
> unuse_mm(net->dev.mm);
> }
>
> +static int peek_head(struct sock *sk)

This routine is doing more than just peeking the head of sk's receive
queue. May be this should be named similar to what qemu calls
'work_around_broken_dhclient()'
> +{
> + struct sk_buff *skb;
> +
> + lock_sock(sk);
> + skb = skb_peek(&sk->sk_receive_queue);
> + if (unlikely(!skb)) {
> + release_sock(sk);
> + return 0;
> + }
> + /* Userspace virtio server has the following hack so
> + * guests rely on it, and we have to replicate it, too: */
> + /* Use port number to detect incoming IPv4 DHCP response packets,
> + * and fill in the checksum. */
> +
> + /* The issue we are solving is that on linux guests, some apps
> + * that use recvmsg with AF_PACKET sockets, don't know how to
> + * handle CHECKSUM_PARTIAL;
> + * The interface to return the relevant information was added in
> + * 8dc4194474159660d7f37c495e3fc3f10d0db8cc,
> + * and older userspace does not use it.
> + * One important user of recvmsg with AF_PACKET is dhclient,
> + * so we add a work-around just for DHCP. */
> + if (skb->ip_summed == CHECKSUM_PARTIAL &&
> + skb_headlen(skb) >= skb_transport_offset(skb) +
> + sizeof(struct udphdr) &&
> + udp_hdr(skb)->dest == htons(68) &&
> + skb_network_header_len(skb) >= sizeof(struct iphdr) &&
> + ip_hdr(skb)->protocol == IPPROTO_UDP &&
> + skb->protocol == htons(ETH_P_IP)) {

Isn't it more logical to check for skb->protocol, followed by ip_hdr and
then udp_hdr?

> + skb_checksum_help(skb);
> + /* Restore ip_summed value: tun passes it to user. */
> + skb->ip_summed = CHECKSUM_PARTIAL;
> + }
> + release_sock(sk);
> + return 1;
> +}
> +
> /* Expects to be always run from workqueue - which acts as
> * read-size critical section for our kind of RCU. */
> static void handle_rx(struct vhost_net *net)
> @@ -222,7 +264,7 @@ static void handle_rx(struct vhost_net *net)
> vq_log = unlikely(vhost_has_feature(&net->dev, VHOST_F_LOG_ALL)) ?
> vq->log : NULL;
>
> - for (;;) {
> + while (peek_head(sock->sk)) {
> head = vhost_get_vq_desc(&net->dev, vq, vq->iov,
> ARRAY_SIZE(vq->iov),
> &out, &in,