[Qemu-devel] [PATCH 17/19] Work around dhclient brokenness

[Mark McLoughlin]

From: Anthony Liguori <address@hidden>

With the latest GSO/csum offload patches, any guest using an unpatched version
of dhclient (any Ubuntu guest, for instance), will no longer be able to get
a DHCP address.

dhclient is actually at fault here.  It uses AF_PACKET to receive DHCP responses
but does not check auxdata to see if the packet has a valid csum.  This causes
it to throw out the DHCP responses it gets from the virtio interface as there
is not a valid checksum.

Fedora has carried a patch to fix their dhclient (it's needed for Xen too) but
this patch has not made it into a release of dhclient.  AFAIK, the patch is in
the dhclient CVS but I cannot confirm since their CVS is not public.

This patch, suggested by Rusty, looks for UDP packets (of a normal MTU) and
explicitly adds a checksum to them if they are missing one.

This allows unpatched dhclients to continue to work without needing to update
the guest kernels.

Signed-off-by: Anthony Liguori <address@hidden>
Signed-off-by: Mark McLoughlin <address@hidden>
---
 hw/virtio-net.c |   29 +++++++++++++++++++++++++++++
 1 files changed, 29 insertions(+), 0 deletions(-)

diff --git a/hw/virtio-net.c b/hw/virtio-net.c
index 3fb10a2..e00c404 100644
--- a/hw/virtio-net.c
+++ b/hw/virtio-net.c
@@ -372,6 +372,34 @@ static int virtio_net_can_receive(VLANClientState *vc)
     return do_virtio_net_can_receive(n, VIRTIO_NET_MAX_BUFSIZE);
 }
 
+/* dhclient uses AF_PACKET but doesn't pass auxdata to the kernel so
+ * it never finds out that the packets don't have valid checksums.  This
+ * causes dhclient to get upset.  Fedora's carried a patch for ages to
+ * fix this with Xen but it hasn't appeared in an upstream release of
+ * dhclient yet.
+ *
+ * To avoid breaking existing guests, we catch udp packets and add
+ * checksums.  This is terrible but it's better than hacking the guest
+ * kernels.
+ *
+ * N.B. if we introduce a zero-copy API, this operation is no longer free so
+ * we should provide a mechanism to disable it to avoid polluting the host
+ * cache.
+ */
+static void work_around_broken_dhclient(struct virtio_net_hdr *hdr,
+                                        const uint8_t *buf, size_t size)
+{
+    if ((hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) && /* missing csum */
+        (size > 27 && size < 1500) && /* normal sized MTU */
+        (buf[12] == 0x08 && buf[13] == 0x00) && /* ethertype == IPv4 */
+        (buf[23] == 17) && /* ip.protocol == UDP */
+        (buf[34] == 0 && buf[35] == 67)) { /* udp.srcport == bootps */
+        /* FIXME this cast is evil */
+        net_checksum_calculate((uint8_t *)buf, size);
+        hdr->flags &= ~VIRTIO_NET_HDR_F_NEEDS_CSUM;
+    }
+}
+
 static int iov_fill(struct iovec *iov, int iovcnt, const void *buf, int count)
 {
     int offset, i;
@@ -399,6 +427,7 @@ static int receive_header(VirtIONet *n, struct iovec *iov, 
int iovcnt,
     if (n->has_vnet_hdr) {
         memcpy(hdr, buf, sizeof(*hdr));
         offset = sizeof(*hdr);
+        work_around_broken_dhclient(hdr, buf + offset, size - offset);
     }
 
     /* We only ever receive a struct virtio_net_hdr from the tapfd,
-- 
1.6.2.5