add/skip packet information on tap send/recv

Unless the IFF_NO_PI flag is set when the tap is created,
the kernel assumes that each packet sent to or received from
a tap fd starts with 4 bytes of packet information.

We previously did not care about this 4 extra bytes, but external
controllers are confused by them (the internal ovs-switch logic was
confused as well, but it defaulted to hub behaviour and the bug
was thus hidden).

Unfortunately, fd_tuntap does not allow us to set IFF_NO_IP, so
we have to manually skip/add the 4 bytes on recv/send.

diff --git a/lib/netdev-pltap.c b/lib/netdev-pltap.c
index 4e010cf..c9252e5 100644 (file)
--- a/lib/netdev-pltap.c
+++ b/lib/netdev-pltap.c
@@ -338,13 +338,16 @@ netdev_pltap_recv(struct netdev *netdev_, void *buffer, size_t size)
 {
     struct netdev_dev_pltap *dev = 
        netdev_dev_pltap_cast(netdev_get_dev(netdev_));
+    char prefix[4];
+    struct iovec iov[2] = {
+        { .iov_base = prefix, .iov_len = 4 },
+       { .iov_base = buffer, .iov_len = size }
+    };
     if (!dev->finalized)
         return -EAGAIN;
     for (;;) {
         ssize_t retval;
-        retval = read(dev->fd, buffer, size);
-       VLOG_DBG("%s: read(%"PRIxPTR", %"PRIu64") = %"PRId64,
-                netdev_get_name(netdev_), (uintptr_t)buffer, size, retval);
+        retval = readv(dev->fd, iov, 2);
         if (retval >= 0) {
             if (retval <= size) {
                return retval;
@@ -376,17 +379,20 @@ netdev_pltap_send(struct netdev *netdev_, const void *buffer, size_t size)
 {
     struct netdev_dev_pltap *dev = 
        netdev_dev_pltap_cast(netdev_get_dev(netdev_));
+    char prefix[4] = { 0, 0, 8, 6 };
+    struct iovec iov[2] = {
+        { .iov_base = prefix, .iov_len = 4 },
+       { .iov_base = buffer, .iov_len = size }
+    };
     if (dev->fd < 0 || !dev->finalized)
         return EAGAIN;
     for (;;) {
         ssize_t retval;
-        retval = write(dev->fd, buffer, size);
-       VLOG_DBG("%s: write(%"PRIxPTR", %"PRIu64") = %"PRId64,
-                netdev_get_name(netdev_), (uintptr_t)buffer, size, retval);
+        retval = writev(dev->fd, iov, 2);
         if (retval >= 0) {
-           if (retval != size) {
-               VLOG_WARN_RL(&rl, "sent partial Ethernet packet (%"PRId64" bytes of "
-                            "%"PRIu64") on %s", retval, size, netdev_get_name(netdev_));
+           if (retval != size + 4) {
+               VLOG_WARN_RL(&rl, "sent partial Ethernet packet (%zd bytes of %zu) on %s",
+                            retval, size + 4, netdev_get_name(netdev_));
            }
             return 0;
         } else if (errno != EINTR) {