tap: reset vnet header size on open

For tap, we currently assume the vnet header size is 10
(the default value) but that might not be the case
if tap is persistent and has been used by qemu previously.
To fix, set vnet header size correctly on open.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

diff --git a/net/tap-linux.c b/net/tap-linux.c
index c6521bec344bec393b6204f9b9a9afb722daf363..3eaedc40ad3d18cb135b6592a5fd9701ae81f070 100644 (file)
--- a/net/tap-linux.c
+++ b/net/tap-linux.c
@@ -39,6 +39,7 @@ int tap_open(char *ifname, int ifname_size, int *vnet_hdr, int vnet_hdr_required
 {
     struct ifreq ifr;
     int fd, ret;
+    int len = sizeof(struct virtio_net_hdr);
 
     TFR(fd = open(PATH_NET_TUN, O_RDWR));
     if (fd < 0) {
@@ -65,6 +66,13 @@ int tap_open(char *ifname, int ifname_size, int *vnet_hdr, int vnet_hdr_required
             close(fd);
             return -1;
         }
+        /*
+         * Make sure vnet header size has the default value: for a persistent
+         * tap it might have been modified e.g. by another instance of qemu.
+         * Ignore errors since old kernels do not support this ioctl: in this
+         * case the header size implicitly has the correct value.
+         */
+        ioctl(fd, TUNSETVNETHDRSZ, &len);
     }
 
     if (ifname[0] != '\0')