macvtap/tun: cross-endian support for little-endian hosts

The VNET_LE flag was introduced to fix accesses to virtio 1.0 headers
that are always little-endian. It can also be used to handle the special
case of a legacy little-endian device implemented by a big-endian host.

Let's add a flag and ioctls for big-endian devices as well. If both flags
are set, little-endian wins.

Since this is isn't a common usecase, the feature is controlled by a kernel
config option (not set by default).

Both macvtap and tun are covered by this patch since they share the same
API with userland.

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

1 files changed, 58 insertions, 1 deletions

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index b210139bef8f..cb376b2d548a 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -111,6 +111,7 @@ do {								\
 #define TUN_FASYNC      IFF_ATTACH_QUEUE
 /* High bits in flags field are unused. */
 #define TUN_VNET_LE     0x80000000
+#define TUN_VNET_BE     0x40000000
 
 #define TUN_FEATURES (IFF_NO_PI | IFF_ONE_QUEUE | IFF_VNET_HDR | \
                       IFF_MULTI_QUEUE)
@@ -206,10 +207,58 @@ struct tun_struct {
         u32 flow_count;
 };
 
+#ifdef CONFIG_TUN_VNET_CROSS_LE
+static inline bool tun_legacy_is_little_endian(struct tun_struct *tun)
+{
+        return tun->flags & TUN_VNET_BE ? false :
+                virtio_legacy_is_little_endian();
+}
+
+static long tun_get_vnet_be(struct tun_struct *tun, int __user *argp)
+{
+        int be = !!(tun->flags & TUN_VNET_BE);
+
+        if (put_user(be, argp))
+                return -EFAULT;
+
+        return 0;
+}
+
+static long tun_set_vnet_be(struct tun_struct *tun, int __user *argp)
+{
+        int be;
+
+        if (get_user(be, argp))
+                return -EFAULT;
+
+        if (be)
+                tun->flags |= TUN_VNET_BE;
+        else
+                tun->flags &= ~TUN_VNET_BE;
+
+        return 0;
+}
+#else
+static inline bool tun_legacy_is_little_endian(struct tun_struct *tun)
+{
+        return virtio_legacy_is_little_endian();
+}
+
+static long tun_get_vnet_be(struct tun_struct *tun, int __user *argp)
+{
+        return -EINVAL;
+}
+
+static long tun_set_vnet_be(struct tun_struct *tun, int __user *argp)
+{
+        return -EINVAL;
+}
+#endif /* CONFIG_TUN_VNET_CROSS_LE */
+
 static inline bool tun_is_little_endian(struct tun_struct *tun)
 {
         return tun->flags & TUN_VNET_LE ||
-                virtio_legacy_is_little_endian();
+                tun_legacy_is_little_endian(tun);
 }
 
 static inline u16 tun16_to_cpu(struct tun_struct *tun, __virtio16 val)
@@ -2062,6 +2111,14 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
                         tun->flags &= ~TUN_VNET_LE;
                 break;
 
+        case TUNGETVNETBE:
+                ret = tun_get_vnet_be(tun, argp);
+                break;
+
+        case TUNSETVNETBE:
+                ret = tun_set_vnet_be(tun, argp);
+                break;
+
         case TUNATTACHFILTER:
                 /* Can be set only for TAPs */
                 ret = -EINVAL;