bpf: xdp: Fix XDP mode when no mode flags specified

7f0a8382 ("bpf, xdp: Maintain info on attached XDP BPF programs in net_device")
inadvertently changed which XDP mode is assumed when no mode flags are
specified explicitly. Previously, driver mode was preferred, if driver
supported it. If not, generic SKB mode was chosen. That commit changed default
to SKB mode always. This patch fixes the issue and restores the original
logic.

Fixes: 7f0a8382 ("bpf, xdp: Maintain info on attached XDP BPF programs in net_device")
Reported-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Tested-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/bpf/20200820052841.1559757-1-andriin@fb.com

net/core/dev.c

@@ -8742,13 +8742,15 @@ struct bpf_xdp_link {
 	int flags;
 };
 
-static enum bpf_xdp_mode dev_xdp_mode(u32 flags)
+static enum bpf_xdp_mode dev_xdp_mode(struct net_device *dev, u32 flags)
 {
 	if (flags & XDP_FLAGS_HW_MODE)
 		return XDP_MODE_HW;
 	if (flags & XDP_FLAGS_DRV_MODE)
 		return XDP_MODE_DRV;
+	if (flags & XDP_FLAGS_SKB_MODE)
 		return XDP_MODE_SKB;
+	return dev->netdev_ops->ndo_bpf ? XDP_MODE_DRV : XDP_MODE_SKB;
 }
 
 static bpf_op_t dev_xdp_bpf_op(struct net_device *dev, enum bpf_xdp_mode mode)
@@ -8896,7 +8898,7 @@ static int dev_xdp_attach(struct net_device *dev, struct netlink_ext_ack *extack
 		return -EINVAL;
 	}
 
-	mode = dev_xdp_mode(flags);
+	mode = dev_xdp_mode(dev, flags);
 	/* can't replace attached link */
 	if (dev_xdp_link(dev, mode)) {
 		NL_SET_ERR_MSG(extack, "Can't replace active BPF XDP link");
@@ -8984,7 +8986,7 @@ static int dev_xdp_detach_link(struct net_device *dev,
 
 	ASSERT_RTNL();
 
-	mode = dev_xdp_mode(link->flags);
+	mode = dev_xdp_mode(dev, link->flags);
 	if (dev_xdp_link(dev, mode) != link)
 		return -EINVAL;
 
@@ -9080,7 +9082,7 @@ static int bpf_xdp_link_update(struct bpf_link *link, struct bpf_prog *new_prog,
 		goto out_unlock;
 	}
 
-	mode = dev_xdp_mode(xdp_link->flags);
+	mode = dev_xdp_mode(xdp_link->dev, xdp_link->flags);
 	bpf_op = dev_xdp_bpf_op(xdp_link->dev, mode);
 	err = dev_xdp_install(xdp_link->dev, mode, bpf_op, NULL,
 			      xdp_link->flags, new_prog);
@@ -9164,7 +9166,7 @@ int bpf_xdp_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
 int dev_change_xdp_fd(struct net_device *dev, struct netlink_ext_ack *extack,
 		      int fd, int expected_fd, u32 flags)
 {
-	enum bpf_xdp_mode mode = dev_xdp_mode(flags);
+	enum bpf_xdp_mode mode = dev_xdp_mode(dev, flags);
 	struct bpf_prog *new_prog = NULL, *old_prog = NULL;
 	int err;