bpf: Fix XDP FD-based attach/detach logic around XDP_FLAGS_UPDATE_IF_NOEXIST

Enforce XDP_FLAGS_UPDATE_IF_NOEXIST only if new BPF program to be attached is
non-NULL (i.e., we are not detaching a BPF program).

Fixes: d4baa936 ("bpf, xdp: Extract common XDP program attachment logic")
Reported-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Tested-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20200812022923.1217922-1-andriin@fb.com

net/core/dev.c

@@ -8913,10 +8913,6 @@ static int dev_xdp_attach(struct net_device *dev, struct netlink_ext_ack *extack
 		NL_SET_ERR_MSG(extack, "Active program does not match expected");
 		return -EEXIST;
 	}
-	if ((flags & XDP_FLAGS_UPDATE_IF_NOEXIST) && cur_prog) {
-		NL_SET_ERR_MSG(extack, "XDP program already attached");
-		return -EBUSY;
-	}
 
 	/* put effective new program into new_prog */
 	if (link)
@@ -8927,6 +8923,10 @@ static int dev_xdp_attach(struct net_device *dev, struct netlink_ext_ack *extack
 		enum bpf_xdp_mode other_mode = mode == XDP_MODE_SKB
 					       ? XDP_MODE_DRV : XDP_MODE_SKB;
 
+		if ((flags & XDP_FLAGS_UPDATE_IF_NOEXIST) && cur_prog) {
+			NL_SET_ERR_MSG(extack, "XDP program already attached");
+			return -EBUSY;
+		}
 		if (!offload && dev_xdp_prog(dev, other_mode)) {
 			NL_SET_ERR_MSG(extack, "Native and generic XDP can't be active at the same time");
 			return -EEXIST;