Andrii Nakryiko [off-list ref] writes:

Not sure if it's more efficient but running
    if ((flags & XDP) & ((flags & XDP) - 1) != 0)

returns whether a number is a multiple of 2.
Should be equivalent to what you checked with hweight32. It is 
less readable though. Just thought I'd throw that in.
Taken from 
https://graphics.stanford.edu/~seander/bithacks.html#DetermineIfPowerOf2

+		NL_SET_ERR_MSG(extack, "Only one XDP mode flag can 
be set");
+		return -EINVAL;
+	}
+	/* old_prog != NULL implies XDP_FLAGS_REPLACE is set */
+	if (old_prog && !(flags & XDP_FLAGS_REPLACE)) {
+		NL_SET_ERR_MSG(extack, "XDP_FLAGS_REPLACE is not 
specified");
+		return -EINVAL;
 	}
 
-	prog_id = dev_xdp_prog_id(dev, mode);
-	if (flags & XDP_FLAGS_REPLACE) {
-		if (expected_fd >= 0) {
-			prog = bpf_prog_get_type_dev(expected_fd,
- 
BPF_PROG_TYPE_XDP,
-						     bpf_op == 
ops->ndo_bpf);
-			if (IS_ERR(prog))
-				return PTR_ERR(prog);
-			expected_id = prog->aux->id;
-			bpf_prog_put(prog);
-		}
-
-		if (prog_id != expected_id) {
-			NL_SET_ERR_MSG(extack, "Active program 
does not match expected");
-			return -EEXIST;
-		}
+	mode = dev_xdp_mode(flags);
+	cur_prog = dev_xdp_prog(dev, mode);
+	if ((flags & XDP_FLAGS_REPLACE) && cur_prog != old_prog) {
+		NL_SET_ERR_MSG(extack, "Active program does not 
match expected");
+		return -EEXIST;
 	}
-	if (fd >= 0) {
+	if ((flags & XDP_FLAGS_UPDATE_IF_NOEXIST) && cur_prog) {
+		NL_SET_ERR_MSG(extack, "XDP program already 
attached");
+		return -EBUSY;
+	}
+
+	if (new_prog) {
+		bool offload = mode == XDP_MODE_HW;
 		enum bpf_xdp_mode other_mode = mode == 
 XDP_MODE_SKB
 					       ? XDP_MODE_DRV : 
 XDP_MODE_SKB;
 
-		if (!offload && dev_xdp_prog_id(dev, other_mode)) 
{
+		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;
 		}
-
-		if ((flags & XDP_FLAGS_UPDATE_IF_NOEXIST) && 
prog_id) {
-			NL_SET_ERR_MSG(extack, "XDP program 
already attached");
-			return -EBUSY;
-		}
-
-		prog = bpf_prog_get_type_dev(fd, 
BPF_PROG_TYPE_XDP,
-					     bpf_op == 
ops->ndo_bpf);
-		if (IS_ERR(prog))
-			return PTR_ERR(prog);
-
-		if (!offload && bpf_prog_is_dev_bound(prog->aux)) 
{
+		if (!offload && 
bpf_prog_is_dev_bound(new_prog->aux)) {
 			NL_SET_ERR_MSG(extack, "Using device-bound 
 program without HW_MODE flag is not supported");
-			bpf_prog_put(prog);
 			return -EINVAL;
 		}
-
-		if (prog->expected_attach_type == BPF_XDP_DEVMAP) 
{
+		if (new_prog->expected_attach_type == 
BPF_XDP_DEVMAP) {
 			NL_SET_ERR_MSG(extack, "BPF_XDP_DEVMAP 
 programs can not be attached to a device");
-			bpf_prog_put(prog);
 			return -EINVAL;
 		}
-
-		if (prog->expected_attach_type == BPF_XDP_CPUMAP) 
{
-			NL_SET_ERR_MSG(extack,
-				       "BPF_XDP_CPUMAP programs 
can not be attached to a device");
-			bpf_prog_put(prog);
+		if (new_prog->expected_attach_type == 
BPF_XDP_CPUMAP) {
+			NL_SET_ERR_MSG(extack, "BPF_XDP_CPUMAP 
programs can not be attached to a device");
 			return -EINVAL;
 		}
+	}
 
-		/* prog->aux->id may be 0 for orphaned 
device-bound progs */
-		if (prog->aux->id && prog->aux->id == prog_id) {
-			bpf_prog_put(prog);
-			return 0;
+	/* don't call drivers if the effective program didn't 
change */
+	if (new_prog != cur_prog) {
+		bpf_op = dev_xdp_bpf_op(dev, mode);
+		if (!bpf_op) {
+			NL_SET_ERR_MSG(extack, "Underlying driver 
does not support XDP in native mode");
+			return -EOPNOTSUPP;
 		}
-	} else {
-		if (!prog_id)
-			return 0;
-		prog = NULL;
-	}
 
-	err = dev_xdp_install(dev, mode, bpf_op, extack, flags, 
prog);
-	if (err < 0 && prog) {
-		bpf_prog_put(prog);
-		return err;
+		err = dev_xdp_install(dev, mode, bpf_op, extack, 
flags, new_prog);
+		if (err)
+			return err;
 	}
-	dev_xdp_set_prog(dev, mode, prog);
+
+	dev_xdp_set_prog(dev, mode, new_prog);
+	if (cur_prog)
+		bpf_prog_put(cur_prog);
 
 	return 0;
 }
 
+/**
+ *	dev_change_xdp_fd - set or clear a bpf program for a 
device rx path
+ *	@dev: device
+ *	@extack: netlink extended ack
+ *	@fd: new program fd or negative value to clear
+ *	@expected_fd: old program fd that userspace expects to 
replace or clear
+ *	@flags: xdp-related flags
+ *
+ *	Set or clear a bpf program for a device
+ */
+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);
+	struct bpf_prog *new_prog = NULL, *old_prog = NULL;
+	int err;
+
+	ASSERT_RTNL();
+
+	if (fd >= 0) {
+		new_prog = bpf_prog_get_type_dev(fd, 
BPF_PROG_TYPE_XDP,
+						 mode != 
XDP_MODE_SKB);
+		if (IS_ERR(new_prog))
+			return PTR_ERR(new_prog);
+	}
+
+	if (expected_fd >= 0) {
+		old_prog = bpf_prog_get_type_dev(expected_fd, 
BPF_PROG_TYPE_XDP,
+						 mode != 
XDP_MODE_SKB);
+		if (IS_ERR(old_prog)) {
+			err = PTR_ERR(old_prog);
+			old_prog = NULL;
+			goto err_out;
+		}
+	}
+
+	err = dev_xdp_attach(dev, extack, new_prog, old_prog, 
flags);
+
+err_out:
+	if (err && new_prog)
+		bpf_prog_put(new_prog);
+	if (old_prog)
+		bpf_prog_put(old_prog);
+	return err;
+}
+
 /**
  *	dev_new_index	-	allocate an ifindex
  *	@net: the applicable net namespace