bpf, xdp: move offload check into dev_xdp_install()

bpf_xdp_link_update() calls dev_xdp_install() directly and bypasses
dev_xdp_attach(), so the offload check that lived in dev_xdp_attach()
does not apply. A user can create an XDP link in SKB or native mode
with a regular program and then replace it via BPF_LINK_UPDATE with an
offloaded program, whose bpf_func is bpf_prog_warn_on_exec(), tripping
the WARN on the first packet.

Move the check from dev_xdp_attach() into dev_xdp_install() so both
the attach path and the link update path are covered by a single check
at the actual install site.

Fixes: 026a4c2 ("bpf, xdp: Implement LINK_UPDATE for BPF XDP link")
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>

Author: mrpre

net/core/dev.c

@@ -10359,6 +10359,11 @@ static int dev_xdp_install(struct net_device *dev, enum bpf_xdp_mode mode,
 
 	netdev_ops_assert_locked(dev);
 
+	if (prog && mode != XDP_MODE_HW && bpf_prog_is_offloaded(prog->aux)) {
+		NL_SET_ERR_MSG(extack, "Using offloaded program without HW_MODE flag is not supported");
+		return -EINVAL;
+	}
+
 	if (dev->cfg->hds_config == ETHTOOL_TCP_DATA_SPLIT_ENABLED &&
 	    prog && !prog->aux->xdp_has_frags) {
 		NL_SET_ERR_MSG(extack, "unable to install XDP to device using tcp-data-split");
@@ -10510,10 +10515,6 @@ static int dev_xdp_attach(struct net_device *dev, struct netlink_ext_ack *extack
 			NL_SET_ERR_MSG(extack, "Native and generic XDP can't be active at the same time");
 			return -EEXIST;
 		}
-		if (!offload && bpf_prog_is_offloaded(new_prog->aux)) {
-			NL_SET_ERR_MSG(extack, "Using offloaded program without HW_MODE flag is not supported");
-			return -EINVAL;
-		}
 		if (bpf_prog_is_dev_bound(new_prog->aux) && !bpf_offload_dev_match(new_prog, dev)) {
 			NL_SET_ERR_MSG(extack, "Program bound to different device");
 			return -EINVAL;