[PATCH net-next] bpf, xdp: move offload check into dev_xdp_install()

[Jiayuan Chen <jiayuan.chen@linux.dev>]

bpf_xdp_link_update() calls dev_xdp_install() directly and skips
dev_xdp_attach(), so the checks in dev_xdp_attach() do not run. A user can
make an XDP link with a normal program and then swap in an offloaded or
device-bound program with BPF_LINK_UPDATE, which puts it on the software
path.

dev_xdp_install() is the one place all three paths go through:
"ip link set xdp" and BPF_LINK_CREATE reach it via dev_xdp_attach(), and
BPF_LINK_UPDATE calls it directly. So move the program checks (offloaded,
bound to another device, device-bound in generic mode, native vs generic,
DEVMAP and CPUMAP) there, and keep only the netlink-flag check
(XDP_FLAGS_UPDATE_IF_NOEXIST) in dev_xdp_attach().

Fixes: 026a4c28e1db3 ("bpf, xdp: Implement LINK_UPDATE for BPF XDP link")
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
1. Previous topic: https://lore.kernel.org/netdev/20260811171047.4e6a409c@kernel.org/
2. Tested with tools/testing/selftests/net/bpf_offload.py and the BPF CI; both pass.
---
 net/core/dev.c | 59 ++++++++++++++++++++++++++------------------------
 1 file changed, 31 insertions(+), 28 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index fd0b445f5d38..b390c2edfb33 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -10360,6 +10360,37 @@ static int dev_xdp_install(struct net_device *dev, enum bpf_xdp_mode mode,
 
 	netdev_assert_locked_ops_compat(dev);
 
+	if (prog) {
+		enum bpf_xdp_mode other_mode = mode == XDP_MODE_SKB
+					       ? XDP_MODE_DRV : XDP_MODE_SKB;
+		bool offload = mode == XDP_MODE_HW;
+
+		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 (!offload && bpf_prog_is_offloaded(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(prog->aux) && !bpf_offload_dev_match(prog, dev)) {
+			NL_SET_ERR_MSG(extack, "Program bound to different device");
+			return -EINVAL;
+		}
+		if (bpf_prog_is_dev_bound(prog->aux) && mode == XDP_MODE_SKB) {
+			NL_SET_ERR_MSG(extack, "Can't attach device-bound programs in generic mode");
+			return -EINVAL;
+		}
+		if (prog->expected_attach_type == BPF_XDP_DEVMAP) {
+			NL_SET_ERR_MSG(extack, "BPF_XDP_DEVMAP programs can not be attached to a device");
+			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");
+			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");
@@ -10499,38 +10530,10 @@ static int dev_xdp_attach(struct net_device *dev, struct netlink_ext_ack *extack
 		new_prog = link->link.prog;
 
 	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 ((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;
-		}
-		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;
-		}
-		if (bpf_prog_is_dev_bound(new_prog->aux) && mode == XDP_MODE_SKB) {
-			NL_SET_ERR_MSG(extack, "Can't attach device-bound programs in generic mode");
-			return -EINVAL;
-		}
-		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");
-			return -EINVAL;
-		}
-		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;
-		}
 	}
 
 	/* don't call drivers if the effective program didn't change */
-- 
2.43.0