kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 341 insertions, 0 deletions

diff --git a/drivers/net/can/vxcan.c b/drivers/net/can/vxcan.c
new file mode 100644
index 000000000..e88225018
--- /dev/null
+++ b/drivers/net/can/vxcan.c
@@ -0,0 +1,341 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * vxcan.c - Virtual CAN Tunnel for cross namespace communication
+ *
+ * This code is derived from drivers/net/can/vcan.c for the virtual CAN
+ * specific parts and from drivers/net/veth.c to implement the netlink API
+ * for network interface pairs in a common and established way.
+ *
+ * Copyright (c) 2017 Oliver Hartkopp <socketcan@hartkopp.net>
+ */
+
+#include <linux/ethtool.h>
+#include <linux/module.h>
+#include <linux/init.h>
+#include <linux/netdevice.h>
+#include <linux/if_arp.h>
+#include <linux/if_ether.h>
+#include <linux/can.h>
+#include <linux/can/dev.h>
+#include <linux/can/skb.h>
+#include <linux/can/vxcan.h>
+#include <linux/can/can-ml.h>
+#include <linux/slab.h>
+#include <net/can.h>
+#include <net/rtnetlink.h>
+
+#define DRV_NAME "vxcan"
+
+MODULE_DESCRIPTION("Virtual CAN Tunnel");
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Oliver Hartkopp <socketcan@hartkopp.net>");
+MODULE_ALIAS_RTNL_LINK(DRV_NAME);
+
+struct vxcan_priv {
+	struct net_device __rcu	*peer;
+};
+
+static netdev_tx_t vxcan_xmit(struct sk_buff *oskb, struct net_device *dev)
+{
+	struct vxcan_priv *priv = netdev_priv(dev);
+	struct net_device *peer;
+	struct net_device_stats *peerstats, *srcstats = &dev->stats;
+	struct can_skb_ext *csx;
+	struct sk_buff *skb;
+	unsigned int len;
+
+	if (can_dropped_invalid_skb(dev, oskb))
+		return NETDEV_TX_OK;
+
+	rcu_read_lock();
+	peer = rcu_dereference(priv->peer);
+	if (unlikely(!peer)) {
+		kfree_skb(oskb);
+		dev->stats.tx_dropped++;
+		goto out_unlock;
+	}
+
+	skb_tx_timestamp(oskb);
+
+	skb = skb_clone(oskb, GFP_ATOMIC);
+	if (skb) {
+		consume_skb(oskb);
+	} else {
+		kfree_skb(oskb);
+		goto out_unlock;
+	}
+
+	/* the cloned skb points to the skb extension of the already cloned
+	 * oskb with an increased refcount. skb_ext_add() creates a copy to
+	 * separate the skb extension data which is needed to start with a
+	 * fresh can_gw_hops counter in the other namespace.
+	 */
+	csx = skb_ext_add(skb, SKB_EXT_CAN);
+	if (!csx) {
+		kfree_skb(skb);
+		goto out_unlock;
+	}
+
+	/* reset CAN GW hop counter */
+	csx->can_gw_hops = 0;
+	skb->pkt_type   = PACKET_BROADCAST;
+	skb->dev        = peer;
+	skb->ip_summed  = CHECKSUM_UNNECESSARY;
+
+	len = can_skb_get_data_len(skb);
+	if (netif_rx(skb) == NET_RX_SUCCESS) {
+		srcstats->tx_packets++;
+		srcstats->tx_bytes += len;
+		peerstats = &peer->stats;
+		peerstats->rx_packets++;
+		peerstats->rx_bytes += len;
+	}
+
+out_unlock:
+	rcu_read_unlock();
+	return NETDEV_TX_OK;
+}
+
+
+static int vxcan_open(struct net_device *dev)
+{
+	struct vxcan_priv *priv = netdev_priv(dev);
+	struct net_device *peer = rtnl_dereference(priv->peer);
+
+	if (!peer)
+		return -ENOTCONN;
+
+	if (peer->flags & IFF_UP) {
+		netif_carrier_on(dev);
+		netif_carrier_on(peer);
+	}
+	return 0;
+}
+
+static int vxcan_close(struct net_device *dev)
+{
+	struct vxcan_priv *priv = netdev_priv(dev);
+	struct net_device *peer = rtnl_dereference(priv->peer);
+
+	netif_carrier_off(dev);
+	if (peer)
+		netif_carrier_off(peer);
+
+	return 0;
+}
+
+static int vxcan_get_iflink(const struct net_device *dev)
+{
+	struct vxcan_priv *priv = netdev_priv(dev);
+	struct net_device *peer;
+	int iflink;
+
+	rcu_read_lock();
+	peer = rcu_dereference(priv->peer);
+	iflink = peer ? READ_ONCE(peer->ifindex) : 0;
+	rcu_read_unlock();
+
+	return iflink;
+}
+
+static void vxcan_set_cap_info(struct net_device *dev)
+{
+	u32 can_cap = CAN_CAP_CC;
+
+	if (dev->mtu > CAN_MTU)
+		can_cap |= CAN_CAP_FD;
+
+	if (dev->mtu >= CANXL_MIN_MTU)
+		can_cap |= CAN_CAP_XL;
+
+	can_set_cap(dev, can_cap);
+}
+
+static int vxcan_change_mtu(struct net_device *dev, int new_mtu)
+{
+	/* Do not allow changing the MTU while running */
+	if (dev->flags & IFF_UP)
+		return -EBUSY;
+
+	if (new_mtu != CAN_MTU && new_mtu != CANFD_MTU &&
+	    !can_is_canxl_dev_mtu(new_mtu))
+		return -EINVAL;
+
+	WRITE_ONCE(dev->mtu, new_mtu);
+	vxcan_set_cap_info(dev);
+	return 0;
+}
+
+static const struct net_device_ops vxcan_netdev_ops = {
+	.ndo_open	= vxcan_open,
+	.ndo_stop	= vxcan_close,
+	.ndo_start_xmit	= vxcan_xmit,
+	.ndo_get_iflink	= vxcan_get_iflink,
+	.ndo_change_mtu = vxcan_change_mtu,
+};
+
+static const struct ethtool_ops vxcan_ethtool_ops = {
+	.get_ts_info = ethtool_op_get_ts_info,
+};
+
+static void vxcan_setup(struct net_device *dev)
+{
+	struct can_ml_priv *can_ml;
+
+	dev->type		= ARPHRD_CAN;
+	dev->mtu		= CANXL_MTU;
+	dev->hard_header_len	= 0;
+	dev->addr_len		= 0;
+	dev->tx_queue_len	= 0;
+	dev->flags		= IFF_NOARP;
+	dev->netdev_ops		= &vxcan_netdev_ops;
+	dev->ethtool_ops	= &vxcan_ethtool_ops;
+	dev->needs_free_netdev	= true;
+
+	can_ml = netdev_priv(dev) + ALIGN(sizeof(struct vxcan_priv), NETDEV_ALIGN);
+	can_set_ml_priv(dev, can_ml);
+	vxcan_set_cap_info(dev);
+}
+
+/* forward declaration for rtnl_create_link() */
+static struct rtnl_link_ops vxcan_link_ops;
+
+static int vxcan_newlink(struct net_device *dev,
+			 struct rtnl_newlink_params *params,
+			 struct netlink_ext_ack *extack)
+{
+	struct net *peer_net = rtnl_newlink_peer_net(params);
+	struct nlattr **data = params->data;
+	struct nlattr **tb = params->tb;
+	struct vxcan_priv *priv;
+	struct net_device *peer;
+
+	struct nlattr *peer_tb[IFLA_MAX + 1], **tbp = tb;
+	char ifname[IFNAMSIZ];
+	unsigned char name_assign_type;
+	struct ifinfomsg *ifmp = NULL;
+	int err;
+
+	/* register peer device */
+	if (data && data[VXCAN_INFO_PEER]) {
+		struct nlattr *nla_peer = data[VXCAN_INFO_PEER];
+
+		ifmp = nla_data(nla_peer);
+		rtnl_nla_parse_ifinfomsg(peer_tb, nla_peer, extack);
+		tbp = peer_tb;
+	}
+
+	if (ifmp && tbp[IFLA_IFNAME]) {
+		nla_strscpy(ifname, tbp[IFLA_IFNAME], IFNAMSIZ);
+		name_assign_type = NET_NAME_USER;
+	} else {
+		snprintf(ifname, IFNAMSIZ, DRV_NAME "%%d");
+		name_assign_type = NET_NAME_ENUM;
+	}
+
+	peer = rtnl_create_link(peer_net, ifname, name_assign_type,
+				&vxcan_link_ops, tbp, extack);
+	if (IS_ERR(peer))
+		return PTR_ERR(peer);
+
+	if (ifmp && dev->ifindex)
+		peer->ifindex = ifmp->ifi_index;
+
+	err = register_netdevice(peer);
+	if (err < 0) {
+		free_netdev(peer);
+		return err;
+	}
+
+	netif_carrier_off(peer);
+
+	err = rtnl_configure_link(peer, ifmp, 0, NULL);
+	if (err < 0)
+		goto unregister_network_device;
+
+	/* register first device */
+	if (tb[IFLA_IFNAME])
+		nla_strscpy(dev->name, tb[IFLA_IFNAME], IFNAMSIZ);
+	else
+		snprintf(dev->name, IFNAMSIZ, DRV_NAME "%%d");
+
+	err = register_netdevice(dev);
+	if (err < 0)
+		goto unregister_network_device;
+
+	netif_carrier_off(dev);
+
+	/* cross link the device pair */
+	priv = netdev_priv(dev);
+	rcu_assign_pointer(priv->peer, peer);
+
+	priv = netdev_priv(peer);
+	rcu_assign_pointer(priv->peer, dev);
+
+	return 0;
+
+unregister_network_device:
+	unregister_netdevice(peer);
+	return err;
+}
+
+static void vxcan_dellink(struct net_device *dev, struct list_head *head)
+{
+	struct vxcan_priv *priv;
+	struct net_device *peer;
+
+	priv = netdev_priv(dev);
+	peer = rtnl_dereference(priv->peer);
+
+	/* Note : dellink() is called from default_device_exit_batch(),
+	 * before a rcu_synchronize() point. The devices are guaranteed
+	 * not being freed before one RCU grace period.
+	 */
+	RCU_INIT_POINTER(priv->peer, NULL);
+	unregister_netdevice_queue(dev, head);
+
+	if (peer) {
+		priv = netdev_priv(peer);
+		RCU_INIT_POINTER(priv->peer, NULL);
+		unregister_netdevice_queue(peer, head);
+	}
+}
+
+static const struct nla_policy vxcan_policy[VXCAN_INFO_MAX + 1] = {
+	[VXCAN_INFO_PEER] = { .len = sizeof(struct ifinfomsg) },
+};
+
+static struct net *vxcan_get_link_net(const struct net_device *dev)
+{
+	struct vxcan_priv *priv = netdev_priv(dev);
+	struct net_device *peer = rtnl_dereference(priv->peer);
+
+	return peer ? dev_net(peer) : dev_net(dev);
+}
+
+static struct rtnl_link_ops vxcan_link_ops = {
+	.kind		= DRV_NAME,
+	.priv_size	= ALIGN(sizeof(struct vxcan_priv), NETDEV_ALIGN) + sizeof(struct can_ml_priv),
+	.setup		= vxcan_setup,
+	.newlink	= vxcan_newlink,
+	.dellink	= vxcan_dellink,
+	.policy		= vxcan_policy,
+	.peer_type	= VXCAN_INFO_PEER,
+	.maxtype	= VXCAN_INFO_MAX,
+	.get_link_net	= vxcan_get_link_net,
+};
+
+static __init int vxcan_init(void)
+{
+	pr_info("vxcan: Virtual CAN Tunnel driver\n");
+
+	return rtnl_link_register(&vxcan_link_ops);
+}
+
+static __exit void vxcan_exit(void)
+{
+	rtnl_link_unregister(&vxcan_link_ops);
+}
+
+module_init(vxcan_init);
+module_exit(vxcan_exit);