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, 77 insertions, 0 deletions

diff --git a/include/net/arp.h b/include/net/arp.h
new file mode 100644
index 000000000..e8747e071
--- /dev/null
+++ b/include/net/arp.h
@@ -0,0 +1,77 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/* linux/net/inet/arp.h */
+#ifndef _ARP_H
+#define _ARP_H
+
+#include <linux/if_arp.h>
+#include <linux/hash.h>
+#include <net/neighbour.h>
+
+
+extern struct neigh_table arp_tbl;
+
+static inline u32 arp_hashfn(const void *pkey, const struct net_device *dev, u32 *hash_rnd)
+{
+	u32 key = *(const u32 *)pkey;
+	u32 val = key ^ hash32_ptr(dev);
+
+	return val * hash_rnd[0];
+}
+
+#ifdef CONFIG_INET
+static inline struct neighbour *__ipv4_neigh_lookup_noref(struct net_device *dev, u32 key)
+{
+	if (dev->flags & (IFF_LOOPBACK | IFF_POINTOPOINT))
+		key = INADDR_ANY;
+
+	return ___neigh_lookup_noref(&arp_tbl, neigh_key_eq32, arp_hashfn, &key, dev);
+}
+#else
+static inline
+struct neighbour *__ipv4_neigh_lookup_noref(struct net_device *dev, u32 key)
+{
+	return NULL;
+}
+#endif
+
+static inline struct neighbour *__ipv4_neigh_lookup(struct net_device *dev, u32 key)
+{
+	struct neighbour *n;
+
+	rcu_read_lock();
+	n = __ipv4_neigh_lookup_noref(dev, key);
+	if (n && !refcount_inc_not_zero(&n->refcnt))
+		n = NULL;
+	rcu_read_unlock();
+
+	return n;
+}
+
+static inline void __ipv4_confirm_neigh(struct net_device *dev, u32 key)
+{
+	struct neighbour *n;
+
+	rcu_read_lock();
+	n = __ipv4_neigh_lookup_noref(dev, key);
+	neigh_confirm(n);
+	rcu_read_unlock();
+}
+
+void arp_init(void);
+int arp_ioctl(struct net *net, unsigned int cmd, void __user *arg);
+void arp_send(int type, int ptype, __be32 dest_ip,
+	      struct net_device *dev, __be32 src_ip,
+	      const unsigned char *dest_hw,
+	      const unsigned char *src_hw, const unsigned char *th);
+int arp_mc_map(__be32 addr, u8 *haddr, struct net_device *dev, int dir);
+void arp_ifdown(struct net_device *dev);
+int arp_invalidate(struct net_device *dev, __be32 ip, bool force);
+
+struct sk_buff *arp_create(int type, int ptype, __be32 dest_ip,
+			   struct net_device *dev, __be32 src_ip,
+			   const unsigned char *dest_hw,
+			   const unsigned char *src_hw,
+			   const unsigned char *target_hw);
+void arp_xmit(struct sk_buff *skb);
+
+#endif	/* _ARP_H */