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, 45 insertions, 0 deletions

diff --git a/include/net/mpls.h b/include/net/mpls.h
new file mode 100644
index 000000000..0bb7944e7
--- /dev/null
+++ b/include/net/mpls.h
@@ -0,0 +1,45 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (c) 2014 Nicira, Inc.
+ */
+
+#ifndef _NET_MPLS_H
+#define _NET_MPLS_H 1
+
+#include <linux/if_ether.h>
+#include <linux/netdevice.h>
+#include <linux/mpls.h>
+
+#define MPLS_HLEN 4
+
+struct mpls_shim_hdr {
+	__be32 label_stack_entry;
+};
+
+static inline bool eth_p_mpls(__be16 eth_type)
+{
+	return eth_type == htons(ETH_P_MPLS_UC) ||
+		eth_type == htons(ETH_P_MPLS_MC);
+}
+
+static inline struct mpls_shim_hdr *mpls_hdr(const struct sk_buff *skb)
+{
+	return (struct mpls_shim_hdr *)skb_network_header(skb);
+}
+
+static inline struct mpls_shim_hdr mpls_entry_encode(u32 label,
+						     unsigned int ttl,
+						     unsigned int tc,
+						     bool bos)
+{
+	struct mpls_shim_hdr result;
+
+	result.label_stack_entry =
+		cpu_to_be32((label << MPLS_LS_LABEL_SHIFT) |
+			    (tc << MPLS_LS_TC_SHIFT) |
+			    (bos ? (1 << MPLS_LS_S_SHIFT) : 0) |
+			    (ttl << MPLS_LS_TTL_SHIFT));
+	return result;
+}
+
+#endif