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, 26 insertions, 0 deletions

diff --git a/include/net/llc_sap.h b/include/net/llc_sap.h
new file mode 100644
index 000000000..b259f5993
--- /dev/null
+++ b/include/net/llc_sap.h
@@ -0,0 +1,26 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef LLC_SAP_H
+#define LLC_SAP_H
+
+#include <asm/types.h>
+
+/*
+ * Copyright (c) 1997 by Procom Technology,Inc.
+ * 		 2001-2003 by Arnaldo Carvalho de Melo <acme@conectiva.com.br>
+ */
+struct llc_sap;
+struct net_device;
+struct sk_buff;
+struct sock;
+
+void llc_sap_rtn_pdu(struct llc_sap *sap, struct sk_buff *skb);
+void llc_save_primitive(struct sock *sk, struct sk_buff *skb,
+			unsigned char prim);
+struct sk_buff *llc_alloc_frame(struct sock *sk, struct net_device *dev,
+				u8 type, u32 data_size);
+
+void llc_build_and_send_test_pkt(struct llc_sap *sap, struct sk_buff *skb,
+				 unsigned char *dmac, unsigned char dsap);
+void llc_build_and_send_xid_pkt(struct llc_sap *sap, struct sk_buff *skb,
+				unsigned char *dmac, unsigned char dsap);
+#endif /* LLC_SAP_H */