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, 30 insertions, 0 deletions

diff --git a/include/net/seg6_local.h b/include/net/seg6_local.h
new file mode 100644
index 000000000..888c1ce6f
--- /dev/null
+++ b/include/net/seg6_local.h
@@ -0,0 +1,30 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ *  SR-IPv6 implementation
+ *
+ *  Authors:
+ *  David Lebrun <david.lebrun@uclouvain.be>
+ *  eBPF support: Mathieu Xhonneux <m.xhonneux@gmail.com>
+ */
+
+#ifndef _NET_SEG6_LOCAL_H
+#define _NET_SEG6_LOCAL_H
+
+#include <linux/percpu.h>
+#include <linux/net.h>
+#include <linux/ipv6.h>
+
+extern int seg6_lookup_nexthop(struct sk_buff *skb, struct in6_addr *nhaddr,
+			       u32 tbl_id);
+extern bool seg6_bpf_has_valid_srh(struct sk_buff *skb);
+
+struct seg6_bpf_srh_state {
+	local_lock_t bh_lock;
+	struct ipv6_sr_hdr *srh;
+	u16 hdrlen;
+	bool valid;
+};
+
+DECLARE_PER_CPU(struct seg6_bpf_srh_state, seg6_bpf_srh_states);
+
+#endif